ArticlePDF Available

The scientific importance of caves in our solar system

Authors:
4 NSS NewS, March 2016
The Scientific Importance of Caves in Our Solar System: Highlights of the
2nd International Planetary Caves Conference, Flagstaff, Arizona
J. Judson Wynne
Department of Biological Sciences, Northern Arizona University and The SETI Institute, Carl Sagan Center
jut.wynne@nau.edu
FIGURE 1: Lunar Reconnaissance Orbiter
(LRO), Mini-RF image of Hadley Rille, which
was visited by Apollo 15 astronauts in 1971.
The rille is over 1000 m wide and about 300
m deep. Courtesy of NASA Goddard Space
Flight Center and Arizona State University.
FIGURE 2: Pit Craters from Mars and the moon. (Left) High-Resolution Imaging Science
Experiment (HiRISE) image of “Jeanne,” Arsia Mons, Mar. This pit crater is 150 meters wide and
at least 178 meters deep. Courtesy of NASA’s Jet Propulsion Laboratory/ California Institute of
Technology and University of Arizona. (Right) LRO image of Mare Tranquillitatis pit crater from
the, north is up. Courtesy NASA Goddard Space Flight Center and Arizona State University.
Background
Researchers and space enthusiasts have
speculated about the presence of caves on
the moon and beyond for over 50 years.
Peering at grainy coarse-resolution images
collected during the 1966-67 Lunar Orbiter
missions, scholars identified curious sinuous
rilles on the surface of the moon [1]. While
there was initial disagreement concerning
their genesis, consensus was largely reached.
These features were most likely ancient lava
tube remnants (Figure 1).
The possibility of caves on Mars came
much later. In 2003, planetary scientists with
the Southwest Research Institute and the
University of Texas, San Antonio hinted at
the possibility of speleogenesis on Mars while
discussing geological faulting and associated
processes [2].
Over the past 10 years, caves have
been discovered nearly everywhere in our
solar system. Many of the same geological
processes that sculpt and shape our planet
are indeed universal. The formation of caves
is no different. The more we look, the more
we find. Once a novel backdrop in science
fiction, the occurrence of extraterrestrial
caves is rapidly evolving into an impressive
body of knowledge.
Today, subterranean features have been
identified on at least seven desert and icy
planetary bodies orbiting our sun. In 2007,
seven spectacular vertical shafts, ranging
from 100 to 225 meters in diameter and
68 to 130 meters in depth, were described
on Mars [3]. I was thrilled to have been part
of this exciting work with colleagues from
the U.S. Geological Survey’s Astrogeology
Science Center. In 2012, a research team
using images from the Japanese Aerospace
Exploration Agency’s SELENE spacecraft
described a similar pit on the moon, which
was 65 meters diameter [4]. With improved
optics and the accelerated search for plan-
etary caves, scientists have now catalogued
more than 2,000 Martian [5] and over 200
lunar cave-like features ([6, 7]; Figure 2).
Vents and fissures associated with water ice
plumes have been found on Saturnian [8],
Jovian [9, 10], and Neptunian moons [11].
On one of Saturn’s moons, Enceladus, these
vents are actually connected to a global
ocean beneath the icy crust [12]. Among
the many breath-taking images from NASA’s
New Horizons’ mission to Pluto are photos
of the primary vents of two possible cryovol-
canoes [13]. These deep shafts may actually
spew ice when they erupt!
Why should scientists and engineers
be interested in caves on other planets and
moons? Beyond the obvious “cool factor” of
extraterrestrial caves, the answer is simple.
These features represent high-priority targets
for future robotic [14, 15] and human
missions [16, 17]. Martian and lunar caves
can provide pre-existing sheltered environ-
ments for the construction of astronaut
bases and/or storage facilities. Use of caves
as astronaut shelters makes sense from the
perspective of launching payloads from Earth
to space. Inflatable habitats are much lighter
than rigid pod designs. Lightweight habitats
could be compacted – providing allowances
for other equipment and provisions for long
duration space travel. Additionally, Martian
caves provide access to the deep subsurface
without the need of a heavy and costly drill-
ing payload. Caves along equatorial Mars
may contain significant water ice deposits
for human consumption and fuel [18].
Furthermore, planetary caves may provide
us with a rare glimpse into ancient geologi-
cal processes, where we can more easily
examine the successive layers of volcanic
activity and sedimentary strata [6, 7]. Without
caves, our ability to one day examine these
processes would be dramatically limited.
