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Planetary caves as astrobiology targets

Authors:

Abstract and Figures

Humans have looked for extraterrestrial biosignatures on the surfaces of other planets and moons. These surfaces are often exposed to conditions and processes that exceed the physical limits of life, e.g., intense cosmic radiation, impact events, and large thermal extremes, that would render difficult the preservation of biosignatures over geologic time. Planetary caves provide protection from cosmic radiation, small-scale impact events, and have relatively stable thermal environments. These characteristics may well permit preservation of biosignatures over long periods of time and make them a prospective astrobiology target for biosignatures beyond Earth (Boston et al., 2001; Léveillé & Datta, 2010; Martins et al., 2017). A cave with natural openings offers direct access to the subsurface without drilling and deeper penetration into subsurface materials than could be obtained from a rover, landed platform, or penetrator launched from orbit. However, current technological and mechanical limitations associated with ingress and navigation make their exploration challenging. Caves form through a number of processes, but those on the moon and Mars identified using satellite data are lava caves. On Earth, lava caves are associated with basaltic lava, a material predicted to be ubiquitous on all rocky planets. On the moon and Mars, hundreds of vertical collapse pits have been identified using a number of remote sensing approaches (Greeley, 1971; Cushing et al., 2007; Haruyama et al., 2009; 2017); many of these may be skylights providing direct access to intact caves that should be substantially larger than those found on Earth due to the combination of lower gravity and higher eruption rates on these smaller planetary bodies (e.g., Blair et al., 2017). Future planetary astrobiology missions would be well-served to include lava caves as a high-priority target for investigation. The purpose of this white paper is to urge support for development of technology needed to enter a planetary cave with a scientific payload for deployment. In the next pages, we review the main challenges associated with: 1) identification of planetary lava caves, 2) subsurface exploration vehicles with advanced subsurface communications/operations techniques, and 3) sensor systems developed for biosignature identification. NASA's SMD currently supports several Earth-based planetary cave analog investigations through its PSTAR (Planetary Science and Technology through Astrobiology Research) program; a table summary of these efforts is also included. We conclude with mention of current and ongoing technology developments both internal and external to NASA that could advance planetary cave identification, access, and exploration.
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Planetary!Caves!as!Astrobiology!Targets!
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A"White"Paper"Submitted"to"the"Space"Studies"Board"of"the"National"Academy"of"Sciences"
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January"8,"2018"
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Jennifer!G.!Blank!
Planetary!Systems!Branch,!Division!of!Space!Sciences!&!Astrobiology!|!Blue!Marble!Space!Institute!of!Science!
NASA!Ames!Research!Center,!MS!245-3,!Moffett!Field!CA!94035!
Tel.!650-604-5349;!415-902-4989!|!Jennifer.g.blank@nasa.gov;!jen@bmsis.org!
!
Ted!L.!Roush,!Carol!L.!Stoker,!Anthony!Colaprete!
Planetary!Systems!Branch,!NASA!Ames!Research!Center!!
!
Saugata!Datta!
Department!of!Geology,!Kansas!State!University!
!
Uland!Wong,!Matthew!Deans!
Intelligent!Robotics!Group,!NASA!Ames!Research!Center!!
!
Terry!Fong!
Space!Technology!Mission!Directorate,!NASA!
!
Richard!Léveillé!
Department!of!Geoscience,!John!Abbot!College!|!Department!of!Earth!&!Planetary!Sciences,!McGill!University!!
!
Diana!E.!Northup!
Department!of!Biology,!University!of!New!Mexico!
!
Aaron!Parness!
Extreme!Environment!Robots!Group,!NASA!Jet!Propulsion!Laboratory!
!
J.R.!Skok!
Carl!Sagan!Center!for!the!Study!of!Life!in!the!Universe,!SETI!Institute!
!
Red!Whittaker,!Nathan!Michael!
The!Robotics!Institute,!Carnegie!Mellon!University!
!
J.!Judson!Wynne!
Department!of!Biological!Sciences,!Northern!Arizona!University!
!
!
!
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/"Planetary"Caves"as"Astrobiology"Targets" " "
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MOTIVATION!
