Fundamental Science and Engineering Questions in Planetary Cave Exploration

Abstract

Nearly half a century ago, two papers postulated the likelihood of lunar lava tube caves using mathematical models. Today, armed with an array of orbiting and fly‐by satellites and survey instrumentation, we have now acquired cave data across our solar system—including the identification of potential cave entrances on the Moon, Mars, and at least nine other planetary bodies. These discoveries gave rise to the study of planetary caves. To help advance this field, we leveraged the expertise of an interdisciplinary group to identify a strategy to explore caves beyond Earth. Focusing primarily on astrobiology, the cave environment, geology, robotics, instrumentation, and human exploration, our goal was to produce a framework to guide this subdiscipline through at least the next decade. To do this, we first assembled a list of 198 science and engineering questions. Then, through a series of social surveys, 114 scientists and engineers winnowed down the list to the top 53 highest priority questions. This exercise resulted in identifying emerging and crucial research areas that require robust development to ultimately support a robotic mission to a planetary cave—principally the Moon and/or Mars. With the necessary financial investment and institutional support, the research and technological development required to achieve these necessary advancements over the next decade are attainable. Subsequently, we will be positioned to robotically examine lunar caves and search for evidence of life within Martian caves; in turn, this will set the stage for human exploration and potential habitation of both the lunar and Martian subsurface.

Full text

Abstract Nearly half a century ago, two papers postulated the likelihood of lunar lava tube caves using
mathematical models. Today, armed with an array of orbiting and fly-by satellites and survey instrumentation,
we have now acquired cave data across our solar system—including the identification of potential cave
entrances on the Moon, Mars, and at least nine other planetary bodies. These discoveries gave rise to the study
of planetary caves. To help advance this field, we leveraged the expertise of an interdisciplinary group to
identify a strategy to explore caves beyond Earth. Focusing primarily on astrobiology, the cave environment,
geology, robotics, instrumentation, and human exploration, our goal was to produce a framework to guide
this subdiscipline through at least the next decade. To do this, we first assembled a list of 198 science and
engineering questions. Then, through a series of social surveys, 114 scientists and engineers winnowed down
the list to the top 53 highest priority questions. This exercise resulted in identifying emerging and crucial
research areas that require robust development to ultimately support a robotic mission to a planetary cave—
principally the Moon and/or Mars. With the necessary financial investment and institutional support, the
research and technological development required to achieve these necessary advancements over the next decade
WYNNE ETAL.
© 2022. The Authors.
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the terms of the Creative Commons
Attribution-NonCommercial-NoDerivs
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adaptations are made.
Fundamental Science and Engineering Questions in Planetary
Cave Exploration
J. Judson Wynne1 , Timothy N. Titus2 , Ali-akbar Agha-Mohammadi3,
Armando Azua-Bustos4,5 , Penelope J. Boston6 , Pablo de León7 , Cansu Demirel-Floyd8,
Jo De Waele9 , Heather Jones10 , Michael J. Malaska3 , Ana Z. Miller11,12 ,
Haley M. Sapers13, Francesco Sauro9, Derek L. Sonderegger14 , Kyle Uckert3 ,
Uland Y. Wong6 , E. Calvin Alexander Jr.15, Leroy Chiao16, Glen E. Cushing2 ,
John DeDecker17, Alberto G. Fairén4,18 , Amos Frumkin19 , Gary L. Harris7,
Michelle L. Kearney20 , Laura Kerber3 , Richard J. Léveillé21,22,
Kavya Manyapu23, Matteo Massironi24 , John E. Mylroie25,
Bogdan P. Onac26,27 , Scott E. Parazynski28 , Charity M. Phillips-Lander29 ,
Thomas H. Prettyman30 , Dirk Schulze-Makuch31,32,33 , Robert V. Wagner34 ,
William L. Whittaker9, and Kaj E. Williams2
1Department of Biological Sciences and Center for Adaptable Western Landscapes, Northern Arizona University,
Flagstaff, AZ, USA, 2U.S. Geological Survey, Astrogeology Science Center, Flagstaff, AZ, USA, 3Jet Propulsion
Laboratory, California Institute of Technology, Pasadena, CA, USA, 4Centro de Astrobiología, CSIC-INTA, Unidad María
de Maeztu, Instituto Nacional de Técnica Aeroespacial Ctra de Torrejón a Ajalvir, Madrid, Spain, 5Instituto de Ciencias
Biomédicas, Facultad de Ciencias de la Salud, Universidad Autónoma de Chile, Santiago, Chile, 6NASA Ames Research
Center, Moffett Field, CA, USA, 7Human Spaceflight Laboratory, Department of Space Studies, University of North
Dakota, Grand Forks, ND, USA, 8School of Geosciences, University of Oklahoma, Norman, OK, USA, 9Department of
Biological, Geological and Environmental Sciences, University of Bologna, Bologna, Italy, 10Robotics Institute, Carnegie
Mellon University, Pittsburgh, PA, USA, 11Laboratório HERCULES, University of Évora, Évora, Portugal, 12Instituto
de Recursos Naturales y Agrobiología, Consejo Superior de Investigaciones Científicas, Seville, Spain, 13Department
of Earth and Space Science and Engineering, York University, Toronto, ON, Canada, 14Department of Mathematics
and Statistics, Northern Arizona University, Flagstaff, AZ, USA, 15Earth and Environmental Sciences Department,
University of Minnesota, Minneapolis, MN, USA, 16Department of Mechanical Engineering, Rice University, Houston,
TX, USA, 17Center for Mineral Resources Science, Colorado School of Mines, Golden, CO, USA, 18Department of
Astronomy, Cornell University, Ithaca, NY, USA, 19Institute of Earth Sciences, The Hebrew University, Jerusalem, Israel,
20Department of Astronomy and Planetary Sciences, Northern Arizona University, Flagstaff, AZ, USA, 21Department of
Earth and Planetary Sciences, McGill University, Montreal, QC, Canada, 22Geosciences Department, John Abbott College,
Ste-Anne-de-Bellevue, QC, Canada, 23NASA Johnson Space Center, Houston, TX, USA, 24Dipartimento di Geoscienze,
Università degli Studi di Padova, Padova, Italy, 25Department of Geosciences, Mississippi State University, Starkville, MS,
USA, 26School of Geosciences, University of South Florida, Tampa, FL, USA, 27Emil G. Racoviță Institute, Babeș-Bolyai
University, Cluj-Napoca, Romania, 28Fluidity Technologies, Inc., Houston, TX, USA, 29Space Science and Engineering
Division, Southwest Research Institute, San Antonio, TX, USA, 30Planetary Science Institute, Tucson, AZ, USA,
31Astrobiology Group, Center of Astronomy and Astrophysics, Technische Universität Berlin, Berlin, Germany, 32Section
Geomicrobiology, GFZ German Research Centre for Geosciences, Potsdam, Germany, 33Department of Experimental
Limnology, Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), Stechlin, Germany, 34School
of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
Key Points:
• Robotics and instrument
advancements identified as linchpin
focal areas for in situ study of
planetary caves
• Research and technological
development required for lunar and/or
Martian cave exploration is achievable
in next decade with proper investment
• First application of systematic and
statistically rigorous social survey
to identify science and engineering
requirements in planetary science
Supporting Information:
Supporting Information may be found in
the online version of this article.
Correspondence to:
J. J. Wynne,
jut.wynne@nau.edu
Citation:
Wynne, J. J., Titus, T. N., Agha-
Mohammadi, A.-a., Azua-Bustos, A.,
Boston, P.J., de León, P., etal. (2022).
Fundamental science and engineering
questions in planetary cave exploration.
Journal of Geophysical Research:
Planets, 127, e2022JE007194. https://doi.
org/10.1029/2022JE007194
Received 13 JAN 2022
Accepted 22 APR 2022
10.1029/2022JE007194
Special Section:
Exploring planetary caves
as windows into subsurface
geology, habitability, and
astrobiology
REVIEW ARTICLE
1 of 32

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1. Introduction
Roughly 50years ago, two companion papers discussed the geologic rationale and provided mathematical mode-
ling to support the likelihood of lava tubes on the Moon (Greeley,1971; Oberbeck etal.,1969). Although Halli-
day(1966) speculated about their existence a few years earlier, these seminal papers reasoned the likelihood for
caves in the Oceanus Procellarum region and mathematically estimated widths, roof thicknesses, and lengths
of potential subsurface features. While these early works were built upon the analysis of low-resolution images
(acquired for Apollo mission landing site selection), planetary missions over the past two decades have acquired
imagery from increasingly higher resolution optical platforms and drastically improved the resolving capabilities
of potential subsurface access points (SAPs) on other planetary bodies.
Today, possible SAPs and terrains likely to support subterranean features have been identified from Mercury to
Pluto (Titus etal.,2021; Wynne etal.,2022). Over 1,000 potential cave openings have been identified on Mars
(Cushing,2017; Cushing et al., 2007) and over 200 potential cave-like features have been documented on the
Moon (Haruyama etal.,2009; Wagner & Robinson,2014,2015,2021). In the outer solar system, vents, pits, and
fissures associated with water ice plumes have been discovered on Saturnian (C. J. Hansen etal.,2011; Porco
etal.,2014), Jovian (Geissler & McMillan,2008; Roth etal.,2014), and Neptunian (Duxbury & Brown,1997)
icy moons. For example, on Enceladus, within the four primary fractures of the south polar region, at least 100
active geysers have been identified (Porco etal.,2014). Incidentally, steep-sided depressions and equatorial pits
have been observed on Titan (Wynne etal.,2022), which may have resulted from hydrocarbon fluids percolating
through thick organic materials in a process similar to karstic dissolution (Malaska etal.,2020). Thus far, 3,545
SAPs have been identified on 11 bodies across our solar system (Wynne etal., 2022). Additionally, possible
volcanic vents have been observed on Triton and Pluto, and numerous pit crater chains on icy and rocky bodies
will require further examination (Wynne etal.,2022). Refer to Wynne etal.(2022) for a complete review of
speleogenic processes and SAPs across the solar system.
Collectively, these features represent a new frontier in planetary exploration. Pits, vents, fissures, and caves
provide access to near surface geology and liquid oceans without the need for drilling or digging (Stamenković
et al., 2019). On Mars, these features may provide access to preserved volatiles including water ice
(Schörghofer,2021; Williams etal.,2010), brines (D. M. Burt & Knauth,2003), and organic matter (Richardson
etal.,2013). More broadly, planetary SAPs may ultimately provide data on volatile delivery and climatic oscil-
lations. Cave climates typically reflect the average annual surface temperature at depth (Cropley,1965; Pflitsch
& Piasecki,2003; Titus etal.,2010), suggesting that planetary subsurfaces likely represent warmer, more stable
environments. Importantly, SAPs contain environments buffered from ionizing space radiation and other hostile
surface conditions (De Angeles etal., 2002; Morthekai etal., 2007; Townsend & Fry,2002). These factors
combined make the planetary subsurface realm one of the most habitable locations to search for evidence of
extinct or perhaps extant life (e.g., Boston etal.,2001; Carrier etal.,2020; Northup etal.,2011; Perkins,2020;
are attainable. Subsequently, we will be positioned to robotically examine lunar caves and search for evidence
of life within Martian caves; in turn, this will set the stage for human exploration and potential habitation of
both the lunar and Martian subsurface.
Plain Language Summary We have now acquired cave data across our solar system—including
the identification of potential cave entrances on the Moon, Mars, and at least nine other planetary bodies. These
discoveries gave rise to the study of planetary caves. To help advance this field, we conducted an expert-opinion
based social survey to identify a strategy to explore caves beyond Earth. We focused primarily on astrobiology,
the cave environment, geology, robotics, instrumentation, and human exploration. First, we assembled a list of
198 science and engineering questions. Then, through a series of social surveys, 114 scientists and engineers
winnowed down the list to the top 53 highest priority questions. This exercise resulted in identifying emerging
and crucial research areas that require robust development to ultimately support a robotic mission to a planetary
cave—principally the Moon and/or Mars. With the necessary financial investment and institutional support, the
research and technological development required to achieve these necessary advancements over the next decade
are attainable. Subsequently, we will be positioned to robotically examine lunar caves and search for evidence
of life within Martian caves; in turn, this will set the stage for human exploration and potential habitation of
both the lunar and Martian subsurface.

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Stamenković etal.,2019). For the Moon and Mars, SAPs arguably represent one of the best locations to establish
astronaut shelters (Blamont,2014; Titus, Phillips-Lander, etal.,2020; von Ehrenfried,2019; Wynne etal.,2008).
Underscoring the growing importance of planetary caves, the 2023–2032 Decadal Survey included four contrib-
uted white papers discussing their significance (e.g., refer to NASEM[2022] for details). These papers likely gave
rise to the steering committee emphasizing the following in the Decadal Survey: (a) the importance of maturing
robotics and instrumentation to access planetary subsurfaces; (b) the need to identify environmental covariates
governing subsurface habitability and diversity on Earth (so that these data can be potentially extrapolated to
other bodies—in particular, Mars); and (c) the potential importance of the Martian subterranean realm in the
search for life (NASEM,2022).
Despite the tremendous potential that planetary subsurfaces represent, we are in the incipient phase of interpret-
ing and examining the subsurface of planetary bodies in the solar system. To effectively explore a planetary cave,
such a mission will ultimately require cross-planetary-body investigations spanning multiple disciplines includ-
ing astrobiology, climatology, geology, robotics and instrumentation, human exploration, and operations. Given
the inherent interdisciplinary nature of planetary cave science and exploration, we assembled a team of engineers,
roboticists, astrobiologists, geologists, and physicists to conduct an expert-opinion-based and systematic social
survey, often referred to as an “horizon scan.” Our goal was to identify emerging and crucial research areas that
require robust development to facilitate and support a successful robotic mission to a planetary cave—principally
the Moon and/or Mars—that could ultimately lay the foundation for human cave exploration and habitation. Here
we present our findings, which we believe will drive the next one to two decades of planetary cave research and
technological development.
2. Horizon Scan Methodology
Over the past decade, horizon scans, which employ an expert-opinion-based paradigm (see Wintle etal.,2020),
have been used to obtain insights and identify future directions into a panoply of medical, societal, and envi-
ronmental research areas. These include identification of emerging technologies in cancer research (Gallego
et al., 2012) and bioengineering (Kemp et al., 2020), storm- and waste-water management (Blumensaat
etal.,2019), improvement of management policies for governmental agencies (Hines etal.,2018), and the iden-
tification of future directions in ecology (Mammola etal.,2020; Patiño etal.,2017; Sutherland etal.,2013), as
well as annual assessments on global biological conservation issues, which have been conducted since 2011 (e.g.,
Sutherland etal.,2021; Sutherland, Bardsley, etal.,2011). To our knowledge, this is the first occasion where a
horizon scan approach was applied to identify research priorities in planetary science or space exploration.
We employed the horizon scan methodology using an expert-opinion-based approach whereby we enlisted the
input of a large interdisciplinary team of scientists and engineers to identify the fundamental questions in plane-
tary cave exploration. While horizon scans are increasingly used to systematically examine available information
to identify both emerging issues and priorities in various areas of research and societal growth (Brown,2007;
Könnölä etal., 2012; Sutherland, Bardsley, etal.,2011), we specifically used this approach to synthesize the
scientific and engineering requirements to make planetary cave exploration a reality.
2.1. Compilation of Initial Survey Questions
For this study, we borrowed elements from Sutherland, Fleishman, et al. (2011), Patiño et al. (2017), and
Mammola etal.(2020). Consisting of online surveys and an interdisciplinary group of scientists and engineers,
we applied a systematic approach to forecast the most important research and technological questions in planetary
cave exploration. In designing this horizon scan, we identified seven subject areas as crucial for advancing our
ability to examine planetary subsurfaces including (a) astrobiology, (b) the cave environment, (c) geology, (d)
instrumentation, (e) robotics, (f) human exploration, and (g) broad concepts. The last subject area was included
to optimally capture the interdisciplinary nature of planetary cave science, while potentially identifying “bigger
picture” concepts that may not fit into the other categories.
Each subject area consisted of one to two panel coordinators and from four to eight panel members (Table1).
Both panel coordinators and members were considered leading experts within each of the subject area groups.
For example, the human exploration section was paneled by space suit engineers, habitation pod designers, and

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retired astronauts. Panel coordinators worked with the lead author (JJW) to identify and invite additional panel
members. We also consulted broadly with colleagues to identify individuals who would contribute complemen-
tary expertise to each panel. Given the interdisciplinary nature of speleological research, planetary science, and
space technology development, some panel members participated on more than one panel. Panel members were
asked to submit 5–10 questions that they identified as fundamental within their subject area and thus likely to
advance the field significantly. However, a few panel members submitted as many as 15 questions. This resulted
in a total of 268 initial questions.
For each subject area, the lead author coordinated directly with panel leads to remove duplicate questions, revise
questions for clarity (e.g., Mammola etal.,2020; Plavén-Sigray etal.,2017), and, in some cases, reassign ques-
tions to other subject areas. This was accomplished by working jointly with the panel coordinator(s) and panel
member(s) who submitted the questions. To the extent possible, jargon was removed, and verbiage was stand-
ardized so that all questions had a similar degree of legibility. Once done, we had a total of 196 questions, which
became the questions for Survey 1.
2.2. Down Selection of Fundamental Questions
All online surveys were developed using Google Forms. Across the three online surveys (Surveys 1 through
3), participants voted for each question as being either of “major” or “minor” importance (sensu Mammola
etal.,2020). For these surveys, the order of survey questions was randomized for each respondent.
For Survey 1 (196 questions), 31 panelists (i.e., most of the coauthors of this paper) participated. All panel
members voted on all questions irrespective of subject area. While Mammola etal.(2020) identified the top 20
questions per subject area based upon the percentage of votes for their first survey, we deemed that approach
somewhat restrictive. Given that planetary cave exploration is an emerging facet of planetary research (Titus
etal.,2021; Titus, Phillips-Lander, etal.,2020), we chose not to risk discarding important questions. Thus, we
applied a more conservative approach whereby questions were retained for the next survey (Survey 2) if more
than 50% of panelists considered a given question of “major importance.” The outcome of this procedure resulted
in 152 questions for the next survey (Survey 2).
For Survey 2, the broader speleological and planetary science community was invited to participate. We solic-
ited participation using a combination of electronic listservs and individual panelists contacted their colleagues
directly via email. Listservs included the SETI Institute's personnel list, and the National Cave and Karst Research
Institute (NCKRI) monthly cave and karst news and announcements listserv. For the NCKRI mailing list alone,
there were nearly 2,000 recipients including several addresses that represent national and regional lists—of these,
between 50% and 60% were international recipients. Barring individuals who may forward a given monthly
update, the estimated reach of this list alone was ∼5,000 people (G. Veni; pers. (written) comm. 2021). We recog-
nize that this likely resulted in some overlap, and that some email addresses were inactive. Overall, we estimated
that Survey 2 reached at least 7,000 unique individuals. Of these, 82 individuals (who were not contributing
Subject area Panelists Members Questions
Instrumentation Uckert*, Malaska, Prettyman, Titus 4 25
Cave Environment Titus*, Cushing, Prettyman, Williams, Wynne 5 16
Geology Sauro*, De Waele*, Cushing, DeDecker, Frumkin, Kerber, Massironi, Onac 8 32
Robotics Jones*, Agha-Mohammadi, Whittaker, Wong 4 22
Astrobiology Demirel-Floyd*, Azua-Bustos, Boston, Fairén, Miller, Phillips-Lander, Sapers, Schulze-Makuch 8 24
Human Exploration de León*, Chiao, Harris, Manyapu
†, Parazynski
†5 27
Broad Concepts Boston*, Alexander, Léveillé, Mylroie, Schulze-Makuch, Wynne 6 6
Other contributors Kearney, Sonderegger, Wagner -- --
Note. An*indicates panel coordinators. Four members served on more than one panel, while two panel members (
†) participated in Survey 1 question development but
were unavailable to partake in survey voting. The “Questions” column indicates the number of questions per group for Survey 1.
Table 1
Summary of Panel Member Names, Total Number of Panel Members per Group, and Number of Questions per Subject Area

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authors on this work) completed Survey 2. The breakdown of Survey 2 respondents by profession includes the
following: geologists (at 47.5%), astrobiologists/biologists (17%), physicists (5%), engineers (14.5%), two chem-
ists, one climatologist, and 12.2% listing “other” as their profession. To gain further insights into the composition
of Survey 2 by institution, we categorized respondents by the email addresses provided. This consisted of 18 indi-
viduals with U.S. academic addresses, 18 with non-U.S. academic addresses, 14 with non-academic professional
addresses (e.g., NASA, ESA, and other governmental and non-governmental agencies), and 32 with personal
email addresses (e.g., Gmail, Yahoo, and Hotmail accounts).
Upon completion of Survey 2, participants had the option of submitting one additional question (sensu Mammola
etal.,2020). This provided us with the opportunity to ensure that we developed the most comprehensive process
for identifying the fundamental questions in planetary cave exploration. Participants submitted a total of 24
additional questions/comments. Of these, three were general comments, two were general questions related to
potential funding sources for a planetary cave mission, and four questions were either vaguely written or had
been addressed theoretically through previous research. Once those nine questions/comments were removed, we
applied the same revisionary approach as in Survey 1, whereby duplicate questions were removed (by crosscheck-
ing the submitted questions with Survey 1 and 2 questions, as well as between the submitted questions), jargon
was removed, and wording standardized for clarity.
Once done, we had eight questions for Survey 3. For this survey, 31 panel members voted (same as for Survey 1).
To identify the highest priority fundamental questions in planetary cave exploration, the results of Surveys 2 and
3 were combined. This resulted in a total of 160 questions. To determine the highest priority (i.e., fundamental)
questions, percentages were calculated using the response data and then rank ordered. All questions from each
survey are provided in Text S1–S3 of Supporting InformationS1 (Wynne,2022).
2.3. Caveats on Interpretation
Several considerations should be addressed when interpreting both the horizon scan approach and the results
of this method. First and foremost, this work applied a methodology that may be most familiar to social and
political scientists (i.e., a general social survey, GSS; refer to R. S. Burt,1984). The backbone of this method-
ology involved: (a) developing questions across the spectrum of planetary cave science and engineering, (b)
scrutinizing over each question through a series of surveys whereby participants either affirmed or negated
a given question's importance via a binary vote, and (c) then rank-ordering the survey results to assemble
the highest priority (or fundamental) questions. As we were merely presenting and subsequently interpreting
the results of this systematic social survey, we do not have the latitude to interject ideas or other considera-
tions beyond what was enumerated as the fundamental questions—doing so would violate the purpose of the
horizon scan—which was to identify the highest priority science and engineering questions based upon the
horizon scan procedures employed.
Concerning the interpretation of horizon scan results, most of these caveats have already been discussed. For
details, refer to Sutherland, Fleishman, etal.(2011), Patiño etal.(2017), and Mammola etal.(2020). However, we
identified three additional caveats that warrant further examination and discussion. First, there may be a potential
for perception limitations and subjectivity of the panelists. For example, none of the panelists in the human explo-
ration group had a strong working knowledge of speleology and/or planetary cave science. Conversely, more than
half (55%) of all panelists had six or more years of scientific and/or recreational caving experience. Concerning
bias, we examined the potential for bias by professional specialty. We suggest any potential shortcomings were,
in part, addressed by the conservative down-selection method we applied to Survey 1. Additionally, including a
relatively large number of panelists from a range of academic backgrounds and career stages (Figure1) may have
further mitigated bias (Sutherland, Fleishman, etal.,2011). Second, while Sutherland etal.(2013) suggested
the use of subject areas (or themes) may facilitate lateral thinking, we do not believe this was the case for this
exercise. Roughly 13% of all panelists served on more than one panel. Moreover, more than a quarter of panelists
submitted questions that were ultimately subsumed into other subject area groups. While purely qualitative, we
suggest this emphasizes both the interdisciplinary nature of the panel, as well as the panelists' ability to think
broadly and dynamically about how best to advance the exploration of planetary caves. Finally, while many of the
questions were presented to address planetary caves in general, lunar and Martian caves represent the most likely
near-term targets for exploration; subsequently, many of the subject area summaries are focused on these bodies.

