Habitability of Lava Tubes o n the Moon and Mars: Lessons From Earth Final Summary of a Research Project Selected by the 2018 MBR Space Settlement Challenge and S upported by a Seed Grant from the Dubai Future Foundation via the Guaana.com Open Research Community and Grant Distribution Platform. Pascal Lee SETI Institut e Submitted: 28 January 2019 Citation: Lee, P. (2019). Habitability of lava tubes on the Moon and Mars: Lessons from Earth. Final Summary to 2018 MBR Space Settlement Challenge , Dubai Future Foundation, Guaana.com.
Habitability of Lava Tubes on the Moon and Mars: Lessons From Earth Pascal Lee SETI Institute, 189 Bernardo Ave, Mountain View, CA 94043, USA Mars Institute, NASA Ames Research Center, MS 245 3, Moffett Field, CA 94035 1000, USA Executive Summary Background. Lava tubes are caves that form when the surface of a lava flow cools and solidifies, while molten rock underneath continues to flow and eventually drains away: a tube shaped underground cave is then create d. Lava tubes are known to exist on the Moon and Mars where they may form i n eithe r volcanic or large impact crater settings. Motivation. Lava tubes on the Moon a nd Mars are of interest for space settlement because caves have been pr oposed as natu ral shelte rs that future human explorers could occupy . C aves w ould in principle protect dwe llers from surface radiation, wide temperature swings , micrometeorite impacts, and rocket exhaust blast . Some lava tubes on the Moon and Mars might also be cold enough t o trap water ice , which, if water ice is actually present, might offer access to an imp ortant resource, and in the case of Mars, also to potential underground habitats for microbial life. Goal and Objectives. The main goal of our study was to begin to learn more about the physical and geological reality of la va tubes on Earth, specifically with respect to their habitability. Our objectives focus on two issues of central relevance to assessing the potential habitability of lava tubes on Earth, and by extension on the Moon and Mars : 1) Geotechnical characteristi cs : How accessible and safe are lava tubes? W hat makes some lava tubes experience collapse? 2) Relation to ice: W hy, when and how does ice occur and evolve in lava tubes? Approach and Findings. 1) To begin survey ing the geotechnical characteristics of lava tubes, we carried out a review of the published technical literature on the topic. Our finding is that while t here is an abunda nce of archaeological and historical reports and maps documenting the access, explorati on, investigation, use , and occupation of lava tubes, including many anecdotal mentions of collapse featu res inside lava tubes, there are essentially no published quantitative studies of the geotechnical propert ies of lava tubes using modern standard geote chnical metrics. 2) To begin examining the relation ship between lava tubes and ice, we carried out a field invest igation of the Lofthellir Lava T ube Ice Cave in Iceland in which massive ice i s known to exist . Our field observations suggest that the ice ins ide Lofthelli r is predominantly meteoric in origin water fr om atmospheric precipitation or condensation rather than from volcanic venting. In other words, the water inside the cave came mostly, if not exclusively , from bove not elow We also report two im portant new findings: a) U nderground micro glaciers are recognized for the first time; they form in the lave tube as a result of accumulation, slow movement , and loss of water and massive ice under the effect of gravity; b) Gelifraction , the fracturing of rocks by freezing of water trapped in joints, is an important cause of breakup and collapse inside this ice rich lava tube. Conclusions. 1) Our literature survey indicates that we still have lit tle quantitative knowledge about the geotechnic al properties of lava tubes on Earth , and therefore limited understanding of the factors that influence their evolution on our planet, let alone on the Moon or Mars ; 2) Our field study of an ice rich lava tube in Iceland suggests that, if ice were able to accumulate inside lava tubes on the Moon or Mars regardless of its origin , a) gravity might produce underground micro glaciers , while b) gelifraction might increase the risks of rock fall and collapse (compared to ice free caves ). Next Steps. T he following is recommended: A) Carry o ut detailed geotechnical assays of a series of lava tubes on Earth that captures their diversity in composition, size, sha pe, age, and weathering history, to understand better the key factors that might control the ev olution of lava tubes on the Moon and Mars ; B) Further i nvestigate Lofthellir and other ice rich lava tubes on Earth to underst and better if and how ice might occur and evolve inside lava tubes on the Moon and Mars; C) Identify, design, and test robotic sy stems and astronaut hardware that will enable safe and productive exploration of lava tubes on the Moon and Mars. Additional Accomplish ments : Our project also enabled, and benefitted from, other research and outreach activities that collectively help advance the human exploration and settlement of space: i) B y comb ining resources from other research projects funded by NASA and other partners , we carried out at Lofthellir the first successful flight of , and 3D mapping by, a lidar equi pped drone in side a lava tube and ice rich cave; ii) o ur fie ldwork at Lofthellir was filmed and will be featured in an upcoming episode of What on Earth? (Season 6) , the award winning TV documentary series and most watched program on the US Science Channel ; iii) Results of our research will be presented at the upcoming 50 th Lunar and Planetary Science Conference in Houston, TX (18 22 March 2019) (Lee et al. 2019).
