Location, Location, Location!
Lava Caves on Mars for Human Habitat,
and the Search for Life
Location, Location, Location!
Lava Caves on Mars for Human Habitat,
and the Search for Life
Journal of Astrobiology and Space Science Reviews, 1, 133-143, 2019

Location, Location, Location!
Lava Caves on Mars for Human Habitat,
and the Search for Life
Penelope J. Boston, Ph.D.
New Mexico Institute of Mining & Technology (NMT) and
The National Cave and Karst Research Institute (NCKRI)
Director of NASA's Astrobiology Center, CA
(reprinted with permission from the publisher)


Over the course of humanity's history as a species, the use of caves, rock shelters, and other natural geological features has played an important role in our survival and cultural development. We suggest that the use of such natural features in future human exploration of Mars and Earth's moon could be a timely and practical solution to a number of potential dilemmas presented by the extreme and challenging nature of the environments on these bodies. Lava tubes, other caves, cavities, and canyon overhangs that are being identified on other planets are sites of intense scientific interest for geology, atmospheric climate records, and potentially biology. They may offer easier subsurface access for direct exploration and drilling, and could provide extractable minerals, gases, and ices. In the past few years, examples of such structures on Mars, the Moon, and potentially other bodies have increasingly come to light. Thus, the real estate is out there waiting for us to modify it for our exploration missions. The present Martian surface environment is extremely cold, dry, chemically active, and high in both ultraviolet and ionizing radiation. Galactic Cosmic Radiation (GCR) and episodic waves of high energy particles from solar proton events (SPE) necessitate the provision of robust radiation protection for habitats, workspaces, vehicles, and personal space suits. The mass penalty of providing this is a major driver in our consideration of the use of natural rock mass for radiation protection for habitats and workspaces, arguably the most massive components of an integrated human exploration equipment suite. Planetary protection considerations emerging from recent studies advocate a localization and zoning of degrees of human impact, much like that being implemented in the Antarctic as Special Regions. Containment of the primary human habitation and work activities within the confines of a subsurface habitat are highly consistent with these new approaches to Planetary Protection forward contamination. To begin to think about caves in the extraterrestrial exploration context, we have developed the notion of a complete, functioning subsurface habitat system. A suite of relatively low technology modifications to caves to improve habitability and safety are suggested. This system can integrate a spectrum of missions from both robotic precursors to human expeditionary missions and ultimately colonization.

Key Words: cave, lava tube, lava cave, habitat, geomicrobiology, astrobiology, habitability, planetary protection, radiation protection, pressure shell

1. Introduction

Over the course of humanity's history as a species, the use of caves, rock shelters, and other natural geological features has played an important role (e.g. Bonsall, 1997; Leroi-Gourhan, 1982; Lommel, 1966; Redondo et al., 2010; Valladas et al., 2001; Whitley, 2009). In fact, much of what we know about early hominids, including Homo neanderthalensis, and early Homo sapiens comes from remains and artifacts that have been preserved for us in caves around the world (Bahn, 2007; Chazan et al., 2008; Dye, 2008; Knudson et al., 2005). These remains include precious DNA-containing tissues (eg. Lalueza-Fox et al, 2007) and apparent artifacts of extreme antiquity in the 60,000-100,000 year range (e.g. Henshilwood et al., 2009; Texier et al., 2010; Vanhaeren et al, 2006). Giving us a picture of more recent millennia, textiles, mummified humans, and other artifacts have shown us aspects of caves in human life ranging from the domestication of cotton to the trade goods that passed through the Silk Road across Asia (e.g. King, 1974; Whitfield, 1996).

The present Martian surface environment is extremely cold, dry, chemically active, and high in both ultraviolet and ionizing radiation (e.g. McKay and Stoker, 1989; Carr, 1981; Jakosky et al., 1997). Indeed, the consensus has been that even organic materials may not be able to survive on the surface (Klein, 1978) although a recent new look at the subject has suggested that perchlorates found by the Phoenix Mission (Kounaves et al. 2010) may have interfered with the Viking organic results (ten Kate, 2010) and necessitates a new look at the question of Martian habitability (Stoker et al., 2010). Nevertheless, the surface is unarguably a very hostile place for our type of life. With all the truly compelling sites on Mars that have been championed as the "best" sites for future robotic and human Mars missions, why have we chosen to focus on caves? Is it just because we are looking for a new twist on the old theme of planetary exploration, life on other planets, or human colonization of extraterrestrial surfaces? No, we have championed this idea because caves provide both unique scientific targets especially for astrobiology (Boston et al., 1992, 2001; Grin et al., 1998; Le'veille & Datta, 2010) and critical practical human support functions in an extremely challenging radiation and physical environment (Boston, 2000; Boston et al., 2003, 2004 a & b).