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2.4. Statistical Analysis
Given the professional specialties of participants across the two surveys were disproportionate in terms of the
number of participants per professional specialty (Figure 1), we examined whether this contributed bias to
the survey results. For example, ∼48% of the Survey 2 participants identified “geology” as their professional
specialty and we wanted to gain inference into whether survey results were skewed toward geology. Because
Survey 3 participants were the same participants as Survey 1, we did not analyze the Survey 3 data set. We used
nonmetric multidimensional scaling (NMDS) with 500 random starts (to obtain the lowest stress result). The
higher the R
2 value, the better the “goodness of fit” of the data. NMDS also enabled us to examine how questions
parsed by professional specialty in graphical space. We then used generalized linear mixed models (GLMM) to
estimate relationships between survey responses and survey participants by professional specialty. To under-
stand the practical significance of this factor, we used the GLMM R
2 (sensu Nakagawa & Schielzeth,2013). All
analyses were conducted using RStudio, version 3.4.0 (2017-04-21) with “vegan” and “glmmTMB” packages,
respectively. GLMM R
2 calculation was completed using the “performance” package.
3. Results and Discussion
We found little evidence to suggest that participants' professional specialties substantially influenced how they
answered the survey questions. NMDS analysis reported low stress and a statistically non-significant effect on
profession for both Survey 1 (stress=0.114, R
2=21.5%, p=0.252) and Survey 2 (stress=0.114, R
2=6.4%,
p=0.54; Figures2 and3). There was no clustering by question group and respective professional specialty. This
is further demonstrated when viewing sawtooth charts depicting how individual respondents answered questions
per professional specialty (Figures2c and3c). Results from GLMM runs for Surveys 1 and 2 produced a statisti-
cally significant effect for the interaction terms only (

2
=63.3.7, df=25, p<3.6×10
−5 and

2
=124.7, df=36,
p<1×10
−10, respectively). To further examine these differences, we then compared all paired comparisons of
Figure 1. Summary by subject area groups, workflow, statistics of panelists (Surveys 1 and 3) and the broader community (Survey 2), and breakdown of the 53
fundamental questions in planetary cave science and engineering by subject area group.

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survey participants by specialty for each question group; this resulted in some significant differences (Table2).
These results indicated that participants voted differently between specialties; thus, further suggesting that
these individual differences contributed to the statistical significance given the interaction effect of professional
specialty and question group.However, this had little practical effect as the R
2 value increased only from 22.2%
to 26.1% and 33.0% to 34.0% for Surveys 1 and 2, respectively.
Upon completion of rank ordering the questions, we found that there was no clean break point at 50 questions.
For questions ranked 48 through 53, the percentages were the same; these questions were all at 74.4% (or a
split of 61 to 21). Thus, we could either have a top 47 or top 53 fundamental questions. We chose the latter
option. The top 53 questions are provided, in rank order, as Table S1 and S2 in Supporting InformationS2
(Wynne,2022).
We now present the 53 top-priority questions in planetary cave science and engineering. In essence, these ques-
tions embody the data produced from the social surveys (and represent the combined results of Surveys 2 and
3). For each question, we provide the rank (#) among all total questions and the percentage of “major impor-
tance” votes received. The two questions submitted by Survey 2 participants are denoted by an asterisk (*). The
purpose of the following seven sections (i.e., subject areas) represent data analysis and interpretations. Here, we
discuss the state of knowledge pertaining to each question, as well as provide scientific and technical guidance
for researchers addressing these issues in the future.
Figure 2. Evaluation of potential bias of survey participants by professional specialty for the 31 survey participants in Survey 1. (a) Whisker plots provide quartile
breakdown; solid line within the box represents the median and dots are outliers. (b) NMDS illustrates the lack of clustering across the various professional specialties.
Analysis ran with 500 random starts. (c) Summary of responses of participants (i.e., the authors) by professional specialty (individual blue lines of varying hues)
identifying questions as “major importance” (y-axis) by question group (x-axis). Each line in this graph represents an individual survey participant. Each point along the
line is the average of how many times the individual chose “major importance” for a given question group.For example, the datapoint for an individual in the robotics
group who considered all broad concepts questions to be of “major importance” would appear as 1.0 (or 100%) on the graph.

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Figure 3. Evaluation of potential bias of survey participants by professional specialty for the 82 survey participants in Survey 2. (a) Whisker plots provide quartile
breakdown; solid line within the box represents the median and dots are outliers. (b) NMDS illustrates the lack of clustering across the various professional specialties.
Analysis ran with 500 random starts. (c) Summary of responses of participants by professional specialty (individual blue lines of varying hues) identifying questions as
“major importance” (y-axis) by question group (x-axis). “Other” represents other profession. Each line in this graph represents an individual survey participant. Each
point along the line is the average of how many times the individual chose “major importance” for a given question group.For example, the datapoint for an individual
in the robotics group who considered all broad concepts questions to be of “major importance” would appear as 1.0 (or 100%) on the graph.
Question group Contrast of professional specialty Odds ratio SE df t-value p-value
Survey 1
Environment Instruments/Astrobiology 11.69 8.703 6,033 3.304 0.012
Environment Instruments/Human Explore 28.07 24.17 6,033 3.874 0.002
Environment Robotics/Instruments 0.072 0.059 6,033 −3.245 0.015
Geology Instruments/Human Explore 17.75 13.77 6,033 3.708 0.003
Survey 2
Geology Environment/Astrobiology 4.287 2.105 12,414 2.965 0.048
Note. Only comparisons with statistical significance (p≤0.05) are presented with odds ratios, standard errors (SE), degrees
of freedom (df), t-distributed test statistic (t-value), and p-values provided.
Table 2
Paired Comparisons of Professional Specialties by Question Groups for Surveys 1 and 2

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3.1. Astrobiology
Q1: Do caves on Mars represent habitable systems for microbial life that once dwelled on the Martian surface?
(#6; 85.4%)
Q2: What lines of evidence are required to conclusively prove life exists/existed in planetary caves? Specifically,
how may we design missions to reduce the likelihood of false negatives? (#7; 84.2%)
Q3: Are speleothems in planetary caves potential archives for past microbial life? If so, how would we confidently
identify these biosignatures? (#11; 82.9%)
Q4: How can we predict the astrobiological value of a planetary cave (i.e., a Martian cave) before exploring it?
Specifically, are there indicators at/near cave entrances or surface expressions over the cave that could be
indicative of microbial metabolic activity? (#16; 81.7%)
Q5: How do we define the preservation potential of biosignatures in Martian caves? Importantly, what are the
most important factors/mechanisms that could facilitate preservation or decomposition of these biosignatures?
(#17; 81.7%)
Q6: If microbial life were confirmed in planetary caves, what can we deduce concerning the origin of life? Impor-
tantly, could we acquire adequate samples to understand how these microbes evolved or diverged throughout
time? (#20; 80.5%)
Q7: How do we expect water/moisture availability to affect biomineral formation or structure in Martian caves?
(#37; 76.8%)
The importance of planetary caves in the search of life elsewhere in the solar system remains largely unstudied.
Investigations will be required to determine whether caves may have been abodes for life on other planets, and if
these features retain preserved biosignatures (or perhaps even support extant life). For the case of Mars, if micro-
bial life arose and prospered on the surface, it may have later used caves as refugia (Q1) as the planet became
increasingly drier and colder (Schulze-Makuch & Irwin,2018). Caves on Earth, because they are protected from
surface processes (such as extreme temperature fluctuations and UV radiation) and have stable physicochem-
ical conditions, harbor a vast diversity of microorganisms able to interact with minerals and exploit different
metabolic pathways (e.g., Boston etal.,2001; Miller et al., 2020). Thus, life, or the biosignatures left behind,
may be protected within Martian caves—remaining more or less unchanged for millions of years (Léveillé &
Datta,2010). If this occurred, several lines of evidence will be required to conclusively demonstrate life exists
or existed in Martian caves, or planetary caves more broadly (Q2). We will need to quantify the physicochem-
ical parameters defining the cave environment and distinguish it from the regional conditions characterizing
the adjacent surface environment. Furthermore, we will need to parameterize how these conditions affect both
the ability of lifeforms to produce biosignatures and the preservation potential of these signatures. Devising a
well-established biogenicity criteria tailored to the geochemical evolution of the cave environment will be of
paramount importance (Azua-Bustos etal.,2020; Neveu etal.,2018; Röling etal.,2015; Rouillard etal.,2021;
Westall etal.,2015).
On Earth, many microbial species actively grow on and within cave sediments and speleothem surfaces, forming
colored microbial mats (Gonzalez-Pimentel etal.,2018; Hathaway etal.,2014) and promoting biomineraliza-
tion processes (Ghezzi etal.,2021; Miller et al., 2014,2016). These geomicrobiological interactions involve
mineral dissolution, precipitation, and changes in the redox state (e.g., Fe
2+ to Fe
3+), which can preserve traces
of microbial features and are considered astrobiologically relevant biosignatures (Northup etal.,2011; Riquelme
etal.,2015; Westall etal.,2015). Thus, mineral deposits such as speleothems could be important repositories for
extant and past microbial life and represent a unique setting to search for putative biosignatures (Q3). For exam-
ple, advanced microscopy techniques, DNA sequencing, and analytical biogeochemistry tools could provide the
necessary information to assess microbe-mineral interactions recorded in Martian speleothems—from molecular
to macroscopic scales (Castro-Wallace etal.,2017; Goordial etal.,2017; Miot etal.,2014; Onstott etal.,2018;
Rouillard etal.,2021).
A priori methods to characterize microbial metabolic activity related to surface expressions and/or cave entrances
have not been fully examined (Q4). However, determining the detectability of various gases and gas compounds
produced by cave microorganisms may be a viable approach. For example, microalgae in Atacama Desert coastal
caves produce oxygen as a photosynthetic byproduct (Azua-Bustos etal.,2009,2010), while Webster etal.(2015)

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and Webster(2019) have shown that chemical and isotopic constituents in cave air are distinguishable from the
surrounding ambient air. While this approach seems promising, more research will be required to examine other
gases (e.g., methane, hydrogen, and carbon monoxide; Lyu etal.,2018; Voordouw,2002; Wang & Wan,2009)
produced by different microbial metabolisms.
Biomineralization involves the deposition, precipitation, or crystallization of minerals either inside or outside
microbial cells, including within living organisms, dead biomass, and biological material—such as extracellular
polymeric substances in biofilms. Thus, biominerals may compose a potential target for identifying microbial
colonization in putatively habitable Martian caves. On Earth, their preservation potential depends on the physic-
ochemical, environmental, geologic, and climatological conditions since their formation (Allwood etal.,2013;
Banfield etal.,2001; Hays etal.,2017; Summons etal.,2011). However, the conundrum is that conditions facil-
itating preservation in one environment can be degrading in another; moreover, environmental factors supporting
habitability typically degrade biosignatures (Hoehler & Westall,2010; Summons etal.,2011). Importantly,noth-
ing is known regarding the preservation potential of biosignatures in Martian caves (Hays etal., 2017) (Q5).
Biomineralization, which occurs at the interface between the cell and its environment, is influenced by water
availability (Q7), as well as the concentration of ions in solution, pH, redox state, and metabolic processes.
Water availability in Martian caves is expected to vary by cave location, size, and morphology (refer to the Cave
Environment section below). In terrestrial subsurfaces, at least an order of magnitude more cells are physically
attached to a substrate than free-living cells in the aqueous phase (Bar-On etal.,2018). Using terrestrial cave
microbes as an analog, cave microorganisms on Mars may employ a similar strategy and would most likely be
rock associated provided water is available at the appropriate threshold to sustain life (Q7). For example, cryp-
toendolithic microorganisms living in the shallow subsurface of the McMurdo Dry Valleys of Antarctica were
identified by the biomineralization of cells and cell casts following cell death, which suggested that fossilization
processes could produce biosignatures in extremely water limited environments using organic matter as nuclea-
tion templates in the absence of active microbial metabolism (Wierzchos etal.,2005). Similar processes may be
expected in Martian caves with transiently habitable conditions.
Terrestrial life requires liquid water, nutrients, and energy sources to sustain life functions. These requirements
have varied spatially and temporally on Mars. If life exists or existed on Mars, this variation would have limited
the origin and perhaps the continuous existence and evolution of life (Westall etal.,2013). For this reason, the
likelihood of finding subsurface life on other planets (in particular, on Mars) will be dependent on defining the
spatial and temporal variations of habitability—and how this variation pertains specifically to isolated environ-
ments such as caves (Q6). Additionally, well-established biogenicity criteria (elucidated in Q2 above) will be
required.
While we first need to detect evidence of extraterrestrial life before we gain inference into how extraterrestrial
microbes may have evolved and speciated over time, addressing both life detection and evolution will most likely
require a similar approach. Developing a terrestrial data library of cave geologic, structural, and climatic infor-
mation, as well as defining environmental zones (within a given cave(s) of interest) will be required to ultimately
model habitability potential of caves beyond Earth. The highest likelihood of successfully identifying putative
cave biosignatures will require knowledge of the optimal location(s) within a cave to sample, and the number of
samples required for statistical significance. The ability to remotely define cave environmental zones (refer to
Howarth[1980,1982] for zonal definitions) should be developed for Martian caves. For example, on Mauna Loa,
Big Island, Hawaii, active coralloids (or “cave popcorn,” a speleothem that forms by the precipitation or evap-
oration of water) demarcate the cave transition zone, while the deep zone is defined as beyond the point where
these speleothems are found (F. G. Howarth, pers. (written) comm. 2021). Additionally, deep zones of some
Mauna Loa caves support perennial ice (Schörghofer etal.,2018). Thus, knowing where to sample within a given
planetary cave will require a robust understanding of environmental zones within terrestrial analog caves, as well
as how to accurately model and transfer those environmental conditions to another planetary body. Moreover, an
appropriate instrument payload (i.e., temperature, relative humidity, and radiation sensors) with adequate time
to collect environmental measurements represents a viable alternative to facilitate the characterization of distinct
cave zones. For sampling intensity, accurately detecting past or extant extraterrestrial microbial life will require
adequate statistical power to avoid contradictory results or false positives (Rouillard etal.,2021). For cave micro-
biological studies, limited samples are often collected to minimize impacts to the cave environment, which may
fail to detect some microorganisms (Fletcher etal.,2011; Shao & Wang,2009). In this context, it is important to

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identify the minimum number of samples required to optimize life detection. Future terrestrial cave studies that
collect robust datasets to develop and test models to determine sample size requirements most accurately will be
required.
3.2. The Cave Environment
Q8: What are the best terrestrial analogs for planetary caves? (#8; 84.2%)
Q9: How do geological events and extreme surface conditions affect planetary cave environments? (#12; 82.9%)
Q10: What is the typical extent of water ice formations within Martian caves? (#21; 80.5%)
Q11: As temperature and relative humidity are considered the primary and readily measurable meteorological
variables driving microbial activity in caves, how do we acquire these data in a manner to identify the optimal
locations to sample within a Martian cave from a rover platform? (#27; 79.3%)
Q12: For a given Martian cave, what is the diurnal and seasonal temperature and relative humidity variations of
the cave interior? Importantly, what sections of a given cave are most variable and what sections are most
stable? (#38; 76.8%)
Q13: How far does cosmic radiation attenuate beyond the cave entrance? In other words, how deep within the
cave must either a rover or human traverse to reach an area insulated from surface radiation? (#42; 75.6%)
Q14: What are the prevalent gases within lunar and Martian caves? How might their presence affect the search
for life (specifically, on Mars)? (#50; 74.4%)
Presently, Earth is the only planetary body where we can monitor the full range of processes that characterize the
cave environment. The terrestrial subsurface encompasses a range of cave types, which are defined by formation
processes and resulting structure (Boston,2004; Titus etal.,2021; Titus, Phillips-Lander, etal.,2020; Wynne
etal.,2022). Surface conditions also vary widely as caves are distributed globally, occurring in nearly every
biome on land and underwater. Which of these locations supports the best caves for analog studies remains a key
question (Q8)—as highlighted by the fact that it ranked 9th in our survey. Analogs provide a test bed for both
technology demonstrations and validation of cave climate models (Léveillé & Datta,2010). In addition to natural
caves, human-made caves (such as tunnels and mines) would provide a more controlled environment. Impor-
tantly, model caves could be constructed inside temperature and pressure-controlled chambers (e.g., the Ames
Planetary Aeolian Laboratory for simulating windborne processes), which could be used for both cave climate
model validation, as well as to simulate predicted cave conditions on other planetary bodies.
The remaining prioritized questions for the cave environment were grouped into three categories: external driv-
ers, cave geometry, and remote sensing of cave interiors. On Earth, cave climate is driven by a combination of
surface climate including temperature (Cropley,1965; Pflitsch & Piasecki,2003), airflow, humidity, precipita-
tion, cave geometry (Badino,2010), and in some cases, subsurface conditions such as geothermal heat flux in
volcanic regions. For other planetary bodies, cave climate (or the stability thereof) determines both habitability
and the potential preservation of evidence of past habitability (Williams etal.,2010). In short, understanding
the interactions of multiple abiotic processes influencing the cave environment is key to understanding potential
habitability (Q9).
Characterizing radiation levels within planetary caves will be necessary for both habitability assessments and
potential human exploration (e.g., Blank etal.,2018; Boston etal.,2001; Northup etal.,2011) (Q13). Measure-
ments should include emissions from the decay of naturally occurring radioisotopes found in the subsurface, and
the cascade of particles produced from the interaction of cosmic rays with the surface and/or atmosphere. The
latter consists of electromagnetic radiation and energetic ions and neutral particles that can be deleterious to most
terrestrial life (Atri & Melott,2014). Galactic cosmic rays and their secondaries are highly penetrating and could
reach cave interiors through thin ceilings (≤1–2m; De Angeles etal.,2002; Morthekai etal.,2007; Townsend &
Fry,2002) and/or via cave entrances. The latter mechanism may be important for solar energetic particles, which
are less penetrating but can result in harmful surface exposures (e.g., Hu etal.,2009). Surface radiation meas-
urements should also be considered—as this would provide a boundary condition for particle transport models to
predict cave radiation levels. Accurate simulations will also require knowledge of the physical and chemical prop-
erties of the surrounding rock (i.e., rock density and average atomic mass) and cave geometry. Direct internal cave
measurements could be conducted with compact instrumentation (e.g., Hassler etal., 2014) and supplemented