Main Results or O utputs Background Lava tubes are caves that form when the surface of a lava flow cools and solidifies, while molten rock underneath continues to flow and eventually drains away: a tube shaped underground cave is then created. Lava tubes are known to exist on the Moon and Mars where they may form in either volcanic lava flows (vol canic lava tubes or in large impact structures (impact melt lava tubes Wagner & Robinson 2014, eill & Datta 2014, Lee 2018 a,b ). Motivation Lava tubes on the Moon and Mars are of interest to space settlement because caves have been proposed as natural shelters that future human explorers could occupy (e.g., Von Ehrenfried 2019 and references therein . Caves would in principle protect dwellers from surface ionizing radiation, wide temperature swings, micrometeorite imp acts, and rocket exhaust bla st. Some lava tubes on the Moon and Mars might also be cold enough to trap water ice, which, if water ice is actually present, might offer access to an important resource . Access to water on the Moon and Mars is considered key to sustainable long term huma n settlement. Water serves as a life critical hydration fluid and solvent, and also as coolant and fuel source Hydrogen H 2 and oxygen O 2 are rocket fuels that may be derived from H 2 O). Access to extractable water will likley drive the location and site se lection of settlements on the Moon and Mars. In the case of Mars, any water or ice in caves might also represent potential underground habitats for microbial life . To date, over 300 pits and caves have been identified on the Moon, all at latitudes less than about 50 o N or S. (Robinson and Wagner 2014, Wagner 2018). In addition, a few candidate pits were recently reported inside Philolaus Crater near the North Pole of the Moon, and interpreted as possible impa ct melt lava tube skylights (Lee 2018). (A sky light is a usually round opening above a lava tube resulting from the coll apse of a limited section of the lava tubes roof). If confirmed, the lava tubes in Philolaus Crater would be at such high latitude (~72 o N) that sunlight would never enter the caves to heat u p the rocks below their skylights . As a result, por tions of these caves could be cold enough for H 2 O ice, if present, to accumulate and remain trapped and stable on timescales of millions to possibly hu ndreds of millio ns of years (Fig. 1). In the case of Mars, most lava tubes and their associated skylights or pit craters are found on the flanks of the planets giant volcanoes, and therefore at relatively high altitude. The interiors of these caves could also be cold eno ugh to trap H 2 O ice on similar geologic timescales (Fig. 2). In spite of the a priori promising prospects offered by lava tubes on the Moon and Mars for human exploration and habitation (Fig. 3) , it remains unclear if se ttling lava tubes would be practical. Many lava tubes on Earth are difficult to access and/or present awkward geometries to move through, and like all caves, may be subject to dangerous r ock falls and collapse. It is also unclear how H 2 O ice, if present, might build up and evolve o ver time in side lava tubes on the Moon and Mars . Goal and Objectives The main goal of our study was to begin to learn more about the physical and geologica l reality of lava tubes on Earth, specifically the factors that affect their habitability , in order to establish a rigorously informed basis for assessing the potential habitability of lava tubes on the Moon and Mars . Our objectives focus ed on two issues of central rele vance to assessing the habitability of lava tubes on Earth , and by (cautious ex tension, on the Moon and Mars : Objective 1 : Geotechnical characteristics: How accessible and safe are lava tubes? What makes some lava tubes experience collapse? Our Objective 1 was to understand the geotechnical characteristics of lava tubes on Earth, the factors that determine these, and potential implications for the habitability of lava tubes on the Mo on and Mars. W e aim ed to identify, in the case of t errestrial lava tubes, the range of variation in the geometry, scale, internal surface roughness (of floors, ceilings and walls, insulation (as affected by fractures, fissures, and diffusive exchanges with the surface, and stability (as measured by ceiling and wall collapses and their frequency, and the factors that determine these. Objective 2 : Relation to ice: Why, when and how does ice occur and evolve in lava tubes? Our Objective 2 was to understand why, when and how H 2 O ice occurs in lava tubes on Earth, and potential implications for H 2 O ice in lava tubes on the Mo on and Mars. W e aim ed to identify, in the case of