Perhaps most compelling is the search for life
beyond Earth; if life existed or exists on other
planetary bodies, we will find that evidence
in protected underground environments.
NSS NewS, March 2016 5
The conference
In October of 2015, nearly 40
astrophysicists, biologists, geochemists,
meteorologists, roboticists and engineers
convened in Flagstaff, Arizona, to further
advance our understanding of caves in our
solar system. Attendees discussed their
research and expressed ways to heighten
awareness of the importance of planetary
caves for future robotic and human missions.
This gathering brought scientists and engi-
neers together for several days providing
a rare opportunity to discuss how to best
improve robotic planetary cave exploration
capabilities.
The conference launched with a plenary
talk by Dr. Leroy Chiao, former NASA
shuttle astronaut and commander of the
International Space Station. He empha-
sized the need for humans to return to the
moon before journeying to Mars. Dr. Chiao
suggested the establishment of a lunar base
would enable humans to fully develop tech-
nologies for long-duration space missions,
while having the luxury of being close to
Earth. Cave exploration robotics and human
habitation technologies would benefit simi-
larly by using lunar caves as a test bed before
applying these technologies on other more
distant missions.
Conference planners included an educa-
tional outreach component consisting of a
primary school (grades 6–12) “space caves”
art contest and a presentation from a local
high school robotics team. Capitalizing on
the “A” in STEAM (Science, Technology,
Engineering, Art and Math), the art contest
was held several weeks before the conference
for Flagstaff area schools. Artwork included
alien cave life (Figure 3), speleonauts and
robots exploring caves, and stunning spac-
escapes viewed skyward through imaginative
planetary cave entrances. Contest winners
met astronaut Leroy Chiao, and their artwork
was displayed during the conference. Lowell
Observatory in Flagstaff hosted an exhibit
showcasing the contest winners’ artwork
from January through
March of 2016.
A team of “future
NASA engineers” from
the Coconino High
School Robotics Group,
Flagstaff was asked
to develop a robotic
concept for planetary
cave exploration. The
students chose Martian
caves for their mission.
Delivering their first
professional presen-
tation, these young
roboticists discussed a
hybrid robotic motility
consisting of traditional
wheeled and climb-
ing technologies ([19];
Figure 4). Perhaps one
day, this concept (or one
similar) may be used on a
surface-to-cave rover on the moon or Mars.
Researchers presented on topics rang-
ing from how caves form to how we might
find them using instruments from space-
craft platforms. In addition to cave (and
vent) formation processes associated with
volcanism and tidal forces, the potential
for dissolution caves on other planets
was discussed by NASA’s Jet Propulsion
Laboratory scientist and caver, Dr. Mike
Malaska. Radar imaging identified regions on
Titan indicative of dissolution (Figure 5) – an
indication that dissolution caves might also
be forming at these localities [20].
This could mean cave identification may
be done as part of a multi-stepped process.
Most NASA Flagship Missions typically
capture only low-resolution visible spectrum
or radar imagery. As part of this process,
we first identify areas on other planets
where geological processes conducive to
cave formation occur using Flagship data.
Once these regions have been identified, a
follow-up mission carrying higher resolution
instrumentation to capture speleological data
can be designed. This iterative approach for
identifying cave-bearing regions and finding
caves is likely to be a major area of focus as
we expand our data on the surface geology
of the outer planets and moons.
Conference participants also discussed
novel techniques for detecting planetary
caves. Colleagues and I presented on how to
apply terrain analysis techniques used on visi-
ble spectrum imagery to identify caves within
thermal imagery [22]. We also proposed a
roadmap for planetary cave detection using
remote sensing. For reliable detection of
caves, a combined-data approach incorporat-
ing visible spectrum, thermal-infrared, and
gravimetry data yields considerable promise.
Multiple images acquired from various view-
ing geometries and spectral wavelengths
will be essential to best model cave entrance
structure and genesis.
At present, the availability of multiple
thermal and visible spectrum images of
a given feature is limited, and research is
constrained by slightly off-nadir imaging.
Our interpretation of possible caves is
further hampered by imagery sporadically
captured from spacecraft platforms where
acquisition is dictated by fly-by or orbiter
mission schedules and tempered by other
mission objectives. For most of the lunar and
Martian caves, analysis and interpretation of
these features depends upon a small number
of images.