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by modeling. Such instrumentation could have a dual use, including measurements of depth (e.g., Prettyman
etal.,2020) and the elemental composition of the surrounding rock (refer to the Instrumentation section). The
decay of naturally occurring radioisotopes may dominate the dose within deep caves, while the buildup of radon
gas will be affected by cave climate and external forcing by fluctuations in surface conditions (e.g., temperature,
relative humidity, and barometric pressure; Šebela & Turk,2011).
Cave geometry also determines how surface conditions will influence the cave environment (de Freitas & Little-
john,1987; Tuttle & Stevenson,1978; Williams & McKay,2015; Williams etal.,2017). Acquiring a data set
with adequate statistical power to model how temperature and humidity varies by structure will enable a robotic
platform to both identify the most stable and buffered locations within a cave (Q11), which, in most cases, will
represent the regions of optimal habitability and thus the best locations to sample for evidence of life (Q12).
Moreover, Q10 highlights the importance of cave ice. Airflow, which is also influenced by cave geometry as well
as surface temperature and barometric pressure shifts governs cave temperature and humidity regimes and water/
water ice stability (Perșoiu & Onac,2019; Williams & McKay,2015; Williams etal.,2017).
Our last cave environment question (Q14) was focused on how to measure gases (e.g., radon, methane, CO2, and
water vapor) within lunar and Martian caves, and how these measurements may be used to search for evidence of
life. As the gas composition within a cave could be modified by the presence of life (e.g., Mansor etal.,2018)—
principally on Mars—this opens the possibility of using either spectroscopy via remote sensing or taking direct
measurements at a cave entrance via a robotic platform. However, before researchers can move forward with these
techniques, we need to address how the cave atmosphere could be modified by abiotic processes (e.g., methane;
Klein etal.,2019). Importantly, could a life-modified atmosphere be detectable from inside the cave or possibly
outside the cave if the cave is exhaling constituents produced by life?
The cave environment is the boundary condition for habitability but is determined by the complex interactions of
several physical processes (e.g., conduction, convection, and advection), surface and subsurface conditions, and
the cave geometry. Addressing these questions by investigating the appropriate terrestrial analogs will increase
our understanding of these interactions, so that we may extend this to modeling cave environments on other
planetary bodies.
3.3. Geology
Q15: Is there solid ice or carbon dioxide within caves on the Moon and Mars? (#18; 81.7%)
Q16: How do we accurately identify lunar and Martian pit craters that possess laterally trending passageway (i.e.,
a cave)? (#28; 79.3%)
Q17: Where are soluble formations (evaporites or carbonates) located on other planetary bodies? Once deter-
mined, do accessible dissolution caves exist within soluble formations (e.g., sulfates, carbonates, halides) on
Mars and other planetary bodies? (#32; 78.1%)
Q18: Do dissolution caves formed by rising groundwater levels exist on Mars? If this is a possibility, where
should we search for them? (#39; 76.8%)
Q19: How can we develop quantitative and highly accurate paleoclimate records from planetary cave deposits?
(#43; 75.6%)
Q20: On which planets and satellites do caves actively form today? (#44; 75.6%)
Most of the remaining unanswered geologic questions related to planetary caves involve formation mechanisms,
distribution and occurrence, morphologies, and the potential for internal deposits across different planetary and
geologic settings. These questions should be framed within the scope of cave size and duration of existence,
which can be highly variable (Mylroie,2019). From our survey, 46 geology questions were identified; of these,
six were considered fundamental.
The most important geology question (Q15, which garnered 82% of the votes from respondents) queried whether
water ice deposits or solid carbon dioxide occur in caves on the Moon and Mars. Caves with (water) ice deposits
are well known on Earth and may be present on Mars and the Moon under specific environmental conditions
(Cooper,1990; Perșoiu & Onac, 2019; Williams & McKay,2015; Williams etal., 2010, 2017)—when subli-
mation and frosting dominates (Schörghofer,2021). Sublimation could also generate cavities in water ice or

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solid carbon dioxide on the polar ice caps of Mars, as well as on icy bodies across the solar system (see Wynne
etal.,2022). However, it remains unclear how these caves could form and what factors would contribute to their
stability.
Numerous SAP candidates have been identified on the Moon and Mars (see Wynne etal., 2022). Many of
these features have been detected in volcanic terrains, which suggests some may be associated with lava tubes
(Kempe,2012). For the Moon, at least eight have overhangs with lateral passage extending at least 20m have
been identified (Wagner & Robinson,2021); some lunar features may support passageways several kilometers
in length and volumes of hundreds of millions of cubic meters (Chappaz etal.,2017; Sauro etal.,2020; Q16).
Many lunar pits occur within impact melt deposits, suggesting an origin related to the impact process or subse-
quent melt pond dynamics (Wagner & Robinson,2014). However, another class of feature worth investigating is
volcano-tectonic cavities (Ferrill etal.,2004; Wyrick etal.,2004), which appear to be more common than lava
tubes (Wynne etal.,2022). Additionally, there are anomalous, isolated pits (e.g., Type 1 atypical pit craters;
Cushing etal.,2015) whose formation mechanism(s) remain unclear. Since these include some of the deepest
and most voluminous pits detected on Mars, the presence or absence of accessible lateral passageways remains
an important constraint that has yet to be addressed. Question 16 could be addressed either via orbiter/flyby
gravimetric surveys (e.g., Chappaz etal.,2017), or with rovers or landers equipped with ground penetrating
radar sounders (Carrer etal.,2017; Kaku etal.,2017) or ground electric resistivity arrays (Torrese etal.,2021).
Other high priority questions involved the potential for speleogenic dissolution processes (Q17 and Q18) on
Mars and other planetary bodies (Baioni & Wezel,2010; Baioni etal.,2009; Malaska & Mitchell,2017; Wynne
etal.,2022; Zupan-Hajna etal.,2017). Dissolution caves are the most common on Earth (Palmer,2007), develop-
ing where carbonate and evaporite lithologies predominate. Presently, the two planetary bodies for which disso-
lution has occurred or is likely to occur are Titan and Mars, respectively (Q17). On Titan, karst-like dissolution
processes with liquid hydrocarbon dissolving thick organic deposits have been implicated in the formation of lake
basins and “labyrinth terrains” (Cornet etal.,2015; Malaska etal.,2020). For Mars, recent studies suggest silici-
clastic rocks can give rise to dissolution when specific environmental conditions exist (Wray & Sauro,2017)—
whether these lithologies could have interacted with ancient water sources on the Martian surface or shallow
subsurface remains unknown. We do know that Mars contains extensive sulfate terrains and potential carbonate
outcrops within ancient craters (Barbieri & Stivaletta,2011; Palomba etal.,2009; Vaniman etal.,2004), while
some geomorphic features on Mars resemble terrestrial diapiric dissolution features (Frumkin etal.,2021). Ques-
tion 18 was also related to the potential presence of off-Earth hypogenic caves, formed by fluids rising along
deep tectonic structures (Klimchouk,2009, 2012). On Earth, this speleogenic mechanism is widespread and
considered one of the primary cave-forming processes across a wide range of lithologies (Sauro etal.,2014).
Another important topic is how to identify, measure, and quantitatively examine paleoclimate records from
planetary cave deposits (Q19). For Earth, paleoclimatic research involves examining the carbon and oxygen
isotope composition of cave minerals and speleothems in both volcanic (Ulloa etal.,2013) and dissolution (both
carbonate and evaporite) caves (De Waele etal.,2017; Fairchild & Baker,2012). On the Moon, Mars, and other
planetary bodies, the presence of secondary minerals is likely related to local geochemical processes, possibly
due to the presence of volatiles, which has direct implications in the search of life on Mars. Importantly, could
we apply the same terrestrial techniques to extract paleoclimatic and/or paleoenvironmental information from
speleothems on another planetary body? Terrestrial caves are typically characterized by stable climatic conditions
where evidence of past climates (mainly within sediments and chemical precipitates) can be preserved much
better than on the surface. By comparing similar deposits on Earth (e.g., Fairchild & Baker,2012), Martian cave
deposits may represent key proxies for further characterizing the climatic history of Mars. However, until we can
conduct a robotic mission to explore a Martian cave interior, this remains speculative.
Finally, there is a need to identify planetary bodies actively undergoing speleogenesis (Q20). This has prelimi-
narily been addressed by Wynne etal.(2022), whereby a solar system wide inventory was conducted to identify
past speleogenic processes and resultant products; they reported nine speleogenic processes on 15 planetary
bodies and SAPs on 11 bodies. For example, Enceladus (Porco etal.,2014), Titan (Malaska etal.,2020; Wynne
etal.,2022), and other icy bodies support active speleogenesis associated with tectonic activity and dissolution,
while Mercury (Jozwiak etal.,2018), Venus (Davey etal.,2013), and Io (Wynne etal.,2022) likely support caves
within tectonic and volcanic terrains (Wynne etal.,2022 and additional references therein). Additionally, fissures
responding to seasonal processes may occur in subpolar regions of Mars, with potential subglacial networks

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of brine-filled cavities (e.g., Lauro etal.,2021). Characterizing areas of interest using repeat high resolution
imagery will ultimately be required to ascertain whether a body of interest is undergoing speleogenesis—as the
dynamics of subsurface processes over time will be crucial for understanding the potential role of planetary caves
in the development and preservation of life on other planets.
3.4. Instrumentation
Q21: What suite of instruments would be optimal for detecting biosignatures and characterizing the mineralogy/
chemistry of a subsurface environment? (#1; 96.3%)
Q22: What is the best instrument (or suite of instruments) required for the in situ study of microbe-mineral inter-
actions? (#9; 84.2%)
Q23: Where are the primary science targets located (i.e., in the entrance zone, twilight zone, and/or deep zone)
within a planetary cave? (#24; 80.5%)
Q24: What instrument resolution (spatial, spectral, and temporal) will be required to answer the key science
questions in a cave environment? (#30; 79.3%)
Q25: For instruments deployed for long-term, stand-alone monitoring, how will instruments be powered and how
will data be transmitted from cave to surface? (#31; 79.3%)
Q26: Can organic signatures be detected on walls from a distance (e.g., 2–3m) using a stand-off UV Raman
spectrometer? (#35; 78.1%)
Q27: What suite of navigational instruments and associated instrument resolutions would be optimal for both
cave mobility and accessing potential sample locations within a cave? (#52; 74.4%)
Q28: What type of stand-off distance (close contact [<1m], remote [10m]) or in situ access (sample handling/
ingestion or drilling) is required to address key scientific questions in a cave? (#53; 74.4%)
A future mission to a planetary cave will require a suite of instruments requiring innovations in mobility, navi-
gation, and communications systems to collect the data to address key science questions. The most important
question recognized in this survey (Q21; #1 as identified by ∼96% of the respondents) emphasized the impor-
tance of identifying an optimal instrument suite to detect biosignatures and characterize the geochemistry of a
planetary subsurface environment. The highest priority astrobiology, geology, and cave environment (refer to
respective sections) science investigations will likely implement a combination of sampling, contact science,
standoff, and remote sensing instrumentation. Instrument suite selection and their required performance charac-
teristics (e.g., resolution, detection limits, and measurement cadence) (Q24) will be tailored to the science objec-
tives of a specific mission, as well as other considerations including cost, operation requirements, instrument size,
and power. In a subsurface environment, these restrictions may be more significant due to potentially complex
mission architectures, limited communication, and accessibility challenges (refer to the Robotics section below).
Considering the aforementioned constraints, proposed instrument suites should be capable of identifying habita-
ble environments within caves—and to the extent possible—the physical and chemical signatures unique to life.
Due to the preservation potential of subsurface environments and the broad community support for astrobiological
investigations, identifying instruments capable of studying microbe-mineral interactions will be a high priority
(Q22). As a result, community engagement is needed to develop science objectives and instrument requirements
considering the unique environment of planetary caves. This process would leverage considerable experience
with studies of planetary surfaces via orbital and surface observations. For example, flight-qualified instruments
for the assessment of habitability, life detection, and geologic characterization employed in recent Mars rover
missions may be considered. These include: laser-induced breakdown spectroscopy (LIBS; e.g., Mars 2020
(M2020) SuperCam; Wiens etal., 2021); visible-near-infrared spectroscopy (e.g., the ExoMars MicrOmega;
Bibring etal.,2017); Raman fluorescence spectrometers (e.g., the M2020 Scanning Habitable Environments with
Raman and Luminescence for Organics and Chemicals (SHERLOC); Bhartia etal.,2021); the ExoMars Raman
Laser Spectrometer (Rull etal.,2017); X-ray fluorescence mapping spectrometers (e.g., the M2020 Planetary
Instrument for X-ray Lithochemistry; Allwood etal.,2020); and X-ray diffraction (e.g., Mars Science Laboratory
(MSL) Chemistry and Mineralogy (CheMin) instrument; Blake etal.,2012). A detailed chemical analysis could
involve instrumentation similar to the MSL Sample Analysis at Mars (SAM) suite, which includes a gas chroma-
tograph mass spectrometer and a tunable laser spectrometer (Mahaffy etal.,2012). Measurements of elemental

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composition, using nuclear spectroscopy (Prettyman etal., 2019; Trainer etal., 2018) or alpha-particle X-ray
spectrometry demonstrated on Mars rovers (Gellert etal.,2006), could be useful in examining cave chemistry.
Subsurface science objectives and operational constraints may require modifications to flight-qualified instru-
ments or require a different set of instrumentation entirely to accommodate a planetary cave mission.
Access to high value science targets poses a challenge to any planetary science mission. However, in situ explo-
ration of planetary caves may present a more substantial challenge than acquiring measurements on relatively
flat surfaces. Data collection along cave walls or ceilings, where speleothems and other features are expected to
occur (Northup etal.,2011) will require in situ access via hovering or climbing platforms (Uckert etal.,2020) or
standoff instruments, which do not require placement within 1m or contact with the surface (e.g., LIBS; Wiens
etal.,2021). Standoff Raman spectroscopy may be implemented in these cases for organic or mineral characteriza-
tion (Angel etal.,2012) (Q26). However, the acceptable measurement distance is dependent on sample properties
and composition, ambient environmental conditions, and required detection limits. Additional sampling prepara-
tion and extraction may also be required, including drilling, rock abrasion, or instrument-specific preparation, as
required for MSL CheMin and SAM (Q28). Characterization of cave geometry using lidar or three-dimensional
imaging will be necessary to support bulk elemental analyses using nuclear spectroscopy data. The latter is likely
to be included on any navigational platform and would also support characterization of cave geotechnical proper-
ties (refer to the Robotics section below).
In terrestrial caves, microbial distribution and biosignature preservation potential varies with distance to light
sources (i.e., skylights and entrances; B. Jones,2001). Science target locations within a subsurface environment
may depend on the expected thermal gradient, airflow variability, and distribution of resources within a cave,
which may drive instrument and robotic platform requirements (Q23). Investigations beyond a cave entrance may
have limited communication relay rates, disrupting ground in-the-loop operations, and necessitating automated
science decision-making (Q25).
The extreme terrain associated with ingress and accessing science targets within a cave necessitate robotic plat-
forms inherently more complex than traditional wheeled surface rovers. These platforms may impose restrictions
(mass, volume, and power) on instrument payloads due to additional resource consumption required for mobility,
autonomy, and communication subsystems. Multiple mission architectures could be employed on such a mission,
including a single robotic platform (e.g., LEMUR 3; Parness etal., 2017); multiple, more disposable (higher
risk-posture) platforms each carrying a single science payload, such as hopping microbots (Kalita etal.,2017);
deployable instruments for long-term monitoring and communication relay points; or tethered-rappelling rovers
(Kerber etal.,2019; Nesnas et al., 2012). The ability to access a science target may depend on the selected
mission architecture, further restricting a science payload. There are also opportunities for synergy whereby some
of the instruments required for navigating the cave environment (navigational cameras) could be repurposed for
science investigations (Q27). For example, visible spectrum or lidar imagery used for mapping and hazard avoid-
ance could also be used to identify features of interest or determine geologic context. In addition, due to poten-
tially limited communication relay rates, onboard processing or storage and automated science decision-making,
may be required.
3.5. Robotics
Q29: What capabilities and sensors will best position robots to obtain the data required to evaluate a planetary
cave for scientific inquiry, human exploration, in situ resource utilization, and other uses? (#3; 91.5%)
Q30: What strategies enable communication between subsurface robotic cave explorers and the planetary surface
so that humans (whether astronauts or ground controllers) can direct robot operations and examine the science
data? (#4; 89%)
Q31: What are the first scientific instruments that a rover should carry beyond those traditionally used for navi-
gation? What sensors would provide the greatest impact to science inquiry, while not limiting the payload for
mobility? (#5; 87.8%)
Q32: What tasks can robots conduct on the surface to identify planetary caves and/or map cave entrances? (#13;
82.9%)
Q33: As the technical readiness levels for most of the technology required for a cave explorer rover are quite low
(i.e., TRL <4), how do we develop a competitive planetary caves mission within the cost cap of NASA science
missions? (#14; 82.9%)

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Q34: How can robots effectively and efficiently perceive geometric and non-geometric hazards that must be
traversed or avoided to enter and explore caves? (#15; 82.9%)
Q35: What mobility solutions enable traversal of planetary surface regolith as well as vertical, steep, and/or
blocky cave entrances and interiors? (#19; 81.7%)
Q36: What level of variations in terrain shape and type of planetary caves can state-of-the-art mobility systems
handle? What developments will be required to optimize traversability of cave interiors? (#22; 80.5%)
Q37: What are the range of obstructions a rover should be expected to encounter within a lunar or Martian cave?
(#23; 80.5%)
Q38: Beyond carrying instruments into caves, what capabilities of robotic cave explorers (e.g., precise localiza-
tion of instrument readings or multimodal context) provide the greatest benefit for cave science? (#29; 79.3%)
Q39: Since cave robots have high power requirements for mobility, perception, and substantial processing for
autonomous operation in the dark, what power source must these robots carry and/or how will it be supplied?
(#33; 78.1%)
Q40: What near-term actions, such as technology development on Earth, technology demonstration missions, and
precursor missions position us best for robotic planetary cave exploration? (#34; 78.1%)
Q41: What autonomy strategies and algorithms will be most effective for planetary cave exploration? (#45;
75.6%)
Q42: How can planetary protection be ensured for cave robots that may require complex mobility systems beyond
anything we've used with surface rovers? In other words, how do we reduce the risks of contamination and
false positives in the search for evidence of subterranean life? (#51; 74.4%)
Robots are likely to be the first explorers of planetary caves (Huber etal., 2014; Husain et al., 2013; Titus
etal.,2021; Titus, Phillips-Lander, etal.,2020; Titus, Wynne, etal.,2020). To effectively explore these targets,
future robotic systems will require the functionality to: (a) properly sense their environment; (b) support and
deliver scientific payloads to sites of interest; (c) plan actions and movements; and (d) negotiate a complex
landscape to execute these actions. These functionalities will be challenged by the unique features within cave
interiors including low light to aphotic conditions, indirect line-of-sight communication requirements, subsurface
power considerations, and rough, uneven, three-dimensional terrain that precludes satellite pre-mapping.
The third most fundamental question, acquiring support of 91.5% of respondents, was related to robotic capa-
bilities and payloads (Q29). Ultimately, these parameters will be determined by mission objectives (i.e., life
detection, evaluation for human exploration, and in situ resource utilization (ISRU)). Reconnaissance with highly
specialized instrumentation could potentially address the most pressing investigations early on, but broad, multi-
purpose sensing will provide the best information for site selection and mission planning. However, as sensor
instruments will serve as the robot's “eyes and ears,” the instrumentation suite will directly dictate how data are
collected and the mission is executed (Nesnas etal.,2019; Rossi etal.,2021).
Constrained by limited payloads, cave robotic designs will represent a barter between every gram of mobility
and navigational sensing technologies and scientific instrumentation (Goel etal., 2021; Yoshida etal.,2013)
(Q31). Limited payload capacity driven by navigational and mobility systems better suited to challenging
cave interiors will preclude the more comprehensive science laboratories common to previous Mars surface
rover missions (Uckert etal.,2020). While payloads will be determined by mission objectives, highly accurate
three-dimensional maps of caves will represent the geospatial backbone of any planetary cave mission (Q38)—
as navigation and mobility will be reliant on how well the rover can “sense” its surroundings. For example, an
astrobiology-focused mission may feature a combined mobility-navigation-life detection payload that leverages
dual-purpose mapping.
Advancing onboard survey instrumentation to obtain high-resolution three-dimensional cave geometries to both
establish safe traverse routes and avoid hazards will be required (Q34). For surface rovers in relatively benign
environments, stereo vision sensors have proven highly effective and efficient. However, many unique hazards,
including complex interior geometry, dust, and darkness, will challenge sensor arrays in Martian caves and
will likely require the development of new sensors (Wong etal.,2011). For example, micro-depth sensors have
been used to map and navigate around obstacles (Santamaria-Navarro etal.,2019), and lidar has been used for
navigation of multi-limbed climbing robots (Uckert etal., 2020). Additionally, a multi-instrument navigation