terrestrial lava tubes in cold regions, the conditions under whic h, and the forms in which, H 2 O ice occurs inside lav a tubes. Sources for the H 2 O ice, whether exogenic meteoric , i.e. , from bove or endogenic (volcanic, i. e., from elow , would need to be identified as well. Approach (Methods and Processes Geotechnical Properties of Lava Tubes Lava tubes on Earth are expected to display significant variations in geotechnical properties. They present a wide range in geometry and scale, in internal surface roughness (floors, ceilings, and walls, in the insulation they offer (many are fractured, fissured, and/or in diffusive contact with the surface environment, and in their stability (many experience collapse. Altho ugh lava tubes have been widely studied, their geotechnical properties and the factors that determine these, have yet to be analyzed systematically and synthesized into a coherent understanding. To begin surveying the geotechnical characteristics of lava t ubes, we carried out a systematic review of the published scientific and tec hnical literature on the topic . Ice in Lava Tubes Little is known about the conditions under which lava tubes may harbor water ice, the forms in which ice is found (distributed vs massive, and the origin(s of the ice. The relative contributions of exogenou s sources meteoric, i.e., atmospheric precipitati on or condensation and endogenous sou rces volcanic volatile release have not been studied systematically. Chemical alteration and devitrification of volcanic glass in contact with water in lava tubes may also trap water in hydrated minerals on geological time scales. To begin examining the relati onship between lava tubes and ice, we carried out a field investigation of th e Lofthellir Lava Tube Ice Cave in Iceland , one of few ice rich lava tube systems (with massive ice known on Earth , and one associated with a currently active volcanic region. We focused our field investigation on the geology , hydrology, and cryology of Lofthellir, and installed a series of envir onmental data loggers to monitor atmospheric pressure, t emperature, lighting, and relative humidity, both outside and inside the cave for an entire year , to characterize the thermodynamics of the cave system. Findings and Results Geotechnical Properties of Lava Tubes : A Review Regarding the geotechnical properties of lava tubes and the factors that determine these, our finding is that while there is an abunda nce of archaeological and historical publications and maps documenting the access , exploration, investigation, occupation, and use of lava tubes on Earth , including many anecdotal mentions of collapse features inside l ava tubes, there are as yet no published quantitative studies of the geotechnical properties of lava tubes using modern standard geotechnical metrics. A recent review of the archaeology and history of human Homo sapiens occupation of lava tubes since pre historic ti mes can be found in Von Ehrenfrieds (2019 monograph , From Cave Man to Cave Martian . Bu t it is difficult to extract from the many original accou nts consistent geotechnical information. To illustrate the cha llenge , we examine here the example of published studies on the human occupation of lava tubes in Iceland. Of the approximately 500 lava tubes known in Iceland, 200 present evidence of former human occupation. The oldest record is the case of the Surtshellir and Vigishellir lava tube system (Patel 2017) . At Surtshellir lackenes Cave, there are ruins of a 4.5 m tall human made wall spanning the 12 m width of the cave, made of cave collapse blocks weighing up to 4 metric tons each . This wall is the largest known Viking Age stone structu re in Iceland . In adjacent Vigishellir ortress Cave, a smaller and older lava tube branching off of Surtshellir , there is a 6.7 m x 3.3 m oval sha ped stone wall considered the oldest standing Viking structure . Radiocarbon dating of animal bone remains at the site yield dates ranging from 890 to 930 A.D., i.e., shortly after the arrival of the first Norse migrants , Icelands earliest long term settlers . I n spite of these clear signs o f human use , t here is no evidence for long term occupation of the lava tubes, possi bly because of the ongoing danger presented by rock fall from the roof and walls of the caves, the lack of running water nearby, and /or the extremely rugged terrain outside the caves which would have made hunting or raising livestock difficult . Indeed, m os t lava caves in Iceland with evidence of human occupation usually show signs of only brief, transient us e (Patel 2017). A potential lesson he re for Moon and Mars exploration and settlement might be that , in spite of the availability of natural shelter s such as caves , what ultimately might determ ine the location of a habitat fo r early settlers is saf ety/reliability of the shelter , and a ccessibility to local resources (water in particular ). Reports of lava tube rock falls and collapse often imply that they ar e/were the result of seismic activity and weat hering (chemical and /or physical. As a result of our fieldwork for Objective 2, we now show that gelifraction (rock fracturing by freezing of water may also be a significant factor in rock fal ls and collap se in lava tub es under cold climate regimes (see next section.