2. Why Caves?

Caves in general are poorly understood and underappreciated by the vast majority of the population. Scientists and engineers are no exception to this generalization. Because we are surface-inhabiting creatures, we bring a certain amount of surface chauvinism to our perception of caves as well as the oceans and the upper atmosphere. However, many modern and ancient indigenous peoples have been well acquainted with the properties of the caves in their environments. They made extensive use of them for shelter, materials acquisition, water and ice repositories, burial chambers, ritual sites, protection from temperature extremes, and refuge from human enemies (e.g. Adler, 1996; Arnold, 1971; Arnold and Bohor, 1975; Hatt et al., 1953; Sieveking, 1979; Tankersley, 1997; Watson, 1986; Wright, 1971). Interestingly, a tent-shaped hut was constructed inside a French cave (the Grotte du Lazaret near Nice) about half a million years ago by members of some early human groups, perhaps Homo heidelbergensis, one of many possible progenitors of both Homo sapiens and Homo neanderthalensis (Jelinek, 1975). Evidently, early hominid inhabitants built a fireplace and other amenities at this site. Obviously, the benefits of construction within a cave were clear to many who have preceded us!

Conditions in cave interiors are typically radically different from and in many ways more benign than the surface environment (Boston et al., 2001). This has enabled microorganisms, larger organisms and even humans to gain protection by using caves as habitat. Many microbial forms are unique to the subsurface and have developed into countless novel strains (Northup and Lavoie, 2001; Boston et al., 2001, Barton & Northup, 2007). Caves also function as sole habitats for a wide array of highly specialized and unique native cave animals (Culver & Pipin, 2009).

The tendency of the uninitiated to imagine all caves as nasty, dank, smelly and rather creepy places akin to the dungeons of fairy tales has made them seem unappealing to some. Indeed, some caves are like that, but many caves, especially in arid environments, are relatively dry, easily accessible, geologically and tectonically stable, and quite "homey". People as diverse as the mushroom growing epicures of the Loire Valley, the Dogon people of Mali, and the miners of Naica, Mexico, respectively, use caves routinely for highly specific economic purposes, to shelter from an otherwise unsupportable surface environment, and sometimes as a source of immense wealth. In recent times, the use of caves on extraterrestrial bodies for human habitation has been suggested by several groups. Lunar lava tube bases received much of the early attention (Horz, 1985; Walden, 1988; Kokh, 1996; Taylor,1998) because lavatube-like structures were clearly visible in early lunar image data (Greeley,1969). Mars lava tubes have also been considered to have great potential as habitat and greenhouse structures (Walden et al., 1988; Frederick, 1999; Boston, 2000; Boston et al., 2003, 2004 a & b).

3. Lava Caves and Other Cave Types on Earth and Solar System Bodies

Contrary to popular belief and the advertisements of tourist caverns, caves on Earth are not rare! They are a globally distributed geological phenomenon that occurs in every major rock type (including salt!) and even in polar and high altitude ices. We have argued that this is not surprising because on any planet with a surface that has an internal or external source of energy (including impact cratering), there will be cracks in that surface. Such cracks form the basis for cave formation by a variety of terrestrial and non-terrestrial mechanisms from simple tectonic caves to highly complex solutional structures (Boston, 2004). Additional cave types are produced by melting within a solid as in the case of ices and lavas. Table 1 shows the plethora of different physical and chemical mechanisms currently known that produce cavities in Earth's crust and may be even more common on other Solar System bodies.

Table 1 -A systematic physics and chemistry based view of cave formation mechanisms derived from Earth examples and extrapolated to Martian conditions. Lavatubes are the only confirmed cave type on Mars to date, however, numerous plausible mechanisms may have produced other varieties of cave that are more difficult to detect remotely