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payload including lidar, visible, and thermal sensors is being advanced for terrestrial cave rotorcraft drones
(Dharmadhikari etal.,2021). Importantly, if these technologies represent the most viable approach for navigating
and mapping caves, this approach will need to be matured.
Navigational capabilities will also vary according to the mobility platform used. A wide range of mobility
platforms have been developed to negotiate cave interiors (Q35) including tethered single axle rovers (Kerber
etal.,2019; Nesnas etal.,2019), legged platforms (Bouman etal.,2020), multi-limbed climbing robots (Parness
etal.,2017; Uckert etal.,2020), micro-robot swarms (Funabiki etal.,2020; Lowe etal.,2018; Otsu etal.,2020),
and rotorcraft (Aoki etal.,2018; Goel et al.,2021; Radotich etal., 2021). Each platform provides a unique
combination of benefits, capabilities, and constraints. Alternatively, rather than choosing a singular platform,
developing architectures involving multiple robotic platforms to either accomplish single mission objectives more
thoroughly (i.e., mapping; Husain etal.,2013) or multiple mission objectives concurrently (Agha-Mohammadi
etal.,2018,2021) could be considered—as such an approach would also increase the breadth of science payloads
entering a Martian cave.
Despite our rather thorough inventories of SAPs on the Moon and Mars (Titus, Wynne, Malaska, etal.,2021;
Wynne etal.,2022), our knowledge of these features is founded primarily upon the remotely sensed examina-
tion on surface expressions—save for limited gravimetric analysis of lunar SAPs (refer to Chappaz etal.,2017).
The next step will be to characterize and prioritize these features for both additional imagery acquisition (e.g.,
Kearney etal.,2021; Wagner & Robinson,2021) and develop and/or expound upon gravimetric analysis (where
appropriate) in support of ultimate robotic precursor missions (Q32). Provided the surface terrain surrounding a
cave entrance is stable, traditional rovers may be used for entrance inspection (Titus etal.,2021; Titus, Wynne,
etal.,2020). More committed, tethered single-axle rovers (e.g., Nesnas etal.,2019) may be lowered into cave
entrances to both examine and map the entrance and the surrounding interior. Also, on worlds with atmospheres
such as Mars, rotorcraft drones (e.g., Aoki etal.,2018; Radotich etal.,2021) could be used for inspecting cave
entrances, and potentially examining and mapping the deeper reaches of a high priority cave. In all cases, these
platforms would offer the first inspection of subsurface geologic stratigraphy and could potentially collect and
examine rock samples.
Ingress, achieving mission objectives, and egress of a lunar or Martian cave will require navigating a complex
three-dimensional landscape. Obtaining a detailed three-dimensional model of the target cave's interior prior to
entry to identify obstacles and establish traverse routes will further improve mission success (Fan etal.,2021;
Thakker etal., 2021) (Q36). Remotely sensed gravimetric data (sensu Chappaz etal.,2017) may be used for
developing a coarse first-order rendition of a cave interior, while multiple robotic platforms could be used for
mapping cave entrances and potentially characterizing shallow cave interiors (e.g., dual- and single-axle rovers
and rotorcraft). The same navigational challenges that occur in terrestrial caves are expected to occur within SAPs
on the Moon and Mars including uneven terrain, boulder-strewn breakdown fields, steep slopes, pits, cracks, and
narrow passages (Q37). Additionally, for both Martian (e.g., Cushing,2012) and lunar features, large accumula-
tions of dust (likely meters thick), especially within entrances, will also have to be overcome.
While several of the aforementioned platforms are capable of negotiating cave interiors, most still require human
assistance or direct navigation. Overall, successful exploration in a three-dimensional landscape will require the
tight integration and co-design of mobility systems, traversability assessment instruments, and innovative auto-
mation/AI algorithms (Agha-Mohammadi etal.,2018,2021; Ahmed etal.,2019; Sauder etal.,2017). Robots
will need to traverse terrain that is at best partially characterized or at worst completely unknown using onboard
autonomy (Otsu etal.,2020) and perception capabilities (Ebadi etal.,2020; Santamaria-Navarro etal.,2019), as
well as respond to off-nominal and unexpected events during operations (Agha-Mohammadi etal.,2018; Kim
etal.,2021) (Q41).
Further challenging robotic exploration will be the availability of a communications link between mission control
and the robot, and the powering of systems while underground. Wireless communication is impaired by the lack
of line of sight both in communication with the surface and within the cave; these impairments can result in
signal fading, multipath effects, and diminished signal strength at the boundaries (Walsh etal.,2019). Various
solutions have been proposed to address some of these challenges including bundling power delivery within
a tether connected to a surface lander or rover (Kerber etal., 2019; Nesnas et al., 2019), data muling where
the mobile robotic systems repeatedly come near the cave entrance to establish line of sight communication

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(Agha-Mohammadi etal.,2021), and a set of repeaters deployed along a line of sight to construct a wireless mesh
network (Q30). A repeater network could be established from the cave to the surface and may be either static or
locally mobile (Ginting etal.,2020,2021; Vaquero etal.,2020). These solutions may be used singularly or in
combination based on data bandwidth requirements, maximum tolerable latency, mission duration, endurance
and power requirements, mass and size limitations, and environmental considerations.
Given the demanding requirements of maintaining high mobility and the processing requirements for autonomous
operations, subsurface robotics can be powered via lithium-ion batteries, limited on-demand hydrogen fuel cells,
and/or laser-driven power beaming from surface to cave interior (Himangshu & Thangavelautham,2020) (Q39).
Over time, lithium-ion batteries will fail, while hydrogen fuel cell life is dependent on the amount (i.e., mass) of
LiH and LiClO4 transported into the cave (for hydrolysis) and the hydrolysis instrumentation. Power beaming
is also both technologically and mass intensive. The number of relay stations or microbots will be dependent on
the required minimum distance between stations due to line-of-sight constraints and the depth traversed within
the cave; these relays would be equipped with both photovoltaic panels for receiving power and a laser for trans-
mitting power. Energy would be beamed from the surface to the last relay point within the cave. Determining the
best power source is both platform and mission dependent. Furthermore, a combination of power sources could
provide both redundancy and enable deeper subsurface ingress. However, the power source and associated mass
requirements would become part of the trade-space when weighing mission objectives against power needs.
All the technologies discussed thus far require significant investment and maturation before robotic cave explora-
tion beyond Earth can become a reality. A lunar precursor mission with a traditional rover might even be achieved
within 5years under the lower-cost Payloads and Research Investigations on the Surface of the Moon (PRISM)
framework, if precision landing is employed (Whittaker etal.,2020,2021). For Mars, based on a mission concept
study, Phillips-Lander etal.(2020) found that a near-term cave life detection mission would exceed the NASA
New Frontiers cost cap and would require funding at the level of a Flagship Mission. However, with sagacious
site selection, advances in terrestrial robotics and autonomous sampling in conjunction with leveraging Mars2020
precision-landing heritage and Mars Sample Return technologies, a Martian cave mission could fit within the
New Frontiers cost cap this decade (Phillips-Lander etal.,2020) (Q33). However, to have the greatest latitude
in mission objectives and scope, the aforementioned robotic platforms should be advanced in tandem (Titus
et al., 2021) (Q40), while AI/autonomous navigation and decision-making programming, and cave-robotics
instrument payloads progress toward flight-qualified/proven status (TRL 8–9).
Most space-faring nations have scientific representation on the Committee on Space Research (COSPAR), the
international organization responsible for Planetary Protection Policy (NASA,2021). Their decisions include
drafting guidelines for both current robotic and future human exploration planetary missions (Q42). Addition-
ally, COSPAR is charged with designating “Special Regions” (i.e., protected areas) on Mars and other planetary
bodies. These areas represent regions where terrestrial microorganisms are likely to survive and replicate and
have a high likelihood for supporting extant indigenous life (NASEM,2015; Rummel etal.,2014). During the
2014 NASA Mars Exploration Program Analysis Group meeting, Martian caves were classified as “Uncertain
Regions.” As such, these features will be treated as Special Regions until data can be acquired to formally clas-
sify them as either “Special” or “Non-Special” Regions (Rummel etal.,2014). Under both designations, robotic
operations and associated hardware that may encounter these regions must undergo stringent cleaning proce-
dures prior to launch to avoid forward contamination as per planetary protection categorization IVc (Rummel
etal.,2014).
In addition to forward contamination, robots need to avoid damaging life that might be present in the cave or
leaving non-biologic contamination that might cause problems for follow-on missions. For example, a drop over
an unexpected precipice or a rotorcraft crash could result in scattered robot debris and contamination associated
with fluid from instruments, hydraulics, and thermal control heat pipes and batteries. These failures may have
particularly damaging consequences in an enclosed cave chamber (Goh,2021). While developing innovative
robots to meet cave mobility challenges, careful attention must be paid to materials selection and design to mini-
mize environmental impacts related to mission-ending crashes.
Technical hurdles for a robotic cave exploration mission are indeed significant. However, robots represent the best
chance for humans to examine a lunar and/or Martian cave within the next one to two decades (Titus etal.,2021;
Titus, Wynne, etal.,2020; Wynne etal.,2022). To make robotic exploration of a planetary cave possible, critical

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investment will be required to: (a) miniaturize and ruggedize robotic sensory systems suitable for low-light to
completely dark environments; (b) further develop autonomy for monitoring overall robot health and function-
ality, and traverse route planning; (c) mature mobility systems; (d) develop communications systems; (e) create
efficient power systems for subsurface use; (f) minimize potential for contamination; and (g) miniaturize science
payloads.
3.6. Human Exploration
Q43: Do Martian caves (or a subset of them) support water ice, which is reasonably accessible and can be extri-
cated efficiently for life support? (#25; 80.5%)
Q44: How will Martian dust within caves adversely affect robotics, instrumentation, and the outer layers of habi-
tation pods and spacesuits? Importantly, how will astronaut crews living/working in a Martian cave manage
dust (i.e., to reduce both damage to equipment and inhalation threat)? (#46; 75.6%)
Q45: What modifications are required to existing extravehicular activity (EVA) tools, and/or what tools need to
be developed, to work and live safely in lunar and Martian caves? (#48; 74.4%)
Q46: For lava tubes used as human habitats, what internal structural modifications will be required to maintain
the structural integrity of the pressurized habitat? (#49; 74.4%)
Human exploration, ISRU, and potential human habitation represent the culmination of planetary cave explora-
tion activities (Titus etal.,2021; Titus, Wynne, etal.,2020; Wynne etal.,2022). For long-duration human plan-
etary exploration missions (specifically to the Moon and Mars), access to, and the habitation of SAPs will be of
paramount importance (e.g., Davila etal.,2015; Titus, Phillips-Lander, etal.,2020; Titus, Wynne, etal.,2020). In
most cases, the cave system will need to be fully examined (refer to the Robotics section) prior to human entry—
for any use. First, the system will need to be thoroughly modeled via remote sensing assets and evaluated to deter-
mine if the necessary resources are likely to exist. Thereafter, the cave will require a robotic survey to confirm
the presence of adequate water ice deposits (see Cave Environment section), life (see Astrobiology section), and/
or geologic stability (see Geology section), and protection from the surface environment (see Cave Environment
section). If life is detected, planetary protection protocols will be required (see Broad Concepts section).
The most important question for human exploration (as identified by ∼81% of the respondents) was whether
Martian caves support water ice, and how this resource may be accessed and used for life support (Q43). To do
this, we must first apply mathematical models to identify caves with the greatest likelihood of supporting neces-
sary quantities of water ice (see Cave Environment section). Thereafter, either robotic or human examination
will be required to confirm the presence of water ice. If confirmed, the ice will need to be analyzed to determine
whether it contains evidence of life (refer to Astrobiology section); if extant lifeforms are identified, planetary
protection protocols would be invoked (see Broad Concepts section). If extraction can move forward, water ice
could be robotically mined and then heated to a liquid state. For human consumption, the liquid water (likely
a liquid brine), will require an intermediary desalinization step (refer to Jackson etal.,2014). Additionally, if
there is potential for life, water should be sufficiently filtered as unknown life (life as we don't know it or can't
detect it) may still exist and represents a potential threat to human health. For other purposes, liquid H2O can
be split into its atomic components (oxygen and hydrogen). Oxygen would then be used for life support, while
hydrogen can be stored and used for fuel cells (Belz,2016). Both oxygen and hydrogen could be used for rocket
fuel for the return trip to Earth (Titus, Phillips-Lander, etal., 2020), while hydrogen fuel cells could power
various life support systems including recycling management (e.g., carbon dioxide removal and waste combus-
tion; Belz,2016) and other power requirements. Incidentally, given the majority of Mars' atmosphere is carbon
dioxide, oxygen recovery via CO2 electrolysis (Burke & Jiao,2016; Hecht etal.,2021) would likely represent
the primary component of a life support system with oxygen from water electrolysis serving as a byproduct from
energy production.
For human habitation of a lunar or Martian cave to occur, a robotic assessment would first be required. This
assessment would involve an examination of the cave internal structure (i.e., volume, depth, and roof height),
structural stability (i.e., low likelihood of roof fall/collapse), and an assessment to determine whether the cave
is sufficiently buffered from the surface (see Cave Environment section). Once determined, a cave may then
be considered for the insertion of a habitation module (or series of modules) (Q46). While some inflatable and

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hybrid inflatable-rigid habitation prototypes have been tested and proof-of-concept models developed (Daga
etal.,2010; Krishnan,2021; Litteken,2019), these systems were not designed for caves. Modifications would be
required to ensure these modules are sufficiently ruggedized for underground use. Another possibility would be
to develop habitation modules using in situ resources (Naser & Chen,2021); however, ISRU and their potential
is in a nascent developmental stage (Starr & Muscatello,2020), and a cave-centric focus has not been explored.
Regardless of the platform selected for human habitation, the structure should have a ruggedized roof and/or be
placed beneath a free-standing rigid roof structure to protect the habitation module, and the structure should be
situated clear of cave walls. These considerations would reduce the likelihood of damage to the structure from
roof fall and/or partial collapse.
Other considerations are how dust within caves would affect instrumentation, habitation modules, and spacesuit
exterior layers, as well as the dust mitigation strategies required for astronaut crews living/working in a Martian
cave (Q44). Typically, if a terrestrial cave occurs in a dust ridden environment, the cave will also be dust ridden.
Because both lunar and Martian surfaces are dusty environments, we anticipate their caves will be equally dusty.
The impacts of lunar and Martian dust have challenged surface operations for decades. During the Apollo
missions, the deleterious effects of lunar dust on spacesuits, buggies and other critical hardware are well estab-
lished (e.g., Gaier,2007; Wagner,2006). On Mars, the hand lens imager onboard Curiosity rover required the
development of both dust mitigation strategies and estimating dust contamination from imaged samples (Yingst
etal.,2020). Additionally, solar panels on Martian rovers were periodically covered in dust and damaged by dust
abrasion (McMillon-Brown etal.,2019); these impacts can compromise power acquisition and mission objec-
tives. Similarly, life support systems and equipment will be challenged by dust because dust mobilization and
adhesion (particularly the finest dust fraction (<45μm)) reduce equipment performance and may cause health
complications.
In addition to developing methods to estimate dust contamination, some mitigating factors include electrostatic
precipitators (Calle etal.,2011; Manyapu & Peltz,2018; Margiotta etal.,2010) and scroll media filters (Linne
etal.,2017); these systems can be built into life support and other instrumentation to reduce dust contamina-
tion. Regarding habitation modules and spacesuits, both will require ruggedization against highly pervasive and
damaging dust. To reduce dust within human habitats, dust mitigating technologies (such as electrostatic precipi-
tators, electrostatic air filters, vacuum suction removal, and gas showers within airlocks) could be used to remove
damaging dust particles; however, to our knowledge, these technologies have not been tested for this purpose.
Developing a tool kit for humans to work and live safely underground off Earth has not been examined (Q45).
However, we offer the following considerations. First and foremost, a ruggedized EVA spacesuit permitting high
dexterity will be required. Current spacesuit technology (e.g., EVA spacesuits used on the International Space
Station) restricts mobility and would be at a high risk of suit breach if used for underground operations. One design
that shows promise for underground use is BioSuit technology (Bethke etal.,2004)—a svelte and flexible alter-
native to current spacesuits. However, these suits and their associated donning and doffing technologies are at the
proof-of-concept stage. To become “SpeleoSuits,” BioSuits must be immensely ruggedized to become puncture
and abrasion resistant and should include reinforced/ballistic material applied to knee, elbow, and hand regions.
For tool development, a first step would be to evaluate flight proven (TRL 9) EVA (Fullerton,1993) and Apollo
lunar sampling tools (Allton,1989). Additionally, NASA is designing EVA tools (based on Apollo-era heritage
designs) for the Artemis program with greater emphasis on sustained operations and long-duration exploration on
the Moon (Coan,2020). Some of this equipment may be either suitable or retrofitted for use on a planetary cave
mission. In terms of technical caving equipment (i.e., rappelling and ascending devices, harnesses, protection
devices, anchor systems, etc.), presently there have been no technical climbing/caving equipment developed for
underground extraterrestrial human operations, nor have there been any studies to examine or test how terrestrial
equipment may be modified for extraterrestrial use.
3.7. Broad Concepts
*Q47: In case life is discovered, what planetary protection protocols are most applicable to ensure that we will
not negatively impact these lifeforms? (#2; 93.6%)
Q48: To optimize planetary cave candidate selection, what remote sensing instruments will be required (or need
to be developed) to determine cave structure and depth with a suitable degree of accuracy? (#10; 84.2%)

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Q49: For candidate selection for both the search for life and human habitation, how do we systematically evaluate
planetary caves for both applications? (#26; 80.5%)
*Q50: How does cave exploration integrate with other scientific/exploration mission goals on Mars? In what
mission framework could a cave exploration mission be conducted? (#36; 77.4%)
Q51: What is the range of types of subterranean features that occur on planetary bodies within our solar system?
(#40; 76.8%)
Q52: Can we develop a data-derived method to predict the likelihood of different types of caves using the funda-
mental properties of a planetary body (e.g., presence of volatiles, viscosity of lava when flowing, presence or
absence of internal tectonism of some variety, etc.)? (#41; 76.8%)
Q53: What are the primary factors contributing to the apparently high microbial biodiversity observed in the
Earth's subsurface? (#47; 75.6%)
Fundamental questions in the broad concepts subject area ranged from planetary protection to leveraging our
knowledge of terrestrial microbial diversity to gain inference into the possibilities to extraterrestrial subsurface
life—well-encapsulating big picture ideas and concepts. Importantly, the only two questions identified by our
Survey 2 participants were within this subject area group (denoted with an *).
Our number two fundamental question dealt with protecting extraterrestrial life (should it exist) from lifeforms
introduced from Earth (Q47). A general overview of planetary protection procedures as it relates to robotic oper-
ations is provided above in question 42. Our inability to adequately “clean” spacecraft prior to planetary missions
has been well-established (refer to Nicholson etal.[2009] and Fairén etal.[2017]). Thus, some have reasoned
that given our stringent protocols for cleaning spacecraft prior to missions, we have already sent the most resilient
terrestrial microbes to other planetary bodies (Fairén etal.,2017; Nicholson etal.,2009). Similarly, “superbugs”
have been documented in hospitals subjected to intensive decontamination procedures (Dancer,2009; Humphries
& McDonnell,2015; Muscarella,2014). Overall, the impacts of introducing terrestrial microbes to Mars or other
planetary bodies is expected to be tantamount to nonnative species introductions on Earth (e.g., Simberloff &
Rejmánek,2011); release from terrestrial competition and predation compounded with potentially being super-
charged by rigorous cleaning procedures, these terrestrial microbes may thus be particularly pernicious and resil-
ient should they become established on other planetary bodies. Concomitantly, others (e.g., Glavin etal.,2004;
McKay,2009; Siefert etal., 2012) have suggested planetary protection concerns become moot once humans
arrive on another planetary body—as forward contamination will be unavoidable. Regardless of this undesirable
outcome, given the availability of next generation genetic/genomic techniques, we should be able to discern
terrestrial from Martian microbes (Fairén etal.,2017); if life beyond Earth has a different biochemistry, it will be
much easier to determine that forward contamination did not occur.
Concerning habitats of possible cave-dwelling extraterrestrial microbes, the range of formation processes and
the diversity of planetary bodies in our solar system gives rise to a plethora of planetary cave types (Titus,
Phillips-Lander, etal.,2020; Wynne etal.,2022) (Q51). Speleogenic processes include thermal erosion/deposi-
tion (lava tube formation), dissolution (karst), sublimation, phase change to create void space, erosional removal
(suffusion), and block break down (Boston,2020; Titus, Phillips-Lander, etal.,2020; Wynne etal.,2022). Key
properties influencing solar system-wide cave formation include temperature, pressure, gravity field, and if
there is a liquid cycle, the properties of that liquid (e.g., water vs. supercritical CO2 vs. methane; Malaska &
Hodyss,2014; Malaska etal.,2011). Host rock (or ice) properties will also influence cave formation processes;
these include the strength (of various moduli) and deformation characteristics of substrate materials at local
environmental conditions. While we have made considerable strides toward identifying and categorizing the
assorted cave-formation processes and their associated properties (Titus, Wynne, etal.,2020), our knowledge of
planetary caves in the solar system remains incomplete. For example, formation processes for SAPs on Venus
are unknown (Wynne etal.,2022). Three upcoming missions, NASA's VERITAS and DAVINCI+ and ESA's
EnVision may serve to partially address this knowledge gap and may potentially reveal novel Venusian cave
formation processes. VERITAS will acquire a global three-dimensional surface map at 15–30m resolution using
a synthetic aperture radar instrument (InSAR; Freeman etal.,2016), while DAVINCI+ will include a descending
probe designed to examine the uncharacterized lower atmosphere and collect imagery to produce high-resolution
(from altitudes of 5km to near the surface) digital elevation models of the highly geologically deformed Tesserae