Humans!have!looked!for!extraterrestrial!biosignatures!on!the!surfaces!of!other!planets!and!
moons.!These!surfaces!are!often!exposed!to!conditions!and!processes!that!exceed!the!physical!
limits! of! life,! e.g.,! intense! cosmic! radiation,! impact! events,! and! large! thermal! extremes,! that!
would!render!difficult!the!preservation!of!biosignatures!over!geologic!time.!!
Planetary! caves! provide! protection! from! cosmic! radiation,! small-scale! impact! events,! and!
have!relatively!stable!thermal!environments.!These!characteristics!may!well!permit!preservation!
of!biosignatures!over!long!periods!of!time!and!make!them!a!prospective!astrobiology!target!for!
biosignatures!beyond!Earth!(Boston!et!al.,!2001;!Léveillé!&!Datta,!2010;!Martins!et!al.,!2017).!A!
cave!with! natural!openings! offers!direct! access!to! the!subsurface! without! drilling! and! deeper!
penetration!into!subsurface!materials!than!could!be!obtained!from!a!rover,!landed!platform,!or!
penetrator! launched! from! orbit.! However,! current! technological! and! mechanical! limitations!
associated!with!ingress!and!navigation!make!their!exploration!challenging.!
Caves!form!through!a!number!of!processes,!but!those!on!the!moon!and!Mars!identified!using!
satellite!data! are! lava!caves.!On! Earth,!lava!caves! are! associated!with!basaltic! lava,!a!material!
predicted!to! be!ubiquitous! on!all! rocky!planets.! On!the! moon! and! Mars,! hundreds!of! vertical!
collapse!pits!have!been!identified!using!a!number!of!remote!sensing!approaches!(Greeley,!1971;!
Cushing!et! al.,!2007;! Haruyama!et! al.,! 2009;!2017);! many!of! these!may! be! skylights! providing!
direct!access!to!intact!caves!that!should!be!substantially!larger!than!those!found!on!Earth!due!to!
the!combination!of! lower!gravity!and! higher! eruption!rates!on! these! smaller!planetary!bodies!
(e.g.,!Blair!et!al.,!2017).!Future!planetary!astrobiology!missions!would!be!well-served!to!include!
lava!caves!as!a!high-priority!target!for!investigation.!
The!purpose!of!this!white!paper!is!to!urge!support!for!development!of!technology!needed!to!
enter!a!planetary!cave!with!a!scientific!payload!for!deployment.!In!the!next!pages,!we!review!the!
main! challenges! associated! with:! 1)! identification! of! planetary! lava! caves,! 2)! subsurface!
exploration!vehicles!with!advanced!subsurface! communications/operations! techniques,!and! 3)!
sensor!systems!developed!for!biosignature!identification.!NASA’s!SMD!currently!supports!several!
Earth-based! planetary! cave! analog! investigations! through! its! PSTAR! (Planetary! Science! and!
Technology!through! Astrobiology! Research)!program;!a! table!summary!of! these! efforts!is!also!
included.! We! conclude! with! mention! of! current! and! on-going! technology! developments! both!
internal! and! external! to! NASA! that! could! advance! planetary! cave! identification,! access,! and!
exploration.!
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1. IDENTIFICATION!OF!LAVA!CAVES!
Lava!caves!are!generated!from!basaltic!eruptions,!when!lava!discharging!from!a!volcanic!vent!
or!fissure! forms! conduits!that!isolate!the!molten!flow!thermally!from!the!surface!and!delay!its!
cooling! as! the! streaming! material! moves! down! slope.! Ultimately,! the! lava! drains! from! the!
conduit,!leaving!behind!a!hollow!tube.! Lava! caves!form! at! the!surface! initially,! extending!from!
flow!levees!along!a!principal!flow!channel,!and!then!evolve!into!near-surface!features!as!a!roof!is!
generated;!fully!formed!lava!caves!are! typically! less! than!few! meters! below!the! surface! of!the!
lava! flow.! Continued! volcanism! can! lead! to! erosion! or! burial! of! older! caves! beneath! younger!
eruptions,!and!lava! caves! and! their! remnants! can!exist!at!substantial!(>!1!km)!depths!on!Earth!
though!the!majority!extend!relatively!shallow!(<!100!m)!depths,!only.!These!caves!are!most!often!
found!through!the!presence!of!skylights,!where!a!portion!of!the!lava!cave!roof!has!collapsed!and!