Sideways-looking imagery of a Martian
pit crater wall with adequate solar illumination
is required to determine if an over-hanging
pit rim is actually associated with an entrance
to a cave. While images captured slightly
off-nadir from High-Resolution Imaging
Science Experiment (HiRISE) onboard
the Mars Reconnaissance Orbiter have
FIGURE 4: SolidWorks® graphic of CRAWDAD
(CocoNuts Robotics All-terrain Walking and
Driving Articulating Device), courtesy Luke
Peterson and CocoNuts, Coconino High
School, Flagstaff.
FIGURE 3: “Guys, We Found Greg,” 12th grade winner of the Space
Caves Art Contest held in conjunction with the 2nd International
Planetary Caves Conference, Flagstaff, Arizona. Artwork of Antonia
Perkins (Photography courtesy of Melissa Dunstan).
6 NSS NewS, March 2016
FIGURE 6: Field testing the DuAXL rover in the Black Point Lava Flow, northern Arizona. Courtesy
of Dr. Issa Nesnas, NASA’s Jet Propulsion Laboratory/ California Institute of Technology.
been useful in examining candidate cave
entrances, the ability of the spacecraft to
tilt slightly from orbit is not enough to “see”
more than a few meters beyond the pit rim.
Using gravimetric techniques to search for
subsurface anomalies associated with candi-
date cave entrances may assist in solving
this problem. Using Gravity Recovery and
Interior Laboratory (GRAIL) mission data,
researchers from Purdue University recently
identified subsurface anomalies (presumed
void spaces) associated with lunar pits [23].
Their results suggest some pits on the moon
actually connect to large cave systems!
Engineers from NASA’s Jet Propulsion
Laboratory (JPL), Tohoku University, Japan
and Carnegie Mellon University reported
on advances in cave explorer robotic tech-
nologies and prototype development. Any
successful planetary cave robotic mission
should include: (1) 3D computer vision
analysis of the entrance passageway and
associated surface area to determine the
route for entering the cave; (2) a commu-
nications system to data link from a rover
deep within a cave to a surface rover (or
relay station); and, (3) a power supply (e.g.,
radioisotope thermoelectric generators) for
long duration underground operations.
Roboticists reported on prototype
rovers that partially addressed some of
these issues. Two of these prototypes (one
from NASA-JPL, the other from Tohoku
University) used the same approach to solve
some of these problems [15, 24]. The solu-
tion was an “umbilical cord” between the
surface and subsurface rovers to maintain
communications and power (Figure 6).
Astrobiologists discussed robotic payload
requirements for detecting evidence of life
in caves, techniques for detecting subsur-
face biosignatures, as well as Earth-analog
biosignatures of rock-eating microorganisms
[25, 26, 27]. A payload to search for in
situ life should include mass spectroscopy,
laser-induced breakdown spectroscopy,
energy dispersive spectroscopy, and a visible
spectrum camera. Cellular automaton algo-
rithms of visible spectrum imagery of the
cave interior show promise for identifying
biosignatures for Earth caves [26]; such an
approach may one day be useful in the search
for extraterrestrial cave life.
Dr. Penelope Boston, geomicrobi-
ologist and caver, proposed a noteworthy
distinction between Earth biospheres and
biospheres that may be found on desert and
icy planets [27]. Type 1 is a photosynthesis-
driven surface biosphere, while Type 2 is a
chemosynthesis-driven subsurface system.
Because Earth supports rich surface and
subsurface biotic communities, it represents
a Type 1 and Type 2 hybrid. Given inhos-
pitable surface conditions mediated by thin
atmospheres, if life exists on desert and icy
planetary bodies in our solar system, it would
most likely occur within a Type 2 biosphere.
onward To Mars!?