Ice in Lava Tubes Investigation of the Lofthellir Lava Tube Ice Cave, Iceland : A Field Report We carried out a field investigation of the occurrence and effects of ice inside a terrestrial lava tube, with focus on characterizing quantitatively the caves physical environment and its range of ice related geologic features and processes. Field Site. We selected the Lofthellir Lava Tube Ice Cave in Iceland, a young (therefore presenting limited weathering degradation) lava cave known to contain a wide variety of massive ice structures in close to pris tine condition Hrarsson 2006, Waters 2006) . Lofthellir is located in the Lake Myvatn region of Iceland, near where Apollo astronaut field geology training took place (Fig. 4 . M ore information on Apollo era training in Iceland can be found here: http:// bit.ly/2IVGJM8 . The Loft hellir lava flow is only about 3,500 years old. The skylight giving access to the cave was first spotted by a local pilot (and perhaps also formed or wid ened) in summer 1989 (A. F. Birgisson, pers. comm .) . The cave and its skylight entrance are on private property (Fig. 5) . Access is restricted an d protected, and must be coordinated via authorized private field guid es . Our field work in the cave was carried out on 10 11 Oct 2018. Geometry. The Lofthellir Lava Tube is on average 10 meters wide. From its skylight entrance, the cave currently extends ~ 50 m toward the NW and ~120 m toward the SE (main branch), far beyond a lava constriction (1 m wide passage) in the main branch at +20 m (Figs. 5 and 7 . The floor of the lava tube is almost entirely lined with ice. In the main branch, from +20 m to +60 m, the ice floor rises to the point of constricting passage to a crawl space along the caves roof. From +60 m to +120 m, the ice floor level drops by ~10 m in a series of wide sub horizontal steps, revealing the tubes main chamber (Fig. 14) . Ice. Throughou t the length of the lava tube, stalactites and stalagmites of ice occur, in places merged into thick columns, curtains, drapes, and domes of ice (Figs. 8 and 9) . The prevalence of stalactites of ice along the roof of the cave suggests a meteoric origin for most of the H 2 O mainly precipitation at the surface followed by percolation and drip (Fig. 9 Left . In places, the textured surface of ice deposits suggests repeated episodes of partial thawing and refreezing (Fig. 9 Right. Isolated wedges of layer ed ice tucked against elevated nooks in the cave walls are likely relict and suggest earlier episodes of ponding and refreezing and higher past levels of floor ice (Fig. 10) . Breaks in slope and stark boundaries within the cave ice suggest dynamic interpla y between distinct ice masses flowing under gravity via slow creep (viscous flow with limited mixing or merging. Subsurface micro glaciers are identified for the first time. They are only decameters in scale, but present the requisite accumulation and abl ation/melting zones, morainal loads, and lobate flow fronts. The largest example at Lofthel lir is ime Travel Glacier (a name our field team proposed), which descends from the astle ice dome (accumulation zone (Fig. 11 . Rock Falls. There is ample evidence of past and ongoing rock fall activity inside Lofthellir, including ice filled rubble aprons, e.g., Lightspeed Rock Glacier (a name our field team proposed (Fig. 12), ice free rubble aprons (Fig. 13), and the more recent Mars Rock and Moon R ock meter scale drop boulders (names our field team proposed) (Fig s . 15 and 17) . Ice filled joints and fractures throughout the cave suggest gelifraction is a significant process of comminution and desquamation, and thus of cave structural evolution over time (Fig. 13 . Drone Mapping. At Lofthellir , we also carried out the first drone borne lidar mapping of an ice rich lava tube and cave. Astrobotic GPS denied mapping and navigation system, AstroNav, gives drones and small free flying spacecraft the ability to fly in subterranean environments Snyder et al . 2017 (Fig. 16 . Both ice and rock cave features were scanned successfully, providing unprecedented precision in mapping the shape and quantifying the geometry and dimensions of Lofthellir skylight entrance area (Fig. 7 and main chamber (Fig. 17 . Dro ne b ased visible imaging outside the lava tube also capture d the Lofthellir skylight in surrounding lava fields (Fig. 7 A . Environmental Monitoring. A total of 10 Onset HOBO environmenta l data loggers were installed inside and outside Lofthellir , of which 7 were installed inside the cave at a variety of locations: lava tube ceilings, walls, and floors, on and off interior ice, and along and away from fractures a nd fissures (Fig. 15 . These loggers will be retrieved after 1 year of environmental monito ring (in Oct 2019, c/o our field guide A. F. Birgisson, and their data processed then to fully complete the current study. In summary, o ur field observations suggest that the ice inside the Lofthellir Lava Tube Ice Cave is predominantly meteoric in origin (water from atmospheric precipitation or condensation rather than from volcanic venting. In other words, the water inside the cave came mostly, if not exclusively, from above, not elow We also report two significant new findings: a) Unde rground micro glaciers are recognized for the first time; they form in the lave tube as a result of accumulation, slow movement, and loss of water and massive ice under the effect of gravity; b) Gelifraction , the fracturing of rocks by freezing of water tr apped in joints, is an important cause of breakup and collapse inside this ice rich lava tube.