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formations (Garvin etal.,2020; V. L. Hansen & Willis,1998). EnVision will acquire near-global coverage data to
characterize geologic types and weathering, identify regions of active volcanic activity, and collect high-resolution
atmospheric measurements (Helbert etal.,2019).
While many speleogenic processes on Earth are well understood (Dreybrodt etal., 2004), extrapolating this
information to planetary caves will require theoretical modeling and laboratory experiments to understand the
key intrinsic properties (Cornet etal., 2015; Malaska & Hodyss,2014; Raulin,1987) (Q52). To illustrate, the
putative karstic processes on Saturn's moon Titan involve liquid methane and ethane as the working fluid dissolv-
ing organic bedrock and/or other organic minerals (Malaska etal.,2011; Maynard-Casely etal.,2018). While, at
some level, this process is comparable to terrestrial karst formation, some of the specific details are quite different
(Cornet etal.,2017). For example, the complex aqueous equilibrium of carbon dioxide partial pressure affecting
carbonate dissolution is nonexistent on Titan. On Mars, karstic processes likely occur in deposited salts such as
kieserite (MgSO4·H2O) that would exist in different dehydration states on Earth or would be so soluble that they
would quickly dissolve (Baioni & Sgavetti,2013). Locally dry conditions on Mars presumably allow this mate-
rial, and these karstic remnants, to exist. In this Martian example, the relative time scale of fluid availability and
cycling allows caves to exist, where on wetter worlds a similar cave would quickly be dissolved away. Thus, the
kinetics of the cave formation process relative to other processes (e.g., collapse, dissolution, etc.) may also be an
important factor. Thus, we have, and should, continue to use our knowledge of planetary body specific geology
to influence where we search for SAPs (see Wynne etal.,2022).
Question 49 intimates how best to develop systematic procedures for parsing cave types into candidates for
robotic exploration, possible human habitation, or perhaps both. This procedure will be driven by the space-bound
assets available to conduct the evaluation. Overall, the roadmap proposed by Titus, Wynne, Malaska, etal.(2021)
outlines two critical steps identified as prerequisites for planetary cave exploration—identification and charac-
terization. To this end, high-resolution imagery of possible SAPs and their surrounding topography must be
acquired and examined, then near-surface to ground assets will be required to further scrutinize and evaluate
cave candidate sites for both robotic and human exploration. However, in the near term, robotic exploration
will factor more prominently than human use. On Mars, selection criteria for robotics will be chiefly centered
around life detection and should include identifying features with the highest probability for supporting water ice
(e.g., Schörghofer,2021; Williams etal.,2010) and being accessible, as well as the additional selection criteria
elucidated by other workers (refer to Wynne etal.[2014], Kearney etal. [2021], and Titus, Wynne, Malaska,
etal.[2021]).
To examine the “characterization” stage more fully, the realities of limited resources and constrained budgets will
necessitate the selection of multipurpose interdisciplinary missions whenever possible (Q48). From orbit, remote
sensing instrumentation including electrical resistivity (Selim etal.,2014) and gravimetry (Chappaz etal.,2017)
may be used to “map” known subsurface voids and possibly detect new ones. Near-surface and surface assets
should include both drones and traditional rovers. Drones can perform a broad range of non-cave specific science
activities, while also being employed to identify and characterize features of interest using standard science and
navigation payloads (e.g., cameras; Titus, Wynne, etal.,2020; Titus, Wynne, Malaska etal.,2021). Traditional
surface rovers could be assigned the cave-related tasks of further examining a feature of interest. For example, if
the rover payload included ground-penetrating radar, the subsurface void space for features of interest could be
characterized (e.g., Esmaeili etal.,2020; Hamran etal.,2020).
Progressing toward the “exploration” stage, several approaches can be taken to optimize payload/instrumentation
for cave specific missions. For example, a surface communications relay station should also include meteoro-
logical and/or seismic instrumentation. The robotic platform entering the cave should be equipped with envi-
ronmental, geologic, and astrobiology instrumentation. Additional information on this topic is provided in the
Instrumentation and Robotics sections above.
Even in locations without caves, data from cave robotic instrumentation would provide new geologic insights into
the near surface (Q50). For example, the Dynamic Albedo of Neutrons (DAN) onboard the Mars Science Labo-
ratory actively and passively probes the near surface by monitoring neutron and gamma ray flux (e.g., Kerner
etal.,2020). DAN is specifically looking for evidence of water bound to minerals near the surface. Similarly,
depending upon the robotic platform and payload, surface-related science data could be collected during a cave

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mission—including validation of remote sensing interpretations, as well as providing additional insights into
near-surface geology. Thus, such a mission would maximally benefit the science community.
Regarding life potential, recent terrestrial studies revealed that subsurface geological settings provide the most
diverse ecosystems—often dominated by chemolithotrophs and novel microbial lifeforms with significant impli-
cations for the origin and evolution of life on Earth (Escudero etal., 2020; Magnabosco etal., 2018; Miller
etal.,2020; Selensky etal.,2021). These microorganisms play vital roles in almost all biological processes, such
as global biogeochemical cycling, biomineralization, and pedogenesis (Gaboyer etal.,2017; Miller etal.,2018;
Tornos etal., 2014). However, and despite the advent of new high-throughput genetic sequencing technolo-
gies, several fundamental aspects of their distribution and ecology remain unknown. While it has been reason-
ably established that mineralogical composition contributes significantly to subsurface microbial diversity and
biomass (Casar etal.,2020; Escudero etal.,2020; A. A. Jones & Bennett,2017; Rempfert etal.,2017), research-
ers have yet to quantify the environmental variables contributing to high microbial biodiversity in subterranean
ecosystems (Q53). To achieve this, an integrative, multidisciplinary approach involving metagenomics, micros-
copy, mineralogy, and geochemistry (Cockell etal.,2019; Miot etal.,2014), as well as ecological and climato-
logical modeling across a variety of cave lithologies, geometries and disparate study areas represents a viable
research frame to significantly advance our knowledge. Through such an approach, we should be able to robustly
parameterize the drivers of subsurface microbial diversity and ultimately pioneer a viable methodology to iden-
tify the best caves to study, as well as optimally select within-cave locations to sample.
4. Conclusions
To our knowledge, this is the first time a horizon scan approach has been applied to synthesize research needs in
planetary science. We believe this work has shown the viability and importance of expert-opinion-based social
surveys in the planetary sciences. As such, we hope this inspires future efforts to examine other subdisciplines in
planetary science and space exploration using similar techniques.
Evaluations concerning whether horizon scans achieve their stated goals have been rarely, if ever, undertaken
(Wintle etal.,2020). We recommend revisiting this topic periodically to synthesize the progress made, as well
as to potentially recalibrate the research efforts to ensure planetary cave exploration technologies are adequately
progressing. However, we identified fundamental research questions rather than more broadly scoped themes.
Ensuing evaluative exercises will require a theme-based approach (e.g., Brown,2007; Hughes et al., 2020;
Könnölä etal.,2012; Sutherland, Bardsley, etal.,2011). By using the same subject area groups applied here,
future efforts should examine the progress within each subject area through evaluating advances made, identifying
remaining knowledge gaps, and pinpointing the specific steps required to address scientific and/or technological
needs. We recommend scheduling “progress” workshops during the International Planetary Caves Conference,
which has convened biennially to triennially since 2011.
At 14 questions, the robotics group had twice as many questions as the other categories. This suggested that
perhaps the broader community recognized advancing robotic technologies represented the linchpin of a plan-
etary cave mission. Importantly, several recently funded in situ cave investigations designed to further develop
robotic subsystems and advance operational readiness are underway. These include rock-climbing robotics
(Uckert etal.,2020), microbots (Dubowsky etal.,2006; Kalita etal.,2017), tethered rovers (Kerber etal.,2019),
and autonomous quadruped rover technologies (Bouman etal.,2020). As we have evinced, relevancy of a specific
subsystem will be driven by the mission; thus, for a planetary cave mission to have the greatest latitude, all the
aforementioned platforms should be improved concomitantly. To accomplish this, significant investment will
be required to support these innovations maturing to a flight-qualified level (Titus, Wynne, etal.,2020; Titus,
Wynne, Malaska, etal.,2021; Wynne etal.,2022).
Mammola etal.(2020) suggested that voting exercises focused on selecting questions (as we have done here)
often skew toward general rather than specific themes. These authors reported this to be the case and we found a
similar trend in our study. For example, our top 10 questions were broadly focused. While one explanation may
be that given the interdisciplinary composition of survey participants in Survey 2, general or broad-focused ques-
tions may have a broader appeal by an interdisciplinary group and thus be identified as important. However, as
we qualitatively noted a disproportionate focus on robotics questions, this may be indicative of another trend. As

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engineers comprised only ∼15% of Survey 2 respondents, it is possible that participants recognized the substantial
lacunae associated with underdeveloped robotic platforms and cast their votes accordingly. In either case, we felt
that we should provide all questions across all three surveys, so that future workers will be able to further examine
and scrutinize over our findings (refer to Supporting InformationS1; Wynne,2022).
Perhaps one of the most important aspects of this work was harnessing the vast interdisciplinary knowledge of
25 scientists (across multiple disciplines), nine engineers, and two retired astronauts. We assembled a diverse
interdisciplinary group to identify a strategy to advance scientific knowledge and technology to ultimately
explore caves on other planetary bodies. Furthermore, we brought six disparate yet interrelated subjects under
one roof. Such interdisciplinary efforts are essential to catalyze innovations (Moirano etal.,2020) and improve
cross-disciplinary efficiency (e.g., Kreuz etal.,2020; Pfohl etal.,2017). Subsequently, we believe horizon scan-
ning efforts, such as the one presented here, will greatly benefit planetary science research—as most subdisci-
plines are equally reliant on interdisciplinary synergistic collaborations and activities.
We would also like to emphasize that Decadal Survey and National Academy of Science Space Studies Board
(NASSSB) white papers are essentially horizon scans. These efforts aggregate expert opinions on a given plane-
tary topic and/or body into a document that guides future research. However, these guiding documents are typi-
cally drafted by a cadre of lead authors with contributing authors providing input thereafter (as was the case with
Titus, Wynne, etal.,2020). As horizon scans can be designed to engage a broad swath of the community at the
formative stage, we submit such an approach (as the one employed here) can be more encompassing and there-
fore more representative of the broader scientific community. Importantly, our horizon scan approach was both
systematic and statistically rigorous and may be considered a template for similar planetary science exercises.
Thus, we recommend that future Decadal Survey and NASSSB working groups apply an approach that embodies
many of the elements presented in this study.
With the passing of the 50th anniversary of the two seminal mathematical modeling efforts on the potential for
lunar lava tubes (Greeley,1971; Oberbeck etal.,1969), we felt this was the perfect opportunity to take stock of
the significant advances in planetary cave exploration. Our collective knowledge has progressed from musing to
confirming the presence of SAPs beyond Earth—not only on the Moon, but on at least 10 other planetary bodies
(Wynne etal.,2022). To date, detection capabilities have been developed and refined (e.g., Chappaz etal.,2017;
Cushing,2017; Pisani & De Waele,2021; Wynne etal.,2008,2021), and astrobiological sampling techniques and
associated instrumentation suites have been advanced by several key research projects (e.g., Agha-Mohammadi
etal.,2021; Blank,2020; Uckert etal.,2020). Moreover, a bevy of robotic platforms are slowly moving toward
flight-qualified status (see Robotics section above), while astronauts are now being trained to conduct subsurface
missions (Sauro etal.,2021). These accomplishments were unfathomable 50years ago. Incidentally, most of
these achievements have transpired over the past 10years.
We anticipate the next 10years will be as riveting as the past decade. As robotic platforms and instrumentation
suites mature, humans move ever closer to embarking upon a robotic planetary cave mission—most likely to the
Moon first, then to Mars. These advances will further bolster the importance of planetary subsurfaces for human
use, which will inevitably result in humans exploring and perhaps establishing astronaut bases underground.
When this comes to pass, humans will return to sojourning in caves. While humanity abandoned this practice
millennia ago, humans dwelling beyond Earth are likely to seek shelter underground once again for safety and
protection.
Disclaimers
The survey results reported herein was organized and implemented by JJW, Northern Arizona University, and
were not conducted on behalf of the U.S. Geological Survey. Any use of trade, firm, or product names is for
descriptive purposes only and does not imply endorsement by the U.S. Government.
Conflict of Interest
The authors declare no conflicts of interest relevant to this study.

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Data Availability Statement
This is a review paper driven by a series of expert opinion based general social surveys. The questions used for
three individual online surveys, all survey results, the combined results of Survey 1 and 2, and the top 53 funda-
mental questions are available via the Harvard Dataverse (https://doi.org/10.7910/DVN/HJA6QI).
References
Agha-Mohammadi, A., Agarwal, S., Kim, S., Chakravorty, S., & Amato, N. (2018). SLAP: Simultaneous localization and planning for physical
mobile robots via enabling dynamic replanning in belief space. IEEE Transactions on Robotics, 34(5), 1195–1214. https://doi.org/10.1109/
tro.2018.2838556
Agha-Mohammadi, A., Otsu, K., Morrell, B., Fan, D. D., Thakker, R., Santamaria-Navarro, A., etal. (2021). NeBula: Quest for robotic autonomy
in challenging environments; TEAM CoSTAR at the DARPA subterranean challenge. Journal of Field Robotics. Accepted. https://arxiv.org/
abs/2103.11470
Ahmed, M., Hossain, K., & Miles, J. (2019). Analysis, design, and construction of autonomous robots (p.68). NASA Goddard Space Flight
Center. Retrieved from https://njsgc.rutgers.edu/sites/default/files/nasa-giss-research-paper-2Nesnas019.pdf
Allton, J. H. (1989). Catalog of Apollo lunar surface geological sampling tools and containers. JSC-23454, LESC-2276 (p.101). NASA Johnson
Space Center. Retrieved from https://repository.hou.usra.edu/handle/20.500.11753/678
Allwood, A. C., Beaty, D., Bass, D., Conley, C., Kminek, G., Race, M., et al. (2013). Conference summary: Life detection in extraterrestrial
samples. Astrobiology, 13(2), 203–216. https://doi.org/10.1089/ast.2012.0931
Allwood, A. C., Wade, L. A., Foote, M. C., Elam, W. T., Hurowitz, J. A., Battel, S., etal. (2020). PIXL: Planetary instrument for X-ray lithochem-
istry. Space Science Reviews, 216, 1–132.
Angel, S. M., Gomer, N. R., Sharma, S. K., & McKay, C. (2012). Remote Raman spectroscopy for planetary exploration: A review. Applied
Spectroscopy, 66(2), 137–150. https://doi.org/10.1366/11-06535
Aoki, R., Oyama, A., Fujita, K., Nagai, H., Kanazaki, M., Kanou, K., etal. (2018). Conceptual helicopter design for exploration of pit craters and
caves on Mars (p.5362). AIAA Space and Astronautics Forum and Exposition.
Atri, D., & Melott, A. L. (2014). Cosmic rays and terrestrial life: A brief review. Astroparticle Physics, 53, 186–190. https://doi.org/10.1016/j.
astropartphys.2013.03.001
Azua-Bustos, A., Fairén, A. G., Silva, C. G., Carrizo, D., Fernández-Martínez, M. Á., Arenas-Fajardo, C., etal. (2020). Inhabited subsurface wet
smectites in the hyperarid core of the Atacama Desert as an analog for the search for life on Mars. Scientific Reports, 10(1), 19183. https://doi.
org/10.1038/s41598-020-76302-z
Azua-Bustos, A., González-Silva, C., Mancilla, R. A., Salas, L., Palma, R. E., Wynne, J. J., etal. (2009). Ancient photosynthetic eukaryote
biofilms in an Atacama Desert coastal cave. Microbial Ecology, 58(3), 485–496. https://doi.org/10.1007/s00248-009-9500-5
Azua-Bustos, A., González-Silva, C., Salas, L., & Vicuña, R. (2010). A novel subaerial Dunaliella sp.Growing on cave spiderwebs in the
Atacama Desert. Extremophiles, 14(5), 443–452. https://doi.org/10.1007/s00792-010-0322-7
Badino, G. (2010). Underground meteorology-“What’s the weather underground?”. Acta Carsologica, 39(3), 427–448. https://doi.org/10.3986/
ac.v39i3.74
Baioni, D., & Sgavetti, M. (2013). Karst terrains as possible lithologic and stratigraphic markers in northern Sinus Meridiani, Mars. Planetary
and Space Science, 75, 173–181. https://doi.org/10.1016/j.pss.2012.08.011
Baioni, D., & Wezel, F. C. (2010). Morphology and origin of an evaporitic dome in the eastern Tithonium Chasma, Mars. Planetary and Space
Science, 58(5), 847–857. https://doi.org/10.1016/j.pss.2010.01.009
Baioni, D., Zupan-Hajna, N., & Wezel, F. C. (2009). Karst landforms in a Martian evaporitic dome. Acta Carsologica, 38(1), 9–18. https://doi.
org/10.3986/ac.v38i1.132
Banfield, J. F., Moreau, J. W., Chan, C. S., Welch, S. A., & Little, B. (2001). Mineralogical biosignatures and the search for life on Mars. Astro-
biology, 1(4), 447–465. https://doi.org/10.1089/153110701753593856
Barbieri, R., & Stivaletta, N. (2011). Continental evaporites and the search for evidence of life on Mars. Geological Journal, 46(6), 513–524.
https://doi.org/10.1002/gj.1326
Bar-On, Y. M., Phillips, R., & Milo, R. (2018). The biomass distribution on Earth. Proceedings of the National Academy of Sciences, 115(25),
6506–6511. https://doi.org/10.1073/pnas.1711842115
Belz, S. (2016). A synergetic use of hydrogen and fuel cells in human spaceflight power systems. Acta Astronautica, 121, 323–331. https://doi.
org/10.1016/j.actaastro.2015.05.031
Bethke, K., Carr, C. E., Pitts, B. M., & Newman, D. J. (2004). Bio-suit development: Viable options for mechanical counter pressure. SAE Trans-
actions, 113, 426–437. https://doi.org/10.4271/2004-01-2294
Bhartia, R., Beegle, L. W., DeFlores, L., Abbey, W., Hollis, J. R., Uckert, K., etal. (2021). Perseverance’s scanning habitable environments with
Raman and luminescence for organics and chemicals (SHERLOC) investigation. Space Science Reviews, 217, 1–115.
Bibring, J.-P., Hamm, V., Pilorget, C., Vago, J. L., & the MicrOmega Team. (2017). The micrOmega investigation onboard ExoMars. Astrobiol-
ogy, 17(6–7), 621–626. https://doi.org/10.1089/ast.2016.1642
Blake, D., Vaniman, D., Achilles, C., Anderson, R., Bish, D., Bristow, T., etal. (2012). Characterization and calibration of the CheMin miner-
alogical instrument on Mars Science Laboratory. Space Science Reviews, 170(1), 341–399. https://doi.org/10.1007/978-1-4614-6339-9_12
Blamont, J. (2014). A roadmap to cave dwelling on the Moon and Mars. Advances in Space Research, 54(10), 2140–2149. https://doi.org/
10.1016/j.asr.2014.08.019
Blank, J. G. (2020). Robotic mapping and exploration of a terrestrial lava tube: A structured planetary cave mission simulation with a remote
astrobiology science team. In Paper presented at 3rd International Planetary Caves Conference. LPI Contributions No. 1047.
Blank, J. G., Roush, T. L., Stoker, C. L., Colaprete, A., Datta, S., Wong, U., etal. (2018). Planetary caves as astrobiology targets. A white paper
submitted to the Space Studies Board of the National Academy of Sciences (Vol. 5).
Blumensaat, F., Leitão, J. P., Ort, C., Rieckermann, J., Scheidegger, A., Vanrolleghem, P.A., & Villez, K. (2019). How urban storm-and wastewa-
ter management prepares for emerging opportunities and threats: Digital transformation, ubiquitous sensing, new data sources, and beyond-a
horizon scan. Environmental Science & Technology, 53(15), 8488–8498. https://doi.org/10.1021/acs.est.8b06481
Boston, P.J. (2004). Extraterrestrial caves. In J. Gunn (Ed.), Encyclopedia of caves and karst science (pp.355–358). Fitzroy Dearborn.
Acknowledgments
The authors thank Drs. Kevin B. Jones,
Janet L. Slate, Lilian Ostrach, Colin
Dundas, and two anonymous review-
ers for their comments leading to the
improvement of this paper. The following
funding sources are recognized for
supporting several of the contributing
authors: Human Frontiers Science
Program grant # RGY0066/2018 (for
AAB), NASA Innovative Advanced
Concepts Grant # 80HQTR19C0034
(HJ, UYW, and WLW), and European
Research Council, ERC Consolidator
Grant # 818602 (AGF), the Spanish
Ministry of Science and Innovation
(project PID2019-108672RJ-I00) and the
“Ramón y Cajal” post-doctoral contract
(grant # RYC2019-026885-I (AZM)), and
Contract #80NM0018D0004 between
the Jet Propulsion Laboratory, California
Institute of Technology and the National
Aeronautics and Space Administration
(AA, MJM, KU, and LK).