Discussions are flourishing over getting
humans to Mars. Both NASA and private
FIGURE 5: Comparison of Earth karst to Titan karst-like features. (a) Aerial photograph of polygonal karst
in Darai Hills, Papau New Guinea [21]. (b) Cassini (Solstice Mission) RADAR images of karst-like features of
Ecaz Labyrinthus, Titan. Courtesy of NASA’s Jet Propulsion Laboratory/ California Institute of Technology.
space companies are ramping up technolo-
gies to get us there. SpaceX and NASA are
developing and testing rocket systems with
the heavy lift capacity to ferry astronauts
to and from Mars. If all goes according
to schedule, SpaceX’s reusable FALCON
Heavy rocket will lift off sometime this
year [28]. The most powerful rocket (with
a 58 ton lift capacity) since Saturn V, the
FALCON Heavy rocket will carry into space
the same weight as a 747 aircraft loaded with
passengers, luggage, and fuel. Once NASA’s
Space Launch System (SLS) Block 3 rocket
is operational, which is anticipated sometime
in the 2020s, it will carry nearly three times
the payload of the FALCON Heavy rocket.
SLS Block 3 will have a 150-ton lift capacity
[29]! Perhaps the most amazing aerospace
advance, and a potential game-changer, is
the development of the Variable Specific
Impulse Magnetoplasma Rocket (VASIMR®)
by Ad Astra. This electric thruster engine is
poised to revolutionize space travel. Once
operational, this engine could reduce travel
time from Earth to Mars from about seven
months to just 39 days [30]! These advances
in rocket systems will lay the groundwork to
send humans to Mars and beyond.
Before we undertake a human
mission to Mars, we should estab-
lish a permanent presence on the
moon. The “moon first” approach
discussed by NASA astronaut
Leroy Chiao has significant merit.
We can fully develop our robotic
and human habitation pod technol-
ogies using a lunar cave laboratory.
Before this can happen, there must
be substantial investment in the
development of technologies for
the exploration and habitation of
extraterrestrial caves. Presently,
these technologies are dramati-
NSS NewS, March 2016 7
cally underfunded and in the earliest stages
of development.
Through proper investment and a lunar
laboratory setting, we can ultimately apply
these and related technologies to a success-
ful Mars mission. The recent discovery of
possible large cave systems associated with
lunar pits represents our first targets for
exploration on the moon and may one day
serve as such a laboratory.
With all of these efforts properly funded
and advancing in parallel, humanity will once
again break the chains of low-Earth orbit
space stations. We will reach ever higher
and venture further toward becoming a
multi-planet dwelling species. The accom-
plishments of the participants of the 2015
conference held in Flagstaff, Arizona will
help propel this campaign forward. These
engineers and scientists and those that follow
will lead humans to once again take residence
in caves – this time on other planets. Like
our ancestors who sought refuge in Earth
caves, we will return to being cavewomen
and cavemen.
Conference abstracts published by
attendees, may be accessed at www.hou.
usra.edu/meetings/2ndcaves2015/. To
peruse abstracts from the 1st International
Planetary Caves Workshop held in
2011, visit www.lpi.usra.edu/meetings/
caves2011/. For a video on “Colonizing the
Caves of Mars”, go to www.youtube.com/
watch?v=BgbNzqKYcnQ.
references:
[1] Oberbeck, V. R. et al. (1979) On the origin
of lunar sinuous rilles: Mod. Geol. 1. 75–80.
[2] Ferrill, D. A. et al. (2003) Influence of gravity
on the geometry of Martian normal faults: 34th
Lunar Planet. Sci. Conf., 2050.
[3] Cushing, G. E. et al. (2007) THEMIS observes
possible cave skylights on Mars: Geophys. Res.
Lett. 34, L17201.
[4] Haruyama, J. et al. (2009) Possible lunar lava
tube skylight observed by SELENE cameras:
Geophys. Res. Lett. 36.21, L21206.
[5] Cushing, G. E. & C. H. Okubo (2015) The
Mars Cave Database: LPI Contributions
1883, 9026.
[6] Wagner, R. V. & M. S. Robinson (2015)
Update: The Search for Lunar Pits: 2nd
International Planetary Caves Conference
(LPI Contributions 1883), 9021.
[7] Wagner, R. V. & M. S. Robinson (2014)
Distribution, formation mechanisms, and
significance of lunar pits: Icarus 237, 52–60.
[8] Hansen, C. J. et al. (2011) The composi-
tion and structure of the Enceladus plume:
Geophys. Res. Lett. 38, L11202.
[9] Geissler, P. E. & M. T. McMillan (2008) Galileo
observations of volcanic plumes on Io: Icarus
197, 505–518.
[10] Roth, L. et al. (2014) Transient water vapor at
Europa’s south pole: Science 343, 171–174.