Obstacles or Changes in Direction During the Project Review of Geotechnical Properties Given the dearth of scientific or technical literature on the geotechnical properties of lava tubes revealed by our publication survey, we elected to reduce the scope we had originally planned to place on a detailed review and synthesis of the existing literature, and increased instead the scop e of our field investig ations at the Lofthellir Lava Tube Ice Cave in Iceland to include detailed in situ observations of the caves structural geology, and rock fall and collapse history. In addition, we joined forces with separate ongoing research projects on planetary drone science exploration operations supported by NASA and other industry and non profit partners Astrobotic, Mars Institute, SETI Institute) to carry out together the first field tests of a lidar equipped drone at Lo fthellir (Figs. 6 and 16 . As a result of t his collaborative approach, our research project was significantly enhanced by producing the first drone acquired 3 D map of the interior of a lava tube or of any ice rich cave. The 3 D maps of Lofthellir have in turn allowed us to gain a much clearer unde rstanding of the geometry and dimensions of the lava tube , and of many of its structural features (Figs. 7 and 17). Environmental Data Loggers. Our original intent was to carry a short term environmental characterization of the Lofthellir Lava Tube Ice Cave to acquire essentially a snapshot of environmental conditions inside and outside the cave at the time of our visit . To that effect, t he data loggers would have been installed and moved every 2 days over a period of 14 days to investigate different d omains of the ice rich lava tube complex. Once we defined our field plans in more detail, however, and realized the challenge and cost of accessing the lava tube multiple times o ver a period of 2 weeks, and also recognizing the much greater scientific valu e of carrying out a longer term environmental monitoring study of the cave , we opted to install our data loggers just once and to leave them behind and allow them to operate autonomously for an entire year (Fig. 15 . Although this meant that no environment al data would be available for immediate analysis following our field deployment, arrangements were made with our local field guide A. F. Birgisson to retrieve our data loggers in 1 year and to ship them back to our lab for analysis. At that time, an ent ire year of environmental data will be available, which will be of great value for helping us understand the thermodynamics of this ice rich lava tube system. The anticipated data will take more time to acquire, but it will be far more valuable scientifica lly and will have cost much less to obtain. Potential Impact and Opportunities for implementation of the Results Aside from many anecdotal accounts of rock fall and collapse features observed inside lava tubes, o ur survey of the scientific and technical lit erature for quantitative studies of the geotechnical properties of lava tubes shows that this is an area of research that has, as yet, received very little attention. This is not necessarily surprising, as lava tubes have generally not been used on Ear th for long term human settlement. While there is a long history of human exploration and use of lava tubes, occupation of these caves appear s to have been mostly short term, intermittent or transient (e.g., Patel 2017). A hypothesis as to why this short t erm interest has been the general case might be found in the combined fact s that many lava tubes are inherently challenging to access and move through, prone to dangerous rock falls and collapse, located (by their very nature) in seismically active areas w hich promotes structural instability, surrounded by rough terrain and topography, and not necessarily close to sources of water unlike karst caves which were formed in association with water. This does not imply that lava tubes on the Moon and Mars shoul d be dropped from consideration for exploration and settlement. Rather, it indicates that detailed studies of the geotechnical properties of lava tubes on Earth and of the fac tors that influence these must be undertaken first in order to establish the rigorous framework needed to assess the promise and risks of lava tubes on the Moon and Mars. As well, pits and caves on the Moon and Mars should be explored robotically starting now to gain as early as poss ible first hand understanding of their physical a nd geologic reality. Meanwhile, our field investigation of the ice rich Lofthellir Lava Tube Ice Cave opens the exciting pro spect of potentially finding, if H 2 O ice does occur and accumulate inside (some lava tubes on the Moon and Mars regardless of th e H 2 s origin , underground micro glaciers and rock glaciers, the result of viscous creep of ice over time under the effect of gravity , even reduced gravity . The presence of ice inside a cav e, however, would also be expected to be accompanied by gelifrac tion , which would promote rock falls and collapse. Icy caves on the Moon or Mars might therefore likely be rough. In summary , lava tubes on the Moon and Mars would be very exciting to explore, but whether or not any would be safe/reliable enough for long term set tlement remains to be investigated , starting with terrestrial lava tubes first to establish a rigorous basis for comparisons.