Journal of Geophysical Research: Planets
WYNNE ETAL.
10.1029/2022JE007194
26 of 32
Boston, P.J. (2020). Cavities and caves throughout the solar system: Prospects revisited for occurrence and astrobiological significance. In Paper
presented at 3rd International Planetary Caves Conference. LPI Contributions No. 1076.
Boston, P.J., Spilde, M. N., Northup, D. E., Melim, L. A., Soroka, D. S., Kleina, L. G., etal. (2001). Cave biosignature suites: Microbes, minerals,
and Mars. Astrobiology, 1, 25–55. https://doi.org/10.1089/153110701750137413
Bouman, A., Ginting, M. F., Alatur, N., Palieri, M., Fan, D. D., Touma, T., etal. (2020). Autonomous spot: Long-range autonomous exploration
of extreme environments with legged locomotion. IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), 2518–2525.
https://doi.org/10.1109/iros45743.2020.9341361
Brown, D. (2007). Horizon scanning and the business environment—The implications for risk management. BT Technology Journal, 25(1),
208–214. https://doi.org/10.1007/s10550-007-0022-8
Burke, K. A., & Jiao, F. (2016). Game changing development program-next generation life support project: Oxygen recovery from carbon dioxide
using ion exchange membrane electrolysis technology. NASA/TM—2016-219436, NASA STI Program (p.55). NASA Langley Research
Center Hampton. Retrieved from https://ntrs.nasa.gov/api/citations/20160014804/downloads/20160014804.pdf
Burt, D. M., & Knauth, L. P. (2003). Electrically conducting, Ca-rich brines, rather than water, expected in the Martian subsurface. Journal of
Geophysical Research, 108, E4. https://doi.org/10.1029/2002je001862
Burt, R. S. (1984). Network items and the general social survey. Social Networks, 6(4), 293–339. https://doi.org/10.1016/0378-8733(84)90007-8
Calle, C. I., Buhler, C. R., Johansen, M. R., Hogue, M. D., & Snyder, S. J. (2011). Active dust control and mitigation technology for lunar and
Martian exploration. Acta Astronautica, 69(11–12), 1082–1088. https://doi.org/10.1016/j.actaastro.2011.06.010
Carrer, L., Gerekos, C., & Bruzzone, L. (2017). Detection of lunar lava tubes with orbiting radar sounder systems. European Planetary Science
Congress, 11, EPSC2017–290.
Carrier, B. L., Beaty, D. W., Meyer, M. A., Blank, J. G., Chou, L., DasSarma, S., etal. (2020). Mars extant life: What's next? Astrobiology, 20(6),
785–814. https://doi.org/10.1089/ast.2020.2237
Casar, C. P., Kruger, B. R., Flynn, T. M., Masterson, A. L., Momper, L. M., & Osburn, M. R. (2020). Mineral-hosted biofilm communities in the
continental deep subsurface, Deep Mine Microbial Observatory, SD, USA. Geobiology, 18(4), 508–522. https://doi.org/10.1111/gbi.12391
Castro-Wallace, S. L., Chiu, C. Y., John, K. K., Stahl, S. E., Rubins, K. H., McIntyre, A. B., etal. (2017). Nanopore DNA sequencing and genome
assembly on the International Space Station. Scientific Reports, 7(1), 18022. https://doi.org/10.1038/s41598-017-18364-0
Chappaz, L., Sood, R., Melosh, H. J., Howell, K. C., Blair, D. M., Milbury, C., & Zuber, M. T. (2017). Evidence of large empty lava tubes on the
Moon using GRAIL gravity. Geophysical Research Letters, 44(1), 105–112. https://doi.org/10.1002/2016gl071588
Coan, D. (2020). Exploration EVA system concept of operations. EVA-EXP-0042, Revision B (p.175). NASA Headquarters. Retrieved from
https://www.nasa.gov/sites/default/files/atoms/files/eva-exp-0042_xeva_system_con_ops_rev_b_final_dtd_10192020_ref_doc.pdf
Cockell, C. S., Holt, J., Campbell, J., Groseman, H., Josset, J. L., Bontognali, T. R., etal. (2019). Subsurface scientific exploration of extraterres-
trial environments (MINAR 5): Analogue science, technology and education in the Boulby Mine, UK. International Journal of Astrobiology,
18(2), 157–182. https://doi.org/10.1017/s1473550418000186
Cooper, B. L. (1990). Sources and subsurface reservoirs of lunar volatiles. Lunar and Planetary Science Conference Proceedings, 20, 259–269.
Cornet, T., Cordier, D., Bahers, T. L., Bourgeois, O., Fleurant, C., Mouélic, S. L., & Altobelli, N. (2015). Dissolution on Titan and on Earth: Toward
the age of Titan's karstic landscapes. Journal of Geophysical Research: Planets, 120(6), 1044–1074. https://doi.org/10.1002/2014je004738
Cornet, T., Fleurant, C., Seignovert, B., Cordier, D., Bourgeois, O., Le Mouélic, S., etal. (2017). Landscape formation through dissolution on
Titan: A 3D landscape evolution model. In Paper presented at 48th Lunar and Planetary Science Conference. LPI Contributions No. 1835.
Cropley, J. B. (1965). Influence of surface conditions on temperatures in large cave systems. National Speleological Society Bulletin, 27, 1–9.
Cushing, G. E. (2012). Candidate cave entrances on Mars. Journal of Cave and Karst Studies, 74(1), 33–47. https://doi.org/10.4311/2010ex0167r
Cushing, G. E. (2017). Mars global cave candidate catalog archive bundle. U.S. Geological Survey. Retrieved from astrogeology.usgs.gov/search/
map/Mars/MarsCaveCatalog/mars_cave_catalog.zip
Cushing, G. E., Okubo, C. H., & Titus, T. N. (2015). Atypical pit craters on Mars: New insights from THEMIS, CTX, and HiRISE observations.
Journal of Geophysical Research: Planets, 120(6), 1023–1043. https://doi.org/10.1002/2014je004735
Cushing, G. E., Titus, T. N., Wynne, J. J., & Christensen, P. (2007). THEMIS observes possible cave skylights on Mars. Geophysical Research
Letters, 34(17), L17201. https://doi.org/10.1029/2007gl030709
Daga, A., Schneider Puente, I., Uzman, Z., de Leon, P., & Harris, G. (2010). Habitat architecture concept definition for “integrated strategies
for the human exploration of the Moon and Mars” (a NASA-funded study): Interim status report. In Paper presented at 40th International
Conference on Environmental Systems AIAA (pp.2010–6072).
Dancer, S. J. (2009). The role of environmental cleaning in the control of hospital-acquired infection. Journal of Hospital Infection, 73(4),
378–385. https://doi.org/10.1016/j.jhin.2009.03.030
Davey, S. C., Ernst, R. E., Samson, C., & Grosfils, E. B. (2013). Hierarchical clustering of pit crater chains on Venus. Canadian Journal of Earth
Sciences, 50(1), 109–126. https://doi.org/10.1139/cjes-2012-0054
Davila, A. F., Fairén, A. G., Rodríguez, A. P., Schulze-Makuch, D., Rask, J., & Zavaleta, J. (2015). The Hebrus Valles exploration zone: Access
to the Martian surface and subsurface. In First Landing Site/Exploration Zone Workshop for Human Missions to the Surface of Mars. LPI
Contributions No. 1012.
De Angeles, G., Wilson, J. W., Clowdsley, M. S., Nealy, J. E., Humes, D. H., & Clem, J. M. (2002). Lunar lava tube radiation safety analysis.
Journal of Radiation Research, 43, S41–S45. https://doi.org/10.1269/jrr.43.s41
de Freitas, C. R., & Littlejohn, R. N. (1987). Cave climate: Assessment of heat and moisture exchange. Journal of Climatology, 7(6), 553–569.
https://doi.org/10.1002/joc.3370070604
De Waele, J., Carbone, C., Sanna, L., Vattano, M., Galli, E., Sauro, F., & Forti, P. (2017). Secondary minerals from salt caves in the Atacama
Desert (Chile): A hyperarid and hypersaline environment with potential analogies to the Martian subsurface. International Journal of Speleol-
ogy, 46(1), 51–66. https://doi.org/10.5038/1827-806x.46.1.2094
Dharmadhikari, M., Nguyen, H., Mascarich, F., Khedekar, N., & Alexis, K. (2021). Autonomous cave exploration using aerial robots. Interna-
tional Conference on Unmanned Aircraft Systems (ICUAS) (pp.942–949). IEEE.
Dreybrodt, W., Gabrovšek, F., & Romanov, D. (2004). Processes of speleogenesis: A modeling approach. ZRC Publishing.
Dubowsky, S., Iagnemma, K., & Boston, P.J. (2006). Microbots for large-scale planetary surface and subsurface exploration. NIAC Phase II,
Year 1 Report (p.6). Massachusetts Institute of Technology.
Duxbury, N. S., & Brown, R. H. (1997). The role of an internal heat source for the eruptive plumes on Triton. Icarus, 125(1), 83–93. https://doi.
org/10.1006/icar.1996.5554
Ebadi, K., Chang, Y., Palieri, M., Stephens, A., Hatteland, A., Heiden, E., etal. (2020). LAMP: Large-scale autonomous mapping and positioning
for exploration of perceptually-degraded subterranean environments. IEEE International Conference on Robotics and Automation, 80–86.
https://doi.org/10.1109/icra40945.2020.9197082

Journal of Geophysical Research: Planets
WYNNE ETAL.
10.1029/2022JE007194
27 of 32
Escudero, C., Del Campo, A., Ares, J. R., Sánchez, C., Martínez, J. M., Gómez, F., & Amils, R. (2020). Visualizing microorganism-mineral
interaction in the Iberian Pyrite Belt subsurface: The acidovorax case. Frontiers in Microbiology, 11, 2833. https://doi.org/10.3389/
fmicb.2020.572104
Esmaeili, S., Kruse, S., Jazayeri, S., Whelley, P., Bell, E., Richardson, J., etal. (2020). Resolution of lava tubes with ground penetrating radar: The
TubeX project. Journal of Geophysical Research: Planets, 125(5), e2019JE006138. https://doi.org/10.1029/2019je006138
Fairchild, I. J., & Baker, A. (2012). Speleothem science: From process to past environments. Wiley-Blackwell.
Fairén, A. G., Parro, V., Schulze-Makuch, D., & Whyte, L. (2017). Searching for life on Mars before it is too late. Astrobiology, 17(10), 962–970.
https://doi.org/10.1089/ast.2017.1703
Fan, D. D., Otsu, K., Kubo, Y., Dixit, A., Burdick, J., & Agha-Mohammadi, A. (2021). STEP: Stochastic traversability evaluation and planning
for safe off-road navigation. Robotics: Science and Systems. Retrieved from https://arxiv.org/pdf/2103.02828
Ferrill, D. A., Wyrick, D. Y., Morris, A. P., Sims, D. W., & Franklin, N. M. (2004). Dilational fault slip and pit chain formation on Mars. Geolog-
ical Society of America Today, 14(10), 4–12. https://doi.org/10.1130/1052-5173(2004)014<4:dfsapc>2.0.co;2
Fletcher, L. E., Conley, C. A., Valdivia-Silva, J. E., Perez-Montaño, S., Condori-Apaza, R., Kovacs, G. T., etal. (2011). Determination of low
bacterial concentrations in hyperarid Atacama soils: Comparison of biochemical and microscopy methods with real-time quantitative PCR.
Canadian Journal of Microbiology, 57(11), 953–963. https://doi.org/10.1139/w11-091
Freeman, A., Smrekar, S. E., Hensley, S., Wallace, M., Sotin, C., Darrach, M., etal. (2016). VERITAS: A discovery-class Venus surface geology
and geophysics mission (pp.1–11). IEEE.
Frumkin, A., Pe’eri, S., & Zak, I. (2021). Development of banded terrain in an active salt diapir: Potential analog to Mars. Geomorphology, 389,
107824. https://doi.org/10.1016/j.geomorph.2021.107824
Fullerton, R. K. (1993). EVA tools and equipment reference book. JSC-20466, Rev. B, NASA-TM-109350 (p.750). NASA Johnson Space Center.
Retrieved from https://ntrs.nasa.gov/citations/19940017339
Funabiki, N., Morrell, B., Nash, J., & Agha-Mohammadi, A. (2020). Range-aided pose-graph-based SLAM: Applications of deployable ranging
beacons for unknown environment exploration. IEEE Robotics and Automation Letters, 6(1), 48–55. https://doi.org/10.1109/lra.2020.3026659
Gaboyer, F., Le Milbeau, C., Bohmeier, M., Schwendner, P., Vannier, P., Beblo-Vranesevic, K., etal. (2017). Mineralization and preservation
of an extremotolerant bacterium isolated from an early Mars analog environment. Scientific Reports, 7(1), 8775. https://doi.org/10.1038/
s41598-017-08929-4
Gaier, J. R. (2007). The effects of lunar dust on EVA systems during the Apollo missions. NASA/TM-2005-213610 (p.73). NASA Glenn Research
Center.
Gallego, G., Bridges, J. F., Flynn, T., Blauvelt, B. M., & Niessen, L. W. (2012). Using best-worst scaling in horizon scanning for hepato-
cellular carcinoma technologies. International Journal of Technology Assessment in Health Care, 28(3), 339–346. https://doi.org/10.1017/
s026646231200027x
Garvin, J. B., Arney, G., Getty, S., Johnson, N., Kiefer, W., Lorenz, R., etal. (2020). DAVINCI+: Deep Atmosphere of Venus Investigation of
Noble gases, Chemistry, and Imaging Plus. In Paper presented at 51st Lunar and Planetary Science Conference. LPI Contributions No. 2599.
Geissler, P.E., & McMillan, M. T. (2008). Galileo observations of volcanic plumes on Io. Icarus, 197(2), 505–518. https://doi.org/10.1016/j.
icarus.2008.05.005
Gellert, R., Rieder, R., Brückner, J., Clark, B. C., Dreibus, G., Klingelhöfer, G., etal. (2006). Alpha Particle X-ray Spectrometer (APXS): Results
from Gusev crater and calibration report. Journal of Geophysical Research, 111, E2. https://doi.org/10.1029/2005je002555
Ghezzi, D., Sauro, F., Columbu, A., Carbone, C., Hong, P. Y., Vergara, F., etal. (2021). Transition from unclassified Ktedonobacterales to
Actinobacteria during amorphous silica precipitation in a quartzite cave environment. Scientific Reports, 11(1), 3921. https://doi.org/10.1038/
s41598-021-83416-5
Ginting, M. F., Otsu, K., Edlund, J. A., Gao, J., & Agha-Mohammadi, A. (2021). CHORD: Distributed data-sharing via hybrid ROS 1 and
2 for multi-robot exploration of large-scale complex environments. IEEE Robotics and Automation Letters, 6(3), 5064–5071. https://doi.
org/10.1109/lra.2021.3061393
Ginting, M. F., Touma, T., Edlund, J., & Agha-Mohammadi, A. (2020). Deployable mesh network for enabling reliable communication from
within subsurface voids to the planetary surface. American Geophysical Union. P055-0001.
Glavin, D. P., Dworkin, J. P., Lupisella, M., Kminek, G., & Rummel, J. D. (2004). Biological contamination studies of lunar landing sites: Impli-
cations for future planetary protection and life detection on the Moon and Mars. International Journal of Astrobiology, 3, 265–271. https://
doi.org/10.1017/s1473550404001958
Goel, K., Tabib, W., & Michael, N. (2021). Rapid and high-fidelity subsurface exploration with multiple aerial robots. In B. Siciliano, C.
Laschi, & O. Khatib (Eds.), Experimental Robotics. Springer Proceedings in Advanced Robotics (Vol. 19). Springer. https://doi.
org/10.1007/978-3-030-71151-1_39
Goh, J. (2021). A literature review of medical support in cave rescue and confined space medicine–implications in urban underground space
development. IOP Conference Series: Earth and Environmental Science, 703(1), 012042. https://doi.org/10.1088/1755-1315/703/1/012042
Gonzalez-Pimentel, J. L., Miller, A. Z., Jurado, V., Laiz, L., Pereira, M. F., & Saiz-Jimenez, C. (2018). Yellow coloured mats from lava tubes of
La Palma (Canary Islands, Spain) are dominated by metabolically active Actinobacteria. Scientific Reports, 8, 1–11. https://doi.org/10.1038/
s41598-018-20393-2
Goordial, J., Altshuler, I., Hindson, K., Chan-Yam, K., Marcolefas, E., & Whyte, L. G. (2017). In situ field sequencing and life detection in remote
(79°26′N) Canadian high arctic permafrost ice wedge microbial communities. Frontiers in Microbiology, 8, 2594. https://doi.org/10.3389/
fmicb.2017.02594
Greeley, R. (1971). Lava tubes and channels in the lunar Marius Hills. The Moon, 3, 289–314. https://doi.org/10.1007/bf00561842
Halliday, W. R. (1966). Terrestrial pseudokarst and the lunar topography. National Speleological Society Bulletin, 28, 167–170.
Hamran, S. E., Paige, D. A., Amundsen, H. E., Berger, T., Brovoll, S., Carter, L., etal. (2020). Radar imager for Mars’ subsurface experiment—
RIMFAX. Space Science Reviews, 216(8), 1–39. https://doi.org/10.1007/s11214-020-00740-4
Hansen, C. J., Shemansky, D. E., Esposito, L. W., Stewart, A. I. F., Lewis, B. R., Colwell, J. E., etal. (2011). The composition and structure of
the Enceladus plume. Geophysical Research Letters, 38(11), L11202. https://doi.org/10.1029/2011gl047415
Hansen, V. L., & Willis, J. J. (1998). Ribbon terrain formation, southwestern Fortuna Tessera, Venus: Implications for lithosphere evolution.
Icarus, 132(2), 321–343. https://doi.org/10.1006/icar.1998.5897
Haruyama, J., Hioki, K., Shirao, M., Morota, T., Hiesinger, H., van der Bogert, C. H., etal. (2009). Possible lunar lava tube skylight observed by
SELENE cameras. Geophysical Research Letters, 36(21), L21206. https://doi.org/10.1029/2009gl040635
Hassler, D. M., Zeitlin, C., Wimmer-Schweingruber, R. F., Ehresmann, B., Rafkin, S., Eigenbrode, J. L., etal. (2014). Mars’ surface radiation
environment measured with the Mars Science Laboratory’s Curiosity rover. Science, 343, 1244797. https://doi.org/10.1126/science.1244797