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/"Planetary"Caves"as"Astrobiology"Targets" " "
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exposed!the!lava!cave!to!the!surface;!skylights!provide!surface!access!to!lava!caves!and!help!mark!
their!presence.!
Near-surface!lava!caves!can!be!detected!using!a!variety!of!techniques,!including:!
Visual!detection!of!exposed!skylights!using!high-resolution!surface!imaging!methods!!
Detection! of! topographic! irregularities! to! identify! possible! lava! cave!openings! from! surface!
brightness!(albedo)!profiles!!
Thermal!measurements!and! analysis!to! detect!temperature! contrasts!between! cave!openings!
and!the!surrounding!surface!rocks!
Ground-penetrating!radar!measurements!to!detect!the!presence!of!shallow!cavities!!
High-precision!gravity!methods!to!detect!the!presence!of!shallow!lava!caves!!
Table!1!lists!examples!of!recent!projects!that!focus!on!lava!cave!identification!from!satellite!orbit!
or!aircraft!using!these!remote-sensing!techniques.!
!
Table!1:!Lava!cave!detection!!
Technique!
Project/Mission!
Key!Observations!
References!
High-res!
Lunar!Reconnaissance!
Orbiter!!
Subsurface!voids!discovered!with!oblique!imaging!
may!be!extensive!lava!cave!systems,!or!represent!
collapses!created!as!magma!drained!
Allen!(2009);!
Robinson!et!al.!
(2012)!
SELENE!
Discovered!65!m!diameter,!80-90m!vertical!hole!in!a!
lava!cave!with!a!370!m!
Haruyama!et!al.!
(2009)!
Surface!
Laboratory!experiments!and!
modeling!
Models!to!evaluate!brightness!profiles!of!pit!craters!!
Jung!et!al.!
(2016)!
Thermal!IR!
Atacama!Desert,!Chile;!
Mohave!Desert,!USA!
Contrast!between!surface!temperatures!and!near-
constant!cave!interior!temperatures!
Wynne!et!al.!
(2008)!
Mars!Odyssey!Thermal!
Emission!Imaging!System!
7!candidate!cave!skylights!located!with!diam.!100-
225!m!and!predicted!minimum!depths!68-130!m!
Cushing!et!al.!
(2007)!
Mars!Odyssey!Thermal!
Emission!Imaging!System!
Analysis!of!T!changes!of!day/night!for!7!cave!
candidates!
Jung!et!al.!
(2014)!
Thermal!inertia!
LRO!Diviner!Lunar!
Radiometer!
Thermal!inertia!and!Tmax/Tmin!ratio!maps!and!
imaging!were!used!to!identify!4!lunar!sites!associated!
with!skylights!!
Slank!(2016)!
Ground-
SELENE!Lunar!Radar!Sounder!!
Distinctive!echo!patterns!evidence!for!the!existence!
of!a!lava!cave!–!and!correlated!with!gravity!mass!
deficiencies!detected!by!GRAIL!
Kaku!et!al.!
(2017);!Sood!et!
al.!(2016)!
High!precision!
GRAIL!
Relatively!large!linear!features!detected!in!the!
vicinity!of!known!skylights!
Chappaz!et!al.!
(2017)!
!
2. PHYSICAL!EXPLORATION!OF!LAVA!CAVES!
A! variety! of! robotic! approaches! have!been! proposed! to! overcome! key! obstacles! related! to!
entering!and!navigating!inside!caves,!including:!
Entry!from!the!surface!down!into!the!lava!cave!system!through!a!skylight!–!these!entrances!often!
include!a!large!vertical!drop!(>!50!m).!
Traversing!an!irregular!floor!surface!and/or!over!large!blocky!obstacles.!
Operations!in!darkness.!
Autonomous!operation!and!localization!(out!of!line-of-sight!to!surface!communications).!!
In! addition! to! traditional! wheeled! rovers,! robotic! vehicles! using! biomimicry! offer! alternative!
locomotion!in!challenging!subsurface!terrain.! Prototypes! include!fleets! of! coordinating!robotic!
ants,! butterflies,! dragonflies,! and! spiders! optimized! for! relay! communications! away! from! a!
control!center.!Recent! cave!robotic!innovations! are! centered!around!a! few! themes,!sorted!by!
modality!in!Table!2.!Robotics!technologies!that!link!perception,!navigation,!mapping!and!decision!