Conclusion s and Next Steps Conclusions 1) Geotechnical Properties of Lava Tubes. Our survey to date of the scientifi c and technical literature on the geotechnical properties of lava tubes on Earth has not yielded any published study using modern geotechnical metrics. Alth ough substantial literature exists documenting the archae o logy and history of human explora tion, investigation, occupation, and use of lava tubes , the references provide only anecdotal accounts of geotechnically relevant features, not quantitative data that would allow understanding the factors that influence the geotechnical properties o f lava tubes and how they affect these properties. 2) Ice in Lava Tubes. Our field investigation of the ice rich Lofthellir LavaTube Ice Cave in Iceland has allowed detailed documentation of a wide range of ice related features and structures inside th e cave. Ou r observations suggest that the massive ice formations f ound inside the cave are predominantly due to H 2 O of meteoric origin (from atmospheric precipitation at the surface and atmospheric condensation inside the cave, as oppos ed to H 2 O produced by volcanic venting . In addition, two new findings were made: a) Underground micro glaciers are recognized for the first time; they form in the lave tube as a result of accumulation, slow movement, and loss of water and massive ice under the effect of gravity ; b) Gelifraction , the fracturing of rocks by freezing of water trapped in joints, is an important cause of breakup and collapse or rock inside this ice rich lava tube. These two findings are of significance because of their potential relevance to the Moon and Mars. If massive H 2 O ice does occur inside lava tubes or other caves there, we predict that gravity, even if reduced, would act over time to mobilize the ice via viscous creep and produce micro glaciers and rock glaciers. Gelifraction, however, would also be expected to operate and promote rock falls and collapse. 3) Additional Achievements Our project also enabled, and benefitted from, other research and outreach activities that collectively help advance the human exploration and settlement of space: i) By combining resources from other ongoing research projects funded by NASA and other partners, we carried out together with these projects at Lofthellir the first successful flight of, and 3D mapping by, a lidar equipped drone inside a lava tube and ic e rich cave (Lee et al. 2019) . This success should encourage further design and development work on propulsive free flying robotic spacecraft systems for the exploration of pits and caves on the Moon and Mars. ii) Our fieldwork at Lofthellir was filmed and will be featured in an upcoming episode of What on Earth? (Season 6) , the award winning TV documentary series and most watched program on the US Science Channel. iii) Results of our research will be presented at the upcoming 50 th Lunar and Planetary Scien ce Conference in Houston, TX (18 22 March 2019) (Lee et al. 2019) . The lessons learned to date maybe considered transformative in that they provide key new insights, gained from practical field experience, into some of the poten tial wonders (underground m icro glaciers! , and also possible dangers (rock fall, collapse , that await us in the exploration of lava tubes on the Moon and Mars. Next Steps As next steps in this research, we recomm end, and would propose, the following follow up work : Geotechnical Field Survey: A campaign of detailed geotechnical investigations of a select series of lava tubes on Earth, capturing their diversity in com position, size, shape, age, weatherin g history, and ice content, to under stand the key factors that influence their geotechnical proper ties and evolution, and by (cautious extension, the geotechnical properties and evolution of lava tubes on the Moon and Mars . Global Study of Ice Rich Lava Tubes : A broadened global survey of ice r ich lava tubes to inves tigate the origin, mode of occurrence, diversity, effects, evolution through time, and microbiology of ice in lava tubes, and potential implication s for ice in lava tubes on the Moon and Mars. Human & Robotic Exploration of Lava Tubes: Given the special importance lava tubes will likely have in the human exploration of the Moon and Mars, identify and design robotic and astronaut hardware systems that would enable and maximize safe and productive lava tube e xploration (e.g., Fig. 1 8).
Figures Figure 1 . Lava Tube Skylights Near North Pole of the Moon? Left: S hadowed depressions on the floor of Philolaus Crater near the N orth Pole of the Moon interpreted as candidate impact melt lava tube entrances or skylights (N ASA LRO ). Right : These lava tubes would be as such high latitude that sunlight would never enter them. Temperatures in the caves c ould be so low that H 2 O ice, were it present, could be stab le for eons (Lee 2018 b ). Figure 2 : Lava Tubes and Skylights on Mars. Left: Collapsed lava tubes on the flanks of the Pavonis Mons volcano on Mars (ESA MEX. Center: A pit (D~200 m) interpreted as a possible lava tube skylight on Pavonis Mons volcano , Mars (NASA MRO. Right : The eanne p it crater (D~150 m) on Arsia Mons volcano , Mars (NASA MRO. Figure 3 : Future Exploration of Lava Tubes on the Moon and Mars. Left: Robotic d rones (propulsi ve free flying spacecraft could be used in the reconnaissanc e/mapping of caves on the Moon and Mars . Right: Humans would follow for in depth exploration and, potentially, habitation and settlement (Lee 2018 b ).
Figure 4 . Iceland Map , Lofthellir Lava Tube Ice Cave, and Astronaut Training. The Lofthellir Lava Tube Ice Cave is located in North Central Iceland, in the general region in which most of the Apollo astronaut field geology training in Iceland took place. Lofthellir was not known at the time . Given the lava tubes rel evance to the Moon and Mars, this lava tube should be added to any future training curriculum in Iceland for Moon or Mars bound astronauts . Left: Map base by Georeli ef. Annotations by P. Lee ; Right: Lee et al. 2019 ). Figure 5 . Lofthellir Lava Tube Ice Cave Skylight Entrance. Left: O utside view from ground level . Right: View from the twilight zone inside the lava tubes n orth west b ranch . At far left is the ladder that was used by the field team to descend into the lava tube through the skylight. Figure 6: Astrobotics Lidar Equipped Drone: The 0.7 m wide hexacopter drone is seen here descending into the skylight of the Lofthellir Lava Tube Ice Cave.