Journal of Geophysical Research: Planets
WYNNE ETAL.
10.1029/2022JE007194
28 of 32
Hathaway, J. J. M., Garcia, M. G., Balasch, M. M., Spilde, M. N., Stone, F. D., Dapkevicius, M. D. L. N., etal. (2014). Comparison of bacterial
diversity in Azorean and Hawai'ian lava cave microbial mats. Geomicrobiology Journal, 31(3), 205–220. https://doi.org/10.1080/01490451.
2013.777491
Hays, L. E., Graham, H. V., Des Marais, D. J., Hausrath, E. M., Horgan, B., McCollom, T. M., etal. (2017). Biosignature preservation and detec-
tion in Mars analog environments. Astrobiology, 17(4), 363–400. https://doi.org/10.1089/ast.2016.1627
Hecht, M., Hoffman, J., Rapp, D., McClean, J., SooHoo, J., Schaefer, R., etal. (2021). Mars oxygen ISRU experiment (MOXIE). Space Science
Reviews, 217, 1–76. https://doi.org/10.1007/s11214-020-00782-8
Helbert, J., Vandaele, A. C., Marcq, E., Robert, S., Ryan, C., Guignan, G., etal. (2019). The VenSpec suite on the ESA EnVision mission to
Venus. SPIE Infrared Remote Sensing and Instrumentation XXVII (Vol. 11128,p.1112804). International Society for Optics and Photonics.
https://doi.org/10.1117/12.2529248
Himangshu, K., & Thangavelautham, J. (2020). Long term exploration of planetary pits, caves, and lava tubes. In International Symposium on
Artificial Intelligence, Robotics and Automation in Space. LPI Contributions No. 5062.
Hines, A., Bengston, D. N., Dockry, M. J., & Cowart, A. (2018). Setting up a horizon scanning system: A US federal agency example. World
Futures Review, 10(2), 136–151. https://doi.org/10.1177/1946756717749613
Hoehler, T. M., & Westall, F. (2010). Mars exploration program analysis group goal one: Determine if life ever arose on Mars. Astrobiology,
10(9), 859–867. https://doi.org/10.1089/ast.2010.0527
Howarth, F. G. (1980). The zoogeography of specialized cave animals: A bioclimatic model. Evolution, 34(2), 394–406. https://doi.org/
10.2307/2407402
Howarth, F. G. (1982). Bioclimatic and geological factors governing the evolution and distribution of Hawaiian cave insects. Entomologia Gener-
alis, 8(1), 17–26. https://doi.org/10.1127/entom.gen/8/1982/17
Hu, S., Kim, M. H. Y., McClellan, G. E., & Cucinotta, F. A. (2009). Modeling the acute health effects of astronauts from exposure to large solar
particle events. Health Physics, 96(4), 465–476. https://doi.org/10.1097/01.hp.0000339020.92837.61
Huber, S. A., Hendrickson, D. B., Jones, H. L., Thornton, J. P., Whittaker, W. L., & Wong, U. Y. (2014). Astrobotic technology: Planetary pits and
caves for science and exploration. Annual Meeting of the Lunar Exploration Analysis Group (Vol. 1820,p.3065).
Hughes, A. C., Lechner, A. M., Chitov, A., Horstmann, A., Hinsley, A., Tritto, A., etal. (2020). Horizon scan of the belt and road initiative. Trends
in Ecology & Evolution, 35(7), 583–593. https://doi.org/10.1016/j.tree.2020.02.005
Humphries, R. M., & McDonnell, G. (2015). Superbugs on duodenoscopes: The challenge of cleaning and disinfection of reusable devices.
Journal of Clinical Microbiology, 53(10), 3118–3125. https://doi.org/10.1128/jcm.01394-15
Husain, A., Jones, H., Kannan, B., Wong, U., Pimentel, T., Tang, S., etal. (2013). Mapping planetary caves with an autonomous, heterogeneous
robot team. IEEE Aerospace Conference (pp.1–13). https://doi.org/10.1109/aero.2013.6497363
Jackson, W. A., Barta, D. J., Anderson, M. S., Lange, K. E., Hanford, A. J., Shull, S. A., etal. (2014). Water recovery from brines to further close
the water recovery loop in human spaceflight. In International Conference on Environmental Systems, ICES-2014–186.
Jones, A. A., & Bennett, P.C. (2017). Mineral ecology: Surface specific colonization and geochemical drivers of biofilm accumulation, compo-
sition, and phylogeny. Frontiers in Microbiology, 8, 491. https://doi.org/10.3389/fmicb.2017.00491
Jones, B. (2001). Microbial activity in caves−A geological perspective. Geomicrobiology Journal, 18(3), 345–357. https://doi.org/10.1080/
01490450152467831
Jozwiak, L. M., Head, J. W., & Wilson, L. (2018). Explosive volcanism on Mercury: Analysis of vent and deposit morphology and modes of
eruption. Icarus, 302, 191–212. https://doi.org/10.1016/j.icarus.2017.11.011
Kaku, T., Haruyama, J., Miyake, W., Kumamoto, A., Ishiyama, K., Nishibori, T., etal. (2017). Detection of intact lava tubes at Marius Hills on the
Moon by SELENE (Kaguya) Lunar Radar Sounder. Geophysical Research Letters, 44(20), 10155–10161. https://doi.org/10.1002/2017gl074998
Kalita, H., Nallapu, R. T., Warren, A., & Thangavelautham, J. (2017). Guidance, navigation and control of multirobot systems in cooperative cliff
climbing. Advances in the Astronautical Sciences Guidance, Navigation and Control, 159, 1–14.
Kearney, M. L., Wynne, J. J., Cushing, G. E., Bardabelias, N. M., & Barlow, N. G. (2021). Robotic exploration potential of Martian caves. In
Paper presented at 52nd Lunar and Planetary Science Conference. LPI Contributions No. 2078.
Kemp, L., Adam, L., Boehm, C. R., Breitling, R., Casagrande, R., Dando, M., etal. (2020). Point of view: Bioengineering horizon scan 2020.
Elife, 9, e54489. https://doi.org/10.7554/elife.54489
Kempe, S. (2012). Volcanic rock caves. In W. B. White & D. C. Culver (Eds.), Encyclopedia of Caves (2nd ed.,pp.865–873). Elsevier.
Kerber, L., Denevi, B. W., Nesnas, I., Keszthelyi, L., Head, J. W., Pieters, C., etal. (2019). Moon diver: A discovery mission concept for under-
standing secondary crust formation through the exploration of a lunar mare pit cross-section. In Lunar and Planetary Science Conference,
Abstract #1163, LPI Contribution No. 2132. Retrieved from https://www.hou.usra.edu/meetings/lpsc2019/pdf/1163.pdf
Kerner, H. R., Hardgrove, C. J., Czarnecki, S., Gabriel, T. S., Mitrofanov, I. G., Litvak, M. L., etal. (2020). Analysis of active neutron measure-
ments from the Mars Science Laboratory Dynamic Albedo of Neutrons Instrument: Intrinsic variability, outliers, and implications for future
investigations. Journal of Geophysical Research: Planets, 125(5), e2019JE006264. https://doi.org/10.1029/2019je006264
Kim, S. K., Bouman, A., Salhotra, G., Fan, D. D., Otsu, K., Burdick, J., etal. (2021). PLGRIM: Hierarchical value learning for large-scale explo-
ration in unknown environments. Proceedings of the International Conference on Automated Planning and Scheduling (Vol. 31). Retrieved
from https://arxiv.org/pdf/2102.05633.pdf
Klein, F., Grozeva, N. G., & Seewald, J. S. (2019). Abiotic methane synthesis and serpentinization in olivine-hosted fluid inclusions. Proceedings
of the National Academy of Sciences, 116(36), 17666–17672. https://doi.org/10.1073/pnas.1907871116
Klimchouk, A. (2009). Morphogenesis of hypogenic caves. Geomorphology, 106(1–2), 100–117. https://doi.org/10.1016/j.geomorph.2008.09.013
Klimchouk, A. (2012). Speleogenesis, hypogenic. In W. B. White & D. C. Culver (Eds.), Encyclopedia of caves (2nd ed.,pp.748–765). Academic
Press.
Könnölä, T., Salo, A., Cagnin, C., Carabias, V., & Vilkkumaa, E. (2012). Facing the future: Scanning, synthesizing and sense-making in horizon
scanning. Science and Public Policy, 39(2), 222–231. https://doi.org/10.1093/scipol/scs021
Kreuz, F., Juraschek, M., Bucherer, M., Söfker-Rieniets, A., Spengler, A., Clausen, U., etal. (2020). Urban factories—Interdisciplinary perspec-
tives on resource efficiency. In R. Elbert, C. Friedrich, M. Boltze, & H. C. Pfohl (Eds.), Urban freight transportation systems (pp.41–52).
Elsevier.
Krishnan, S. (2021). A polyhedral approach for design of inflatable lunar habitats. In Paper presented at 17th Biennial International Conference
on Engineering, Science, Construction, and Operations in Challenging Environments (pp.1004–1011).
Lauro, S. E., Pettinelli, E., Caprarelli, G., Guallini, L., Rossi, A. P., Mattei, E., etal. (2021). Multiple subglacial water bodies below the south pole
of Mars unveiled by new MARSIS data. Nature Astronomy, 5(1), 63–70. https://doi.org/10.1038/s41550-020-1200-6
Léveillé, R. J., & Datta, S. (2010). Lava tubes and basaltic caves as astrobiological targets on Earth and Mars: A review. Planetary and Space
Science, 58(4), 592–598. https://doi.org/10.1016/j.pss.2009.06.004

Journal of Geophysical Research: Planets
WYNNE ETAL.
10.1029/2022JE007194
29 of 32
Linne, D. L., Sanders, G. B., Starr, S. O., Eisenman, D. J., Suzuki, N. H., Anderson, M. S., etal. (2017). Overview of NASA technology devel-
opment for in-situ resource utilization (ISRU). InPaper presented at 68th International Astronautical Congress, Adelaide, Australia, 25–29
September 2017. IAC-17-D3.3.1. Retrieved from https://ntrs.nasa.gov/api/citations/20180000407/downloads/20180000407.pdf
Litteken, D. A. (2019). Inflatable technology: Using flexible materials to make large structures Electroactive polymer actuators and devices
(EAPAD) XXI (Vol. 10966,p.1096603). SPIE.
Lowe, C. M., Sarnell, J. A., Stuehrmann, L. G., Schwartzman, M. C., & Robbins, T. J. (2018). A Heterogeneous swarm solution for disaster
reconnaissance: A feasibility study. Worcester Polytechnic Institute. Retrieved from https://core.ac.uk/download/pdf/212986929.pdf
Lyu, Z., Shao, N., Akinyemi, T., & Whitman, W. B. (2018). Methanogenesis. Current Biology, 28(13), R727–R732. https://doi.org/10.1016/j.
cub.2018.05.021
Magnabosco, C., Lin, L. H., Dong, H., Bomberg, M., Ghiorse, W., Stan-Lotter, H., etal. (2018). The biomass and biodiversity of the continental
subsurface. Nature Geoscience, 11(10), 707–717. https://doi.org/10.1038/s41561-018-0221-6
Mahaffy, P.R., Webster, C. R., Cabane, M., Conrad, P.G., Coll, P., Atreya, S. K., etal. (2012). The sample analysis at Mars investigation and
instrument suite. Space Science Reviews, 170, 401–478. https://doi.org/10.1007/s11214-012-9879-z
Malaska, M., Radebaugh, J., Mitchell, K., Lopes, R., Wall, S., & Lorenz, R. (2011). Surface dissolution model for Titan karst. In Paper presented
at 1st International Planetary Cave Workshop. LPI Contributions No. 8018.
Malaska, M. J., & Hodyss, R. (2014). Dissolution of benzene, naphthalene, and biphenyl in a simulated Titan lake. Icarus, 242, 74–81. https://
doi.org/10.1016/j.icarus.2014.07.022
Malaska, M. J., & Mitchell, K. (2017). Predicting cave formations in Saturn's moon Titan. GSA Annual Meeting. Retrieved from https://trs.jpl.
nasa.gov/bitstream/handle/2014/48703/CL%2317-5343.pdf?sequence=1
Malaska, M. J., Radebaugh, J., Lopes, R. M., Mitchell, K. L., Verlander, T., Schoenfeld, A. M., etal. (2020). Labyrinth terrain on Titan. Icarus,
344, 113764. https://doi.org/10.1016/j.icarus.2020.113764
Mammola, S., Amorim, I. R., Bichuette, M. E., Borges, P.A. V., Cheeptham, N., Cooper, S. J. B., etal. (2020). Fundamental research questions
in subterranean biology. Biological Reviews, 95(6), 1855–1872. https://doi.org/10.1111/brv.12642
Mansor, M., Harouaka, K., Gonzales, M. S., Macalady, J. L., & Fantle, M. S. (2018). Transport-induced spatial patterns of sulfur isotopes (δ
34S)
as biosignatures. Astrobiology, 18(1), 59–72. https://doi.org/10.1089/ast.2017.1650
Manyapu, K. K., & Peltz, L. (2018). Mitigation system utilizing conductive fibers. U.S. Patent No. 10,016,766. U.S. Patent and Trademark Office.
Dust.
Margiotta, D. V., Peters, W. C., Straka, S. A., Rodríguez, M., McKittrick, K. R., & Jones, C. B. (2010). The Lotus coating for space exploration:
A dust mitigation tool. In Optical System Contamination: Effects, Measurements, and Control (Vol. 7794,p.77940I). International Society for
Optics and Photonics.https://doi.org/10.1117/12.864480
Maynard-Casely, H., Cable, M., Vu, T., Malaska, M., Choukroun, M., & Hodyss, R. (2018). Prospects for mineralogy on Titan. American Miner-
alogist, 103(3), 343–349. https://doi.org/10.2138/am-2018-6259
McKay, C. P. (2009). Planetary ecosynthesis on Mars: Restoration ecology and environmental ethics. In C. M. Bertka (Ed.), Exploring the origin,
extent, and future of life: Philosophical, ethical, and theological perspectives (Vol. 4,pp.245–260). Cambridge University Press.
McMillon-Brown, L., Peshek, T. J., Pal, A., & McNatt, J. (2019). Dust abrasion damage on Martian solar arrays: Experimental investigation and
opportunity rover performance analysis. In IEEE 46th Photovoltaic Specialists Conference (pp.2838–2844). IEEE.
Miller, A. Z., García-Sánchez, A. M., Coutinho, M. L., Costa Pereira, M. F., Gázquez, F., Calaforra, J. M., etal. (2020). Colored microbial coat-
ings in show caves from the Galapagos Islands (Ecuador): First microbiological approach. Coatings, 10(11), 1134. https://doi.org/10.3390/
coatings10111134
Miller, A. Z., Garcia-Sanchez, A. M., Martin-Sanchez, P.M., Costa Pereira, M. F., Spangenberg, J. E., Jurado, V., etal. (2018). Origin of abundant
moonmilk deposits in a subsurface granitic environment. Sedimentology, 65(5), 1482–1503. https://doi.org/10.1111/sed.12431
Miller, A. Z., José, M., Jiménez-Morillo, N. T., Pereira, M. F., González-Pérez, J. A., Calaforra, J. M., & Saiz-Jimenez, C. (2016). Analytical
pyrolysis and stable isotope analyses reveal past environmental changes in coralloid speleothems from Easter Island (Chile). Journal of Chro-
matography A, 1461, 144–152. https://doi.org/10.1016/j.chroma.2016.07.038
Miller, A. Z., Pereira, M. F., Calaforra, J. M., Forti, P., Dionísio, A., & Saiz-Jimenez, C. (2014). Siliceous speleothems and associated
microbe-mineral interactions from Ana Heva Lava Tube in Easter Island (Chile). Geomicrobiology Journal, 31(3), 236–245. https://doi.org/1
0.1080/01490451.2013.827762
Miot, J., Benzerara, K., & Kappler, A. (2014). Investigating microbe-mineral interactions: Recent advances in X-ray and electron micros-
copy and redox-sensitive methods. Annual Review of Earth and Planetary Sciences, 42(1), 271–289. https://doi.org/10.1146/annurev-
earth-050212-124110
Moirano, R., Sánchez, M. A., & Štěpánek, L. (2020). Creative interdisciplinary collaboration: A systematic literature review. Thinking Skills and
Creativity, 35, 100626. https://doi.org/10.1016/j.tsc.2019.100626
Morthekai, P., Jain, M., Dartnell, L., Murray, A. S., Bøtter-Jensen, L., & Desorgher, L. (2007). Modelling of the dose-rate variations with depth in
the Martian regolith using GEANT4. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors
and Associated Equipment, 580(1), 667–670. https://doi.org/10.1016/j.nima.2007.05.118
Muscarella, L. F. (2014). Risk of transmission of carbapenem-resistant Enterobacteriaceae and related “superbugs” during gastrointestinal endos-
copy. World Journal of Gastrointestinal Endoscopy, 6(10), 457–474. https://doi.org/10.4253/wjge.v6.i10.457
Mylroie, J. E. (2019). Caves in space. Journal of Cave and Karst Studies, 81(1), 25–32. https://doi.org/10.4311/2018es0102
Nakagawa, S., & Schielzeth, H. (2013). A general and simple method for obtaining R
2 from generalized linear mixed-effects models. Methods in
Ecology and Evolution, 4(2), 133–142. https://doi.org/10.1111/j.2041-210x.2012.00261.x
NASA. (2021). Planetary protection. NASA. Retrieved from https://sma.nasa.gov/sma-disciplines/planetary-protection/#planetaryProtection_
conferenceDocuments
Naser, M. Z., & Chen, Q. (2021). Extraterrestrial construction in lunar and Martian environments. In Paper presented at 17th Biennial Interna-
tional Conference on Engineering, Science, Construction, and Operations in Challenging Environments (pp.1200–1207).
National Academies of Sciences, Engineering, and Medicine [NASEM]. (2015). Review of the MEPAG report on Mars special regions. The
National Academies Press. https://doi.org/10.17226/21816
National Academies of Sciences, Engineering, and Medicine [NASEM]. (2022). Origins, worlds, and life: A decadal strategy for planetary
science and astrobiology 2023–2032. The National Academies Press. https://doi.org/10.17226/26522
Nesnas, I. A., Kerber, L., Parness, A., Kornfeld, R., Sellar, G., McGarey, P., etal. (2019). Moon diver: A discovery mission concept for under-
standing the history of secondary crusts through the exploration of a lunar mare pit. In IEEE Aerospace Conference (pp.1–23). IEEE. https://
doi.org/10.1109/aero.2019.8741788

Journal of Geophysical Research: Planets
WYNNE ETAL.
10.1029/2022JE007194
30 of 32
Nesnas, I. A., Matthews, J. B., Abad-Manterola, P., Burdick, J. W., Edlund, J. A., Morrison, J. C., etal. (2012). Axel and DuAxel rovers for the
sustainable exploration of extreme terrains. Journal of Field Robotics, 29(4), 663–685. https://doi.org/10.1002/rob.21407
Neveu, M., Hays, L. E., Voytek, M. A., New, M. H., & Schulte, M. D. (2018). The ladder of life detection. Astrobiology, 18(11), 1375–1402.
https://doi.org/10.1089/ast.2017.1773
Nicholson, W. L., Schuerger, A. C., & Race, M. S. (2009). Migrating microbes and planetary protection. Trends in Microbiology, 17(9), 389–392.
https://doi.org/10.1016/j.tim.2009.07.001
Northup, D. E., Melim, L. A., Spilde, M. N., Hathaway, J. J. M., Garcia, M. G., Moya, M., etal. (2011). Lava cave microbial communities within
mats and secondary mineral deposits: Implications for life detection on other planets. Astrobiology, 11(7), 601–618. https://doi.org/10.1089/
ast.2010.0562
Oberbeck, V. R., Quaide, W. L., & Greeley, R. (1969). On the origin of lunar sinuous rilles. Modern Geology, 1, 75–80.
Onstott, T. C., Ehlmann, B. L., Sapers, H., Coleman, M., Ivarsson, M., Marlow, J. J., etal. (2018). Paleo-rock-hosted life on Earth and the search
on Mars: A review and strategy for exploration. Astrobiology, 19(10), 1230–1262. https://doi.org/10.1089/ast.2018.1960
Otsu, K., Tepsuporn, S., Thakker, R., Vaquero, T. S., Edlund, J. A., Walsh, W., etal. (2020). Supervised autonomy for communication-degraded
subterranean exploration by a robot team. In 2020 IEEE Aerospace Conference (pp.1–9).
Palmer, A. N. (2007). Cave geology. Cave Books.
Palomba, E., Zinzi, A., Cloutis, E. A., d’Amore, M., Grassi, D., & Maturilli, A. (2009). Evidence for Mg-rich carbonates on Mars from a 3.9 μm
absorption feature. Icarus, 203(1), 58–65. https://doi.org/10.1016/j.icarus.2009.04.013
Parness, A., Abcouwer, N., Fuller, C., Wiltsie, N., Nash, J., & Kennedy, B. (2017). Lemur 3: A limbed climbing robot for extreme terrain mobility
in space. In IEEE International Conference on Robotics and Automation (ICRA) (pp.5467–5473).
Patiño, J., Whittaker, R. J., Borges, P. A., Fernández-Palacios, J. M., Ah-Peng, C., Araújo, M. B., etal. (2017). A roadmap for island biol-
ogy: 50 fundamental questions after 50 years of The Theory of Island Biogeography. Journal of Biogeography, 44(5), 963–983. https://doi.
org/10.1111/jbi.12986
Perkins, S. (2020). Core concept: Lava tubes may be havens for ancient alien life and future human explorers. Proceedings of the National Acad-
emy of Sciences, 117(30), 17461–17464. https://doi.org/10.1073/pnas.2012176117
Perșoiu, A., & Onac, B. P. (2019). Ice in caves. In W. B. White, D. C. Culver, & T. Pipan (Eds.), Encyclopedia of caves (3rd ed.,pp.553–558).
Academic Press.
Pflitsch, A., & Piasecki, J. (2003). Detection of an airflow system in Niedzwiedzia (Bear) Cave, Kletno, Poland. Journal of Cave and Karst
Studies, 63, 160–173.
Pfohl, H.-C., Berbner, U., & Zuber, C. (2017). Interdisciplinary decisions in production, logistics, and traffic and transport: Measures for over-
coming barriers in interdisciplinary decision-making. In E. Abele, M. Boltze, & H.-C. Pfohl (Eds.), Dynamic and seamless integration of
production, logistics and traffic (pp.13–30). Springer International Publishing.
Phillips-Lander, C. M., Agha-Mohamadi, A., Wynne, J. J., Titus, T. N., Chanover, N., Demirel-Floyd, C., etal. (2020). Mars Astrobiological Cave
and Internal habitability Explorer (MACIE): A new frontiers mission concept. Bulletin of the American Astronomical Society, 53, 4.
Pisani, L., & De Waele, J. (2021). Candidate cave entrances in a planetary analogue evaporite karst (Cordillera de la Sal, Chile): A remote sensing
approach and ground-truth reconnaissance. Geomorphology, 389, 107851. https://doi.org/10.1016/j.geomorph.2021.107851
Plavén-Sigray, P., Matheson, G. J., Schiffler, B. C., & Thompson, W. H. (2017). The readability of scientific texts is decreasing over time. Elife,
6, e27725. https://doi.org/10.7554/elife.27725
Porco, C., DiNino, D., & Nimmo, F. (2014). How the geysers, tidal stresses, and thermal emission across the south polar terrain of Enceladus are
related. The Astronomical Journal, 148(3), 45. https://doi.org/10.1088/0004-6256/148/3/45
Prettyman, T. H., Englert, P.A. J., Yamashita, N., & Landis, M. E. (2019). Neutron, gamma-ray, and X-ray spectroscopy of planetary bodies. In J.
L. Bishop, J. F. Bell, & J. E. Moersch (Eds.), Remote Compositional Analysis (pp.588–603). Cambridge University Press.
Prettyman, T. H., Titus, T. N., Cushing, G. E., Okubo, C. H., Sankey, J. B., Williams, K. E., etal. (2020). Muon overburden gauge for planetary
analog studies of cave ice stability. In Paper presented at 3rd International Planetary Caves Conference. LPI Contributions No. 2197.
Radotich, M., Withrow-Maser, S., deSouza, Z., Gelhar, S., & Gallagher, H. (2021). A study of past, present, and future Mars rotorcraft (p.17).
NASA Ames Research Center. Retrieved from https://rotorcraft.arc.nasa.gov/Publications/files/Radotich_aVTOL_Tech_Meeting_Final_
Revised_012521.pdf
Raulin, F. (1987). Organic chemistry in the oceans of Titan. Advances in Spaces Research, 7(5), 71–81. https://doi.org/10.1016/0273-1177(87)90358-9
Rempfert, K. R., Miller, H. M., Bompard, N., Nothaft, D., Matter, J. M., Kelemen, P., etal. (2017). Geological and geochemical controls on
subsurface microbial life in the Samail Ophiolite, Oman. Frontiers in Microbiology, 8, 56. https://doi.org/10.3389/fmicb.2017.00056
Richardson, C. D., Hinman, N. W., & Scott, J. R. (2013). Evidence for biological activity in mineralization of secondary sulphate deposits in a
basaltic environment: Implications for the search for life in the Martian subsurface. International Journal of Astrobiology, 12(4), 357–368.
https://doi.org/10.1017/s1473550413000256
Riquelme, C., Marshall Hathaway, J. J., Enes Dapkevicius, M. D. L., Miller, A. Z., Kooser, A., Northup, D. E., etal. (2015). Actinobacte-
rial diversity in volcanic caves and associated geomicrobiological interactions. Frontiers in Microbiology, 6, 1342. https://doi.org/10.3389/
fmicb.2015.01342
Röling, W. F., Aerts, J. W., Patty, C. H., Ten Kate, I. L., Ehrenfreund, P., & Direito, S. O. (2015). The significance of microbe-mineral-biomarker
interactions in the detection of life on Mars and beyond. Astrobiology, 15(6), 492–507. https://doi.org/10.1089/ast.2014.1276
Rossi, A. P., Maurelli, F., Unnithan, V., Dreger, H., Mathewos, K., Pradhan, N., etal. (2021). DAEDALUS-Descent And Exploration in Deep
Autonomy of Lava Underground Structures: Open Space Innovation Platform (OSIP) lunar caves-system study. Würzburger Forschungsber-
ichte in Robotik und Telematik. University of Würzburger. Retrieved from https://opus.bibliothek.uni-wuerzburg.de/opus4-wuerzburg/front-
door/deliver/index/docId/22791/file/Rossi_et_al_DAEDALUS.pdf
Roth, L., Saur, J., Retherford, K. D., Strobel, D. F., Feldman, P.D., McGrath, M. A., & Nimmo, F. (2014). Transient water vapor at Europa’s south
pole. Science, 343(6167), 171–174. https://doi.org/10.1126/science.1247051
Rouillard, J., van Zuilen, M., Pisapia, C., & Garcia-Ruiz, J. M. (2021). An alternative approach for assessing biogenicity. Astrobiology, 21(2),
151–164. https://doi.org/10.1089/ast.2020.2282
Rull, F., Maurice, S., Hutchinson, I., Moral, A., Perez, C., Diaz, C., etal. (2017). The Raman laser spectrometer for the ExoMars rover mission to
Mars. Astrobiology, 17(6–7), 627–654. https://doi.org/10.1089/ast.2016.1567
Rummel, J. D., Beaty, D. W., Jones, M. A., Bakermans, C., Barlow, N. G., Boston, P.J., etal. (2014). A new analysis of Mars “special regions”:
Findings of the second MEPAG special regions science analysis group (SR-SAG2). Astrobiology, 11, 888–868. https://doi.org/10.1089/
ast.2014.1227
Santamaria-Navarro, A., Thakker, R., Fan, D. D., Morrell, B., & Agha-Mohammadi, A. (2019). Towards resilient autonomous navigation of
drones. In International Symposium on Robotics Research (pp.922–937). Springer.