[11] Duxbury, N. S. & R. H. Brown (1997) The
role of an internal heat source for the eruptive
plumes on Triton: Icarus 125, 83–93.
[12] Thomas, P. C. et al. (2016) Enceladus’s
measured physical libration requires a global
subsurface ocean: Icarus 264, 3747.
[13] NASA (2016) Possible ice volcano on Pluto
has “Wright Stuff”: http://www.nasa.gov/
feature/possible-ice-volcano-on-pluto-has-the-
wright-stuff/
[14] Parness, A. et al. (2012) Gravity-independent
mobility and drilling on natural rock using
microspines: 2012 IEEE Int. Conf. Robot.
Automat. (ICRA), 34373442.
[15] Kerber L. et al. (2015) Exploring pits and
caves with the Axel extreme terrain rover: LPI
Contributions 1883, 9022.
[16] Wynne, J. J. et al. (2008) On developing
thermal cave detection techniques for Earth,
the moon and Mars: Earth Planet. Sci. Lett.
272, 240–250.
[17] Boston, P. J. et al. (2003) Human utilization
of subsurface extraterrestrial environments:
Gravit. Space Biol. Bull. 16, 121–131.
[18] Williams, K. E. et al. (2010) Do ice caves exist
on Mars?: Icarus 209, 358–368.
[19] Peterson L. et al. (2015) Prototype for a
Caving Robot Drive Mechanism to Explore
Mars: LPI Contributions 1883, 9038.
[20] Malaska M. J. et al. (2015) Titan Karst: LPI
Contributions 1883, 9024.
[21] Williams, P. W. (1972) Morphometric analysis
of polygonal karst with examples from New
Guinea: Geol. Soc. Am. Bull. 83, 761–796.
[22] Wynne, J. J. et al. (2015) Detecting terrestrial
caves by applying topographic analysis tech-
niques to thermal imagery: LPI Contributions
1883, 9029.
[23] Sood R. et al. (2015) Detection of Buried
Empty Lunar Lava Tubes Using GRAIL Gravity
Data: LPI Contributions 1883, 9016.
[24] Yoshida, K. et al. (2015) Moonraker and
Tetris: Japanese Microrovers for Lunar Cave
Exploration: LPI Contributions 1883, 9026.
[25] Boston P. J. et al. (2015) Unique Biosignatures
in Caves of All Lithologies: LPI Contributions
1883, 9028.
[26] Uckert K. et al. (2015) The Characterization
of Biosignatures in Caves Using a Suite of
Instruments: LPI Contributions 1883, 9023.
[27] Boston P. J. et al. (2015) Hypothetical Inverted
Critical Zones for Subsurface Biospheres on
Desert Planets and Icy Ocean Worlds: LPI
Contributions 1883, 9039.
[28] SpaceX (2015) FALCON Heavy: http://
www.spacex.com/falcon-heavy
[29] NASA (2015) Space Launch System Fact
Sheet (PDF): https://www.nasa.gov/sites/
default/files/files/SLS-Fact-Sheet_aug2014-
finalv3.pdf.
[30] Ad Astra. VASIMR: http://www.adas-
trarocket.com/aarc/VASIMR
... From a planetary perspective, detecting caves elsewhere in the solar system will factor prominently into how caves are targeted for future robotic exploration in the search for life [27][28][29][30][31]. Advanced detection capabilities will also enable us to prioritize caves for human habitation on the Moon and Mars [29,[32][33][34], and searching for evidence of life on Mars [29,34]. ...
... From a planetary perspective, detecting caves elsewhere in the solar system will factor prominently into how caves are targeted for future robotic exploration in the search for life [27][28][29][30][31]. Advanced detection capabilities will also enable us to prioritize caves for human habitation on the Moon and Mars [29,[32][33][34], and searching for evidence of life on Mars [29,34]. To date, over 1000 Martian [35,36] and more than 200 lunar cave-like features [37-40] have been confirmed. ...