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/"Planetary"Caves"as"Astrobiology"Targets" " "
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making! have! made! great! advances.! This! means! that! previously! impractical! approaches! for!
exploring!cave!environments!are!near-future!possibilities.!
!
Table!2:!Robotic!approaches!to!subsurface!exploration!of!lava!caves!
Robotic!
Modality!
Description!
Examples!
Micro-
swarms!
Simple,!mass-produced,!redundant,!teams.!Dropped,!shot,!or!hop!into!cave!entrances.!
Redundancy!provides!parallel!exploration!and!risk!reduction.!Each!robot!in!swarm!has!low!
science!capability.!!!!
Pit-bots!(Thangavelautham!et!
al.,!2014),!SphereX!(Kalita!et!al.,!
(2017)!
Rovers!
Drive!over!blocky!collapse!ramps!to!access!caves.!Technology!includes!inflatable!wheels,!
tandem!tethered!teams,!and!pneumatic!hopping.!Rovers!are!usually!limited!to!the!ground!
plane!but!provide!superior!payload!ratio!and!simple!concept!of!operations.!!
Sandflea!(Boston!Dynamics)!
UZUME!(Furutani,!2016),!
Hakuto!(Walker!et!al.,!2015)!
Cable-
deployed!
Use!tethers!and!cables!to!enter!through!vertical!skylights.!Descend!via!rappelling!or!a!
Tyrolean!deployment.!Surface!tether!infrastructure!to!surface!introduces!complexity!but!
can!provide!power!and!communications!for!robots!inside!cave.!Possibility!to!lower!large!
payloads!to!skylight!floors.!
Axel!(Nesnas!et!al.,!2008)!
Cliffbot!(Paulsen!et!al.,!2005)!
Tyrobot!(Wong!et!al.,!2015)!
Climbers!
Negotiate!blocky!hazards!and!climb!cave!walls!and!ceilings!with!gripping!limbs.!Enables!
long-duration!access!to!interesting!features!on!the!ceiling,!but!moderate!payload!and!
risky!because!no!inherent!stability.!!
Lemur!3!(Parness!et!al.,!2017)!
Geckobots!(Sitti!et!al.,!2003)!
Flyers!
Fly!into!caves!through!openings.!Propulsive!or!rotorcraft.!Drones!avoid!all!obstacles!and!
have!good!vantage!points!at!the!center!of!voids!for!scientific!observation.!Range-limited!
or!inefficient!in!thin!planetary!atmospheres.!Limited!payload!capacity!
CMU!has!NASA!STMD!funded!
cave!UAV!exploration!project,!
Mars!Helicopter!will!fly!on!2020!
but!no!plans!to!explore!a!cave.!
!
3. SENSOR!SYSTEMS!DEVELOPED!FOR!CAVE!BIOSIGNATURE!IDENTIFICATION!
Boston!and!others!(2001)!present!a! comprehensive! inventory! of!biological! techniques! that!
are! commonly! used! to! assess! biosignatures! associated! with! microbial! communities! in! caves,!
many!of!which!are!not!currently!feasible!for!a!remote,!robotic!planetary!mission.!A!recent!science!
instrument! suite! for! planetary! cave! exploration! has! been! proposed! by! Uckert! et! al! (2017)!
includes:!
Infrared! reflectance! spectroscopy! (wavelength! range! includes! important! mineralogical! and!
biogeochemical!absorption!features)!
Laser-induced!breakdown!spectroscopy!(provides!elemental!composition!of!target)!
Scanning!electron!microscopy!and!energy!dispersive!X-ray!spectroscopy!!
In!addition!to!these!sensors,!the!robotic!system!could!also!employ!
High-resolution!imaging!(to!record!rock!surfaces!and!assist!in!navigation)!
Environmental!sensors!(temperature,!atmospheric!pressure,!etc.)!
Gas!sensors!(detection!of!key!species,!such!as!methane,!water,!etc.)!
Spectral! imaging! sensors,! including! imaging! in! the! deep! UV! to! detect! organic! and! microbial!
features!(bioluminescence)!and!distinguish!these!from!mineral!coatings!on!rock!surfaces!