Figure 7 : Drone Mapping of Lofthellir Lava Tube Ice Cave Skylight Entrance : A: Surface visual imaging with snow dusting on ground, including at the bottom of the skylight ; B: A with underlying lidar imaged cave; C: Subsurface lidar elevation map (Red: hig hest elevation; Purple: lowest (Lee et al. 2019).
Figure 8 . Massive Ice in the Lofthellir Lava Tube Ice Cave . Left: The icy floor of the lava tube presents several merged domes of ice connected to and clearly fed from the roof via ice stalactites . Right: R ocky debris along the walls of the cave individual boulders or rock piles may become encased in massive ice . Figure 9 . Growth and Metamorphism of Ice Stalagmit es and Ice Domes . Left : The lta is a cluster of ice stalagmi tes in Lofthellirs Main Chamber. Near the center of the image, me ltwater dripping from the cave ceiling produces a splash of liquid water from a small meltwater pool at the top of an ice stalagmite , forming a distinct ejecta pattern. I nsert: Close up of the splash ejecta pattern. The ejected meltwater partially refreezes upon conta ct with colder ice near the base of the ice mass and the flo or of the cave. The wide reach of the meltwater ejecta allows massive, wide based stalagmite clusters to form . Right: The surface of mature ice stalagmites and ice domes presents disruption textures such as polygonal pattern s with cells of lower albedo clear ice surrounded by rims of brighter roug h ice. These are interpreted as be ing due to repeated cycles of thawing and freezing of the ice mass surface zone (over a depth of a few cm . Figure 10 . Old Layered s Ice. Isolated remnants of horizontally layered ice hanging above the current elevation of the caves ice floor attest to earlier higher levels of the floor ice. The left arm of a caver in a yellow jacket in the background provides scale .
Figure 11 : Lofthelli r Subsurface Micro Glaciers. View of the main chamber looking towards ime Travel Glacier. Astrobotics 0.7 m wide hexacopter drone at lower left provides scale. (Lee et al. 2019). Figure 12 : Lofthellir ightspeed Rock Glacier, a Subsurface Rock Micro Glacier. Left: I n r eflected light. Right: Same view with t ransmitt ed light. The rock component visible on the apron is a thin veneer of rocky debris resulting from recent falls and slides . The material lying immediately underneath is mas sive clear ic e. Deeper inside the debris and ice apro n, there might be other ice rich and rock debris layers. Figure 13 : Gelifraction and Collapse . C eiling and wall collapse is an active modification process in the Lofthellir Lava Tube Ice Cave. Left: Gelifraction the fracturing of rock by freezing of water along joints contributes to co mminution (break down in size and desquamati on (flaking of rocks . Right: A recent collapse along a 5 m section of the cave . A curtain of merged ice stalactites nea r the far end of the debris apron suggests ge lifraction played a role in fracturing the ceiling and wal l rocks, leading to collapse .
Fig. 14 . Lofthellir Lava Tube Ice Cave 3D Sketch Map. P erspective sketch map of the Lofthellir Lava Tube Ice Cave sketched on 10 Oct 2018 as the field team investigated the cave along progressively deeper sections . Ice deposits were colorized in blue later. The length shown is approximately 130 meters of the total 200+ meters. The entrance skylight would be to the rig ht (not shown. The Astrobotic lidar mapping drone was flown in the Main C hamber at far left . Figure 15. Rock Falls and Environmental Sensors. Rock fall d ebris aprons incorporating varying amounts of ice create the V shaped narrow seen here across the Main Chamber of the Lofthellir Lava Tube Ice Cave. oon Rock, a 1 meter wide boulder seen here sitting in the dark on the caves icy floor appeared recently as a result of a single mass rock fall (A. F. Birgisson, pers. comm. ). Environmental d ata loggers w ere installed at different locations inside and outside the cave to collect 1 full year of recordings of variations in lighting (from cave visits , temperature, atmospheric pressure, and relative humidity.
Figure 16 : Drone in Flight I nsid e the Lofthellir L ava Tube Ice Cave . The h exacopter drone with integrated l idar developed b y Astrobotic is seen here in flight inside Lofthellirs Main Chamber . This was the first time a drone equipped with a lidar mapped a lava tube, and a cave with ice . Figure 17 : Drone B orne Lidar Map of the Lofthellir Lava Tube Ice Caves Main Chamber. Drone map produced by Astrobotics drone borne lidar. Projected perspective view. (Red: highest elevation; Purple: lowest; Black: no coverage. The main geologic features referred to in this report are indicated in this rendering (Lee et al. 2019).