Article
Full-text available
Since the initial experiments nearly 50 years ago, techniques for detecting caves using airborne and spacecraft acquired thermal imagery have improved markedly. These advances are largely due to a combination of higher instrument sensitivity, modern computing systems, and processor intensive analytical techniques. Through applying these advancements, our goals were to: (1) Determine the efficacy of methods designed for terrain analysis and applied to thermal imagery; (2) evaluate the usefulness of predawn and midday imagery for detecting caves; and (3) ascertain which imagery type (predawn, midday, or the difference between those two times) was most informative. Using forward stepwise logistic (FSL) and Least Absolute Shrinkage and Selection Operator (LASSO) regression analyses for model selection, and a thermal imagery dataset acquired from the Mojave Desert, California, we examined the efficacy of three well-known terrain descriptors (i.e., slope, topographic position index (TPI), and curvature) on thermal imagery for cave detection. We also included the actual, untransformed thermal DN values (hereafter "unenhanced thermal") as a fourth dataset. Thereafter, we compared the thermal signatures of known cave entrances to all non-cave surface locations. We determined these terrain-based analytical methods, which described the "shape" of the thermal landscape hold significant promise for cave detection. All imagery types produced similar results. Down-selected covariates per imagery type, based upon the FSL models, were: Predawn-slope, TPI, curvature at 0 m from cave entrance, as well as slope at 1 m from cave entrance; midday-slope, TPI, and unenhanced thermal at 0 m from cave entrance; and difference-TPI and slope at 0 m from cave entrance, as well as unenhanced thermal and TPI at 3.5 m from cave entrance. Finally, we provide recommendations for future research directions in terrestrial and planetary cave detection using thermal imagery.
Article
Full-text available
The search for life and habitable environments on other Solar System bodies is a major motivator for planetary exploration. Due to the difficulty and significance of detecting extant or extinct extraterrestrial life in situ, several independent measurements from multiple instrument techniques will bolster the community's confidence in making any such claim. We demonstrate the detection of subsurface biosignatures using a suite of instrument techniques including IR reflectance spectroscopy, laser-induced breakdown spectroscopy, and scanning electron microscopy/energy dispersive X-ray spectroscopy. We focus our measurements on subterranean calcium carbonate field samples, whose biosignatures are analogous to those that might be expected on some high-interest astrobiology targets. In this work, we discuss the feasibility and advantages of using each of the aforementioned instrument techniques for the in situ search for biosignatures and present results on the autonomous characterization of biosignatures using multivariate statistical analysis techniques. Key Words: Biosignature suites-Caves-Mars-Life detection. Astrobiology 17, 1203-1218.
Conference Paper
Full-text available
Introduction: Since Rinker’s [1] groundbreaking work on terrestrial cave detection nearly 50 years ago, our ability to find caves using airborne and spacecraft acquired imagery has improved considerably. Due to superior analytical techniques, improved instrument optics, and high resolution imagery, we have furthered terrestrial cave detection capabilities [2,3,4] and confirmed cave-like features on the Moon [5,6,7] and Mars [8,9,10]. Lunar caves may serve as the best locations for human habitation [2,4,6], while Martian caves are of great interest as astrobiological targets, accessing potential water-ice reserves, as well as astronaut bases [2,4]. Further, geothermal vents associated with vapor plumes identified on Saturnian [11], Jovian [12,13] and Neptunian moons [14] hint at additional planetary subterranean access points, and represent high priority targets for future habitability studies. Cave detection is typically most successful when multiple thermal images are acquired during both the warmest (mid-afternoon) and coolest (predawn) times of day [3]. Although data acquisition is logistically easiest on Earth, repeat thermal imagery over short temporal periods is lacking for most terrestrial locations. When searching for caves on other planetary bodies, obtaining multiple images for regions of interest within a limited window of time is challenging. Accordingly, researchers must rely on imagery sporadically captured from spacecraft platforms where acquisition is dictated by fly-by or orbiter mission schedules and tempered by other mission objectives. For extraterrestrial cave detection, we explored two targeting categories using terrestrial analogs. (1) Deep caves, which have sufficient linear length and/or depth to adequately buffer interior environments from harsh surface conditions. On Mars, these caves would be among the best candidates to search for evidence of life. (2) Shallow caves, which extend tens of meters in length, may represent suitable sites for establishing astronaut bases on the Moon and Mars. Within these less protected, but more easily accessable sites, habitat pods may be constructed or inserted a small distance within the cave entrance.