Most!of!these!techniques!have!been!or!will!be!deployed!as!instrument!payload!on!planetary!
missions!and!could!be!adapted!for!use!in!a!subsurface!environment.!
!
4. TERRESTRIAL!NATURAL!ANALOG!STUDIES!OF!PLANETARY!CAVES!
There!is!increasing!interest!in!the!microbiome!of!lava!caves!on!Earth;!studies!to!date!have!focused!
on!microbial!colony!morphology!and!association!with!secondary!mineralogy!(Lavoie!et!al.,!2010),!
16S! rDNA!of! bacteria! (Lavoie! et! al.,! 2017;! Kommedal,! 2017),! and! identifying! potential!
energy/chemical! nutrient! pathways! to! support! microbial! life.! Table! 3! summarizes! recent! and!
current!NASA-supported! lava! cave! astrobiology! studies! that! could! inform! future! planetary!
investigations.!
Other! space! agencies! are! preparing! for! future! planetary! cave! astrobiology! missions!
through!studies!in!terrestrial!lava! caves.! The! Canadian! Space! Agency’s! Astrobiology! Training! in!
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/"Planetary"Caves"as"Astrobiology"Targets" " "
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Lava! Tubes! (ATiLT)! project! (PI! R.! Léveillé)! is! educating! the! next! generation! of! astrobiology!
scientists! through! exercises! coupling! stand-off! life! detection! instruments! (LIBS,! IR)! with!
laboratory! analyses! of! field! microbe-mineral-ice! samples! collected! at! Lava! Beds! National!
Monument.!The!European!Space!Agency’s!PANGAEA!training!program!is!preparing!astronauts!to!
become!effective!partners!of!astrobiology!scientists!and!mission!engineers!through!exercises!in!
analog! settings,! including! lava! caves! (Loredana! Bessone,! pers." comm.,! PANGAEA! program!
director).!!!
!
Table!3:!Selected!NASA-funded!terrestrial!analog!studies!of!lava!caves!
Location!
Description!
References!
El!Mapais!Nat’l!Monument!!
NM,!USA!
Death!Valley!Nat’l!Park!
CA!USA!
Deployed!NIR!and!XRF!spectroscopies,!XRF,!and!Deep-UV!Raman!
instruments!on!rock-climbing!robot.!Use!of!pattern!recognition!AI!
to!discriminate!macroscopic!microbial!patterns!on!lava!cave!walls.!
NASA!PSTAR!Free!Climber!project!
(PI!A.!Parnass)!
Lava!Beds!National!
Monument!
CA,!USA!
Surface!GPR!(ground-penetrating-radar)!to!detect!known!lava!
caves!(simulating!possible!activities!of!the!Mars2020!payload!
instrument!RIMFAX!(Radar!Images!for!Mars!Subsurface!
Exploration)),!compare!with!Lidar-mapped!caves.!Use!of!hand-held!
instruments!to!study!alteration!mineralogy!in!cave!interiors.!
NASA!PSTAR!TubeX!project!
(PI!K.!Young)!
Esmaeili!et!al.!(2017);!Whelley!et!al.!
(2017)!
Cave!astrobiology!using!rover-borne!spectral!imaging!and!
spectrometers.!Simulates!astrobiology!science!mission!operational!
activities!with!remote!team!directing!rover!and!interpreting!data.!
NASA!PSTAR!BRAILLE!project!
(PI!J.!Blank)!
Craters!of!the!Moon!
National!Monument!
ID,!USA!
Lidar!survey!of!the!lava!cave!as!an!analog!to!the!exploration!of!pits!
on!the!Moon!and!Mars.!
NASA!SSERVI!FINESSE!project!!
(PI!J.!Heldmann);!Garry!et!al.!(2017)!
Geochemical!techniques!to!evaluate!the!occurrence!of!biological!
activity!associated!with!the!formation!of!secondary!minerals!in!
lava!caves!and!caves.!Identified!bio/organic!compounds!associated!
with!Na-sulfates.!
NASA!EXOBIOLOGY!
(PI!N.!Hinman)!
Richardson!et!al.!(2013)!