Journal of Geophysical Research: Planets
WYNNE ETAL.
10.1029/2022JE007194
31 of 32
Sauder, J., Hilgemann, E., Johnson, M., Parness, A., Hall, J., Kawata, J., etal. (2017). Automation rover for extreme environments. NASA Inno-
vative Advanced Concepts (NIAC) Phase I, Final Report. NASA Jet Propulsion Laboratory, California Institute of Technology. Retrieved from
https://www.nasa.gov/sites/default/files/atoms/files/niac_2016_phasei_saunder_aree_tagged.pdf
Sauro, F., De Waele, J., Onac, B. P., Galli, E., Dublyansky, Y., Baldoni, E., & Sanna, L. (2014). Hypogenic speleogenesis in quartzite: The case of
Corona 'e Sa Craba Cave (SW Sardinia, Italy). Geomorphology, 211, 77–88. https://doi.org/10.1016/j.geomorph.2013.12.031
Sauro, F., De Waele, J., Payler, S. J., Vattano, M., Sauro, F. M., Turchi, L., & Bessone, L. (2021). Speleology as an analogue to space exploration:
The ESA CAVES training programme. Acta Astronautica, 184, 150–166. https://doi.org/10.1016/j.actaastro.2021.04.003
Sauro, F., Pozzobon, R., Massironi, M., De Berardinis, P., Santagata, T., & De Waele, J. (2020). Lava tubes on Earth, Moon and Mars: A
review on their size and morphology revealed by comparative planetology. Earth-Science Reviews, 209, 103288. https://doi.org/10.1016/j.
earscirev.2020.103288
Schörghofer, N. (2021). Ice caves on Mars: Hoarfrost and microclimates. Icarus, 357, 114271. https://doi.org/10.1016/j.icarus.2020.114271
Schörghofer, N., Businger, S., & Leopold, M. (2018). The coldest places in Hawaii: The ice-preserving microclimates of high-altitude craters
and caves on tropical island volcanoes. Bulletin of the American Meteorological Society, 99(11), 2313–2324. https://doi.org/10.1175/
bams-d-17-0238.1
Schulze-Makuch, D., & Irwin, L. N. (2018). Life in the universe: Expectations and constraints (3rd ed.). Springer.
Šebela, S., & Turk, J. (2011). Local characteristics of Postojna Cave climate, air temperature, and pressure monitoring. Theoretical and Applied
Climatology, 105(3–4), 371–386. https://doi.org/10.1007/s00704-011-0397-9
Selensky, M. J., Masterson, A. L., Blank, J. G., Lee, S. C., & Osburn, M. R. (2021). Stable carbon isotope depletions in lipid biomarkers
suggest subsurface carbon fixation in lava caves. Journal of Geophysical Research: Biogeosciences, 126(7), e2021JG006430. https://doi.
org/10.1029/2021jg006430
Selim, E. I., Basheer, A. A., Elqady, G., & Hafez, M. A. (2014). Shallow seismic refraction, two-dimensional electrical resistivity imaging, and
ground penetrating radar for imaging the ancient monuments at the western shore of Old Luxor city, Egypt. Archaeological Discovery, 2(02),
31–43. https://doi.org/10.4236/ad.2014.22005
Shao, Q., & Wang, Y. G. (2009). Statistical power calculation and sample size determination for environmental studies with data below detection
limits. Water Resources Research, 45, 9. https://doi.org/10.1029/2008wr007563
Siefert, J. L., Souza, V., Eguiarte, L., & Olmedo-Alvarez, G. (2012). Microbial stowaways: Inimitable survivors or hopeless pioneers? Astrobiol-
ogy, 12(7), 710–715. https://doi.org/10.1089/ast.2012.0833
Simberloff, D., & Rejmánek, M. (2011). Encyclopedia of biological invasions (Vol. 3). University of California Press.
Stamenković, V., Beegle, L. W., Zacny, K., Arumugam, D. D., Baglioni, P., Barba, N., etal. (2019). The next frontier for planetary and human
exploration. Nature Astronomy, 3(2), 116–120. https://doi.org/10.1038/s41550-018-0676-9
Starr, S. O., & Muscatello, A. C. (2020). Mars in situ resource utilization: A review. Planetary and Space Science, 182, 104824. https://doi.
org/10.1016/j.pss.2019.104824
Summons, R. E., Amend, J. P., Bish, D., Buick, R., Cody, G. D., Des Marais, D. J., etal. (2011). Preservation of Martian organic and environ-
mental records: Final report of the Mars biosignature working group. Astrobiology, 11(2), 157–181. https://doi.org/10.1089/ast.2010.0506
Sutherland, W. J., Atkinson, P.W., Butchart, S. H., Capaja, M., Dicks, L. V., Fleishman, E., etal. (2021). A horizon scan of global biological
conservation issues for 2022. Trends in Ecology & Evolution, 37(1), 95–104. https://doi.org/10.1016/j.tree.2021.10.014
Sutherland, W. J., Bardsley, S., Bennun, L., Clout, M., Côté, I. M., Depledge, M. H., etal. (2011). A horizon scan of global conservation issues
for 2011. Trends in Ecology & Evolution, 26(1), 10–16. https://doi.org/10.1016/j.tree.2010.11.002
Sutherland, W. J., Fleishman, E., Mascia, M. B., Pretty, J., & Rudd, M. A. (2011). Methods for collaboratively identifying research priorities and
emerging issues in science and policy. Methods in Ecology and Evolution, 2(3), 238–247. https://doi.org/10.1111/j.2041-210x.2010.00083.x
Sutherland, W. J., Freckleton, R. P., Godfray, H. C. J., Beissinger, S. R., Benton, T., Cameron, D. D., etal. (2013). Identification of 100 funda-
mental ecological questions. Journal of Ecology, 101(1), 58–67. https://doi.org/10.1111/1365-2745.12025
Thakker, R., Alatur, N., Fan, D., Tordesillas, J., Paton, M., Otsu, K., etal. (2021). Autonomous off-road navigation over extreme terrains with
perceptually-challenging conditions. In Experimental Robotics: 17th International Symposium (pp.161–171).
Titus, T. N., Phillips-Lander, C. M., Boston, P.J., Wynne, J. J., & Kerber, L. (2020). Planetary cave exploration progresses. Eos Transactions
American Geophysical Union, 101. https://doi.org/10.1029/2020eo152045
Titus, T. N., Wynne, J. J., Boston, P.J., de Leon, P., Demirel-Floyd, C., Jones, H., etal. (2020). Science and technology requirements to explore
caves in our Solar System. Bulletin of the American Astronomical Society, 53(4), 167. https://doi.org/10.3847/25c2cfeb.a68ba8cb
Titus, T. N., Wynne, J. J., Malaska, M. J., Agha-Mohammadi, A., Buhler, P.B., Alexander, E. C., etal. (2021). A roadmap for planetary cave
science and exploration. Nature Astronomy, 5(6), 524–525. https://doi.org/10.1038/s41550-021-01385-1
Titus, T. N., Wynne, J. J., Ruby, D., & Cabrol, N. A. (2010). The Atacama Desert cave Shredder: A case for conduction thermodynamics. In 41st
Lunar and Planetary Science Conference, LPI Contributions No. 1096.
Tornos, F., Velasco, F., Menor-Salván, C., Delgado, A., Slack, J. F., & Escobar, J. M. (2014). Formation of recent Pb-Ag-Au mineralization by
potential sub-surface microbial activity. Nature Communications, 5(1), 4600. https://doi.org/10.1038/ncomms5600
Torrese, P., Pozzobon, R., Rossi, A. P., Unnithan, V., Sauro, F., Borrmann, D., etal. (2021). Detection, imaging and analysis of lava tubes for
planetary analogue studies using electric methods (ERT). Icarus, 357, 114244. https://doi.org/10.1016/j.icarus.2020.114244
Townsend, L. W., & Fry, R. J. M. (2002). Radiation protection guidance for activities in low-Earth orbit. Advances in Space Research, 30(4),
957–963. https://doi.org/10.1016/s0273-1177(02)00160-6
Trainer, M. G., Brinckerhoff, W. B., Freissinet, C., Lawrence, D. J., Peplowski, P.N., Parsons, A. M., etal. (2018). Dragonfly: Investigating the
surface composition of Titan. In Paper presented at 49th Lunar and Planetary Science Conference. LPI Contributions No. 2586.
Tuttle, M. D., & Stevenson, D. E. (1978). Variation in the cave environment and its biological implications. In R. Zuber, J. Chester, S. Gilbert, &
D. Rhodes (Eds.), 1977 National Cave Management Symposium Proceedings (pp.108–121). Adobe Press.
Uckert, K., Parness, A., Chanover, N., Eshelman, E. J., Abcouwer, N., Nash, J., etal. (2020). Investigating habitability with an integrated
rock-climbing robot and astrobiology instrument suite. Astrobiology, 20(12), 1427–1449. https://doi.org/10.1089/ast.2019.2177
Ulloa, A., Campos-Fernández, C. S., & Rojas, L. (2013). Minerals Cave, Irazú volcano, Costa Rica: Description, mineralogy and origin. Revista
Geologica de America Central, 48, 169–187.
Vaniman, D. T., Bish, D. L., Chipera, S. J., Fialips, C. I., Carey, J. W., & Feldman, W. C. (2004). Magnesium sulphate salts and the history of
water on Mars. Nature, 431(7009), 663–665. https://doi.org/10.1038/nature02973
Vaquero, T., Saboia da Silva, M., Otsu, K., Kaufmann, M., Edlund, J., & Agha-Mohammadi, A. (2020). Traversability-aware signal coverage
planning for communication node deployment in planetary cave exploration. In International Symposium on Artificial Intelligence, Robotics
and Automation in Space. LPI Contributions No. 5023.
von Ehrenfried, M. (2019). From Cave Man to Cave Martian: Living in Caves on the Earth, Moon and Mars. Springer.

Journal of Geophysical Research: Planets
WYNNE ETAL.
10.1029/2022JE007194
32 of 32
Voordouw, G. (2002). Carbon monoxide cycling by Desulfovibrio vulgaris Hildenborough. Journal of Bacteriology, 184(21), 5903–5911. https://
doi.org/10.1128/jb.184.21.5903-5911.2002
Wagner, R. V., & Robinson, M. S. (2014). Distribution, formation mechanisms, and significance of lunar pits. Icarus, 237, 52–60. https://doi.
org/10.1016/j.icarus.2014.04.002
Wagner, R. V., & Robinson, M. S. (2015). Update: The search for lunar pits. In Paper presented at 2nd International Planetary Caves Conference.
LPI Contributions 9021.
Wagner, R. V., & Robinson, M. S. (2021). Occurrence and origin of lunar pits: Observations from a new catalog. In Paper presented at 52nd
Lunar and Planetary Science Conference. LPI Contributions No. 2530.
Wagner, S. A. (2006). The Apollo experience lessons learned for constellation lunar dust management. TP-2006-213726 (p.73). NASA Johnson
Space Center.
Walsh, S. M., Strano, M. S., & Stanton, S. C. (2019). Approaching robotics and autonomous systems as an integrated materials, energy, and
control problem. In S. M. Walsh & M. S. Strano (Eds.), Robotic systems and autonomous platforms: Advances in materials and manufacturing
(pp.19–46). Elsevier.
Wang, J., & Wan, W. (2009). Factors influencing fermentative hydrogen production: A review. International Journal of Hydrogen Energy, 34(2),
799–811. https://doi.org/10.1016/j.ijhydene.2008.11.015
Webster, K. D. (2019). Developing cave air as a biosignature. Mars extant life: What’s next? Conference. LPI Contributions No. 5048.
Webster, K. D., Drobniak, A., Sauer, P.E., Mastalerz, M., & Schimmelmann, A. (2015). Cave air as a biosignature. In Paper presented at 2nd
International Planetary Caves Conference. LPI Contributions No. 9009.
Westall, F., Foucher, F., Bost, N., Bertrand, M., Loizeau, D., Vago, J. L., etal. (2015). Biosignatures on Mars: What, where, and how? Implica-
tions for the search for Martian life. Astrobiology, 15(11), 998–1029. https://doi.org/10.1089/ast.2015.1374
Westall, F., Loizeau, D., Foucher, F., Bost, N., Betrand, M., Vago, J., & Kminek, G. (2013). Habitability on Mars from a microbial point of view.
Astrobiology, 13(9), 887–897. https://doi.org/10.1089/ast.2013.1000
Whittaker, W. L., Jones, H. L., Ford, J. S., Sharif, K., & Wong, U. Y. (2021). Skylight: A mission concept for in-situ investigation of the morphol-
ogy, geology and mineralogy of lunar pits. In Lunar and Planetary Science Conference. LPI Contributions No. 2644.
Whittaker, W. L., Jones, H. L., Wong, U. Y., Ford, J. S., Whittaker, W. C., Khera, N., etal. (2020). Micro-rover exploration of lunar pits deployable
by commercial lander. In Paper presented at 3rd International Planetary Caves Conference. LPI Contributions No. 1074.
Wiens, R. C., Maurice, S., Robinson, S. H., Nelson, A. E., Cais, P., Bernardi, P., etal. (2021). The SuperCam instrument suite on the NASA Mars
2020 rover: Body unit and combined system tests. Space Science Reviews, 217, 1–87.
Wierzchos, J., Sancho, L. G., & Ascaso, C. (2005). Biomineralization of endolithic microbes in rocks from the McMurdo Dry Valleys of
Antarctica: Implications for microbial fossil formation and their detection. Environmental Microbiology, 7(4), 566–575. https://doi.
org/10.1111/j.1462-2920.2005.00725.x
Williams, K. E., & McKay, C. (2015). Comparing flow-through and static ice cave models for Shoshone Ice Cave. International Journal of
Speleology, 44(2), 115–123. https://doi.org/10.5038/1827-806x.44.2.2
Williams, K. E., McKay, C. P., Toon, O., & Head, J. W. (2010). Do ice caves exist on Mars? Icarus, 209(2), 358–368. https://doi.org/10.1016/j.
icarus.2010.03.039
Williams, K. E., Titus, T. N., Okubo, C. H., & Cushing, G. E. (2017). Martian cave air-movement via Helmholtz resonance. International Journal
of Speleology, 46(3), 439–444. https://doi.org/10.5038/1827-806x.46.3.2130
Wintle, B. C., Kennicutt, M. C., II, & Sutherland, W. J. (2020). Scanning horizons in research, policy and practice. In W. J. Sutherland, P.N. M.
Brotherton, Z. G. Davies, N. Ockenden, N. Pettorelli, & J. A. Vickery (Eds.), Making a difference in conservation: Linking science and policy
(pp.30–47). Cambridge University Press.
Wong, U., Morris, A., Lea, C., Lee, J., Whittaker, C., Garney, B., & Whittaker, R. (2011). Comparative evaluation of range sensing technologies
for underground void modeling. IEEE/RSJ International Conference on Intelligent Robots and Systems, 3816–3823. https://doi.org/10.1109/
iros.2011.6094938
Wray, R. A., & Sauro, F. (2017). An updated global review of solutional weathering processes and forms in quartz sandstones and quartzites.
Earth-Science Reviews, 171, 520–557. https://doi.org/10.1016/j.earscirev.2017.06.008
Wynne, J. J. (2022). Supplemental information for: Fundamental science and engineering questions in planetary cave exploration. Harvard
Dataverse. https://doi.org/10.7910/DVN/HJA6QI
Wynne, J. J., Ashley, J. W., Boston, P.J., Cushing, G. E., & Parness, A. (2014). Target selection and evaluation criteria for caves on the Moon and
Mars. In 2014 NASA/JPL Planetary Cave Workshop (p.2).
Wynne, J. J., Jenness, J., Sonderegger, D. L., Titus, T. N., Jhabvala, M. D., & Cabrol, N. A. (2021). Advancing cave detection using terrain analysis
techniques and thermal imagery. Remote Sensing, 13(18), 3578. https://doi.org/10.3390/rs13183578
Wynne, J. J., Mylroie, J. E., Titus, T. N., Malaska, M. J., Buczkowski, D. L., Buhler, P.B., etal. (2022). Planetary caves: A solar system view of
processes and products. Journal of Geophysical Research: Planets. https://doi.org/10.1029/2022JE007303
Wynne, J. J., Titus, T. N., & Chong Diaz, G. (2008). On developing thermal cave detection techniques for Earth, the Moon and Mars. Earth and
Planetary Science Letters, 272(1–2), 240–250. https://doi.org/10.1016/j.epsl.2008.04.037
Wyrick, D., Ferrill, D. A., Morris, A. P., Colton, S. L., & Sims, D. W. (2004). Distribution, morphology, and origins of Martian pit crater chains.
Journal of Geophysical Research, 109, E6. https://doi.org/10.1029/2004je002240
Yingst, R. A., Bray, S., Herkenhoff, K., Lemmon, M., Minitti, M. E., Schmidt, M. E., etal. (2020). Dust cover on Curiosity's Mars Hand Lens
Imager (MAHLI) calibration target: Implications for deposition and removal mechanisms. Icarus, 351, 113872. https://doi.org/10.1016/j.
icarus.2020.113872
Yoshida, K., Britton, N., & Walker, J. (2013). Development and field testing of moonraker: A four-wheel rover in minimal design. In Proceedings
of the 12th International Symposium on Artificial Intelligence, Robotics and Automation in Space. Retrieved from http://robotics.estec.esa.
int/i-SAIRAS/isairas2014/Data/Session%202a/ISAIRAS_FinalPaper_0114.pdf
Zupan-Hajna, N., Baioni, D., & Tramontana, M. (2017). Karst landforms within Noctis Labyrinthus, Mars. Acta Carsologica, 46, 73–82.