A B C D Figure 18 . EVA Boot Sole Design for Icy Surface Exploration . A: Tintin explores an icy cave on the Moon but loses traction (Note his tread less and cleat less soles Herge/Moulinsart; B: Apollo Moon boot o vershoes (Smithsonian); C: Treaded but c leat less s ole of Apollo Moon boot o vers hoe NASA/ Smithsonian; D: Rubber b oot w ith treads and c leats used by our field team at Lofthellir Lava Tube Ice Cave, Iceland . The d esign of future planetary EVA boot shoe soles should include opti ons with cleats and crampons to ensure safe traction on ev entual icy surfaces . Acknowledg ments This research was selected by the 2018 MBR Space Settlement Challenge and supported by a seed grant from the Dubai Future Foundation via the Guaana.com open research community and grant distribution platform. This research also leveraged support for other related research, in particular for UAS/drone science and exploration operations from NASA to P. Lee via NASA Cooperative Agreement NNX14AT27A, and from NASA to Astrobotic via NASA STTR NNX16CK16C. Additional support was provided by the SETI In stitute, the Mars Institute, and the Icelandic Centre for Research (Rannis). P. Lee also wishes to thank h is research collaborators and field companions (Fig. 19 ), in particular Eirik Kommedal graduate student at Norwegi an University of Life Sciences, SETI Institute student intern, and Mars Institute Fridtjof Nansen Fellow, Andrew Horchler, Fraser Kitchell, Eric Amoroso, and Kerry Snyder of Astrobotic, Tom Douglas of U.S. Army Cold Regions Research and Engineering Lab, Julian Watson, Tony Mews, and Daddi Bjarnason with WAG TV, Anton F. Birgisson of Geo Travel Iceland, and Baldvin E. Einarsson of Saga Travel. Figure 19 : Field Team that investigated the Lofthell ir Lava Tube Ice Cave, Iceland.
Abbreviations DFF Dubai Future Foundation E SA European Space Agency (ESA Member Countries HMP Haughton Mars Project JAXA Japan Aerospace Exploration Agency (Japan Lidar Light Detection and Ranging LRO Lunar Reconnaissance Orbiter MBR Mohammed Bin Rashid MEX Mars Express MRO Mars Reconnaissance Orbiter NASA National Aeronautics and Space Administration (USA Rannis Rannsknami!st! lands (Icelandic Centre for Research (Iceland STTR Small business Technology TRansfer UAE United Arab Emirates UAESA United Arab Emirates Space Agency (UAE References Hrarsson, B. (2006 Islenskir Hellar (Icelands Caves ISBN 9979219726. Lee, P. (2018a). Philolaus Crater: Exploring candidate lava tubes and skylights near the Lunar North Pole. Lunar Science for Landed Missions Workshop , Jan 2018, NASA SSERVI. Lee, P. (2018 b ). Possible lava tube skylights near the North Pole of the Moon. 49 th Lunar & Planet. Sci. Conf. , Abstract #2982. Lee, P. (2019). Foreword, in From Cave Man to Cave Martian. Living in Caves on the Earth, Moon and Mars , by M. utch von Ehrenfried. Springer Pra xis, p1 3. Lee, P., E. Kommedal, A. Horchler , E. Amoroso, K. Snyder, and A. F. Birgisson (2019). Lofthellir lava tube ice cave: Subsurface micro glaciers, rockfalls, drone lidar 3D mapping, and implications for the exploration of potential ice rich lava tubes on the Moon and Mars. 50 th Lunar & Planet. Sci. Conf., The Woodlands, TX, 18 22 Mar 2019. Submitted. eill R. & S. Datta, (2009 ). Lava tubes and basaltic cav es as astrobiological targets on Earth and Mars: A review. Planet. Space Sci. 58, 592 598. Patel, S. S. (2017). The Blackeners Cave: Viking age outlaws, taboos, and ritual in Icelands lava fields. Archaeology, May 2017. Snyder, K., E. Amoroso, A. D. Horchler, F. Kitchell, (2017 . AstroNav: Robust, high rate SLAM for planetary exploration. 16 th ACM/IEEE Inl. Conf. IPSN , Pittsburgh, PA. Von Ehrenfried, M. utch (2019). From Cave Man to Cave Martian. Living in Caves on the Earth, Moon and Mars . Springer Praxis, 300 pp. Wagner, R. V. (2018). Lunar Science for Landed Missions Workshop, Jan 2018, NASA SSERVI. Wagner, R. V. and M. S. Robinson (2014). Distribution, formation mechanisms, and significance of lunar pits. Icaru s 237, 52 60. Waters, E. (2006). Recent contributions to Icelandic cave exploration by the Shepton Mallet Caving Club (UK). AMCS Bulletin 19 , 188 196.