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5. FUTURE!LAVA!CAVE!ASTROBIOLOGY!RESEARCH!TOPICS!!
The!past!decade!has!seen!significant!progress!made!on!the!detection!of!lava!cave!caves!on!the!
Moon! and! Mars,! but! new! advances! are! needed! to! facilitate! their! exploration! and!
characterization.! The! Defense! Advance! Research! Projects! Agency! has! announced! (12/2017)!
underground!operations!as!the!next!frontier!for!national!technology!investment;!planetary!cave!
astrobiology! science! could! benefit! greatly! from! rapid! and! significant! new! technology!
achievements!spurred!on!by!this!new!DARPA!SubT!challenge.!!!
Below!are!a!few!of! the! areas!of! research! that!could! be!supported! through!current! and!
future!NASA!astrobiology!program!initiatives.!Advancements!in!these!areas!would!also!provide!
benefit!toward!exploration!of!countless!other!difficult!terrains!throughout!the!solar!system.!
!
REMOTE!SENSING!FOR!LAVA!CAVE!CAVE!DETECTION!
Machine!learning!(ML)!techniques!to!process!planetary!orbital!imaging!data!to!identify!lava!cave!
skylights!(Wagner!et!al.,!2017)!
Remote!sensing!data!fusion!(such!as!thermal,!gravity,!and!radar)!to!improve!detection!sensitivity!
of!near-surface!caverns!!
Improved!sensor!technology!and!spectral!resolution!to!increase!skylight!detection!success!
Application!of! new!computational! methods! for! analysis!of! massively! large! data! sets!(planetary!
imagery)!and!quantum!computing!
DEVELOPMENT!OF!AUTONOMOUS!ROBOTICS!ABILITIES!FOR!LAVA!CAVE!CAVE!EXPLORATION!
Sample!collection!and!retrieval!capabilities!
Highly!mobile!robotic!units!to!facilitate!access!into!lava!caves!
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/"Planetary"Caves"as"Astrobiology"Targets" " "
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Sensing! and! control! systems! to! facilitate! autonomous! characterization! and! navigation! in!
subsurface!environments!
A!reference!library!of!images!(or!chemistries)!of!terrestrial!cave!biosignatures!to!serve!as!a!training!
set!for!future!robotics!missions!with!enhanced!AI!and!ML!capabilities!
IMPROVEMENTS! IN! INSTRUMENTATION! FOR! AUTONOMOUS! IDENTIFICATION! AND! CHARACTERIZATION! OF!
BIOSIGNATURES!
Low-mass,! power,! and! volume! sample! preparation! and! instrumental! techniques! capable! of!
onboard!analysis!of!organic!molecules/compounds!at!low!detection!thresholds!
In"situ!microstructural/textural!analysis!capabilities!of!mineral!surface!
!
REFERENCES!
Allen,!C.C.!(2009)!Lunar!lava!tubes!–!The!promise!of!new!orbital!data,!Geological"Soc."Am."Abstr."41:192.!
Blair,!D.M.,!et!al.!(2017)!The!structural!stability!of!lunar!lava!tubes,!Icarus!282:47-55.!
Boston,!P.J.,!et!al.!(2001)!Cave!biosignature!suites:!Microbes,!minerals,!and!Mars,!Astrobiology"1:25-55.!
Chappaz,!L.,!et!al.!(2017)!Evidence!of!large!empty!lava!tubes!on!the!Moon!using!GRAIL!gravity,!Geophys."Res."Lett.!44:105112.!
Cushing,!G.E.,!et!al.!(2007)!THEMIS!observes!possible!cave!skylights!on!Mars,!Geophys."Res."Lett.!34:L17201.!
Esmaeili,!S.,!et!al.!(2017)!Resolution!of!lava!tubes!with!ground!penetrating!radarTubeX!proj.,!2017"AGU"Fall"Mtg.,"NS23A-0021.!
Garry,!W.B.,!et!al.!(2017)!Planetary!exploration!of!lava!tubes!with!LIDAR!at!Craters!of!the!Moon,!Lunar"Planet."Sci."Conf."48:1207.!
Furutani,!K.!(2016)!…Inflatable!outer!wheel!rover!for!exploration.planetary…subsurface!caverns,!Int."J."Auto.Tech.!10:!584-590.!
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