Article
Full-text available
In November and December 2012 the Hubble Space Telescope (HST) imaged Europa's ultraviolet emissions in the search for vapor plume activity. We report statistically significant coincident surpluses of hydrogen Lyman-α and oxygen OI130.4 nm emissions above the southern hemisphere in December 2012. These emissions are persistently found in the same area over ~7 hours, suggesting atmospheric inhomogeneity; they are consistent with two 200-km-high plumes of water vapor with line-of-sight column densities of about 10(20) m(-2). Nondetection in November and in previous HST images from 1999 suggests varying plume activity that might depend on changing surface stresses based on Europa's orbital phases. The plume was present when Europa was near apocenter and not detected close to its pericenter, in agreement with tidal modeling predictions.
Article
Full-text available
The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed an occultation of the Sun by the water vapor plume at the south polar region of Saturn's moon Enceladus. The Extreme Ultraviolet (EUV) spectrum is dominated by the spectral signature of H2O gas, with a nominal line-of-sight column density of 0.90 0.23 × 1016 cm-2 (upper limit of 1.0 × 1016 cm-2). The upper limit for N2 is 5 × 1013 cm-2, or <0.5% in the plume; the lack of N 2 has significant implications for models of the geochemistry in Enceladus' interior. The inferred rate of water vapor injection into Saturn's magnetosphere is ∼200 kg/s. The calculated values of H2O flux from three occultations observed by UVIS have a standard deviation of 30 kg/s (15%), providing no evidence for substantial short-term variability. Collimated gas jets are detected in the plume with Mach numbers of 5-8, implying vertical gas velocities that exceed 1000 m/sec. Observations at higher altitudes with the Cassini Ion Neutral Mass Spectrometer indicate correlated structure in the plume. Our results support the subsurface liquid model, with gas escaping and being accelerated through nozzle-like channels to the surface, and are consistent with recent particle composition results from the Cassini Cosmic Dust Analyzer.
Article
Several planetary satellites apparently have subsurface seas that are of great interest for, among other reasons, their possible habitability. The geologically diverse Saturnian satellite Enceladus vigorously vents liquid water and vapor from fractures within a south polar depression and thus must have a liquid reservoir or active melting. However, the extent and location of any subsurface liquid region is not directly observable. We use measurements of control points across the surface of Enceladus accumulated over seven years of spacecraft observations to determine the satellite's precise rotation state, finding a forced physical libration of 0.120 $\pm$ 0.014{\deg} (2{\sigma}). This value is too large to be consistent with Enceladus's core being rigidly connected to its surface, and thus implies the presence of a global ocean rather than a localized polar sea. The maintenance of a global ocean within Enceladus is problematic according to many thermal models and so may constrain satellite properties or require a surprisingly dissipative Saturn.
Article
Lunar Reconnaissance Orbiter Camera images reveal the presence of steep-walled pits in mare basalt (n = 8), impact melt deposits (n = 221), and highland terrain (n = 2). Pits represent evidence of subsurface voids of unknown extents. By analogy with terrestrial counterparts, the voids associated with mare pits may extend for hundreds of meters to kilometers in length, thereby providing extensive potential habitats and access to subsurface geology. Because of their small sizes relative to the local equilibrium crater diameters, the mare pits are likely to be post-flow features rather than volcanic skylights. The impact melt pits are indirect evidence both of extensive subsurface movement of impact melt and of exploitable sublunarean voids. Due to the small sizes of pits (mare, highland, and impact melt) and the absolute ages of their host materials, it is likely that most pits formed as secondary features.
Article
To grip rocks on the surfaces of asteroids and comets, and to grip the cliff faces and lava tubes of Mars, a 250 mm diameter omni-directional anchor is presented that utilizes a hierarchical array of claws with suspension flexures, called microspines, to create fast, strong attachment. Prototypes have been demonstrated on vesicular basalt and a'a lava rock supporting forces in all directions away from the rock. Each anchor can support >160 N tangent, >150 N at 45°, and >180 N normal to the surface of the rock. A two-actuator selectively-compliant ankle interfaces these anchors to the Lemur IIB robot for climbing trials. A rotary percussive drill was also integrated into the anchor, demonstrating self-contained rock coring regardless of gravitational orientation. As a harder-than-zero-g proof of concept, 20mm diameter boreholes were drilled 83 mm deep in vesicular basalt samples, retaining a 12 mm diameter rock core in 3–6 pieces while in an inverted configuration, literally drilling into the ceiling.
Article
Consideration of the effects of gravity on lithostatic stress on Mars indicates that dilational faulting found in the upper 2 km on Earth may extend to depths of 5 km on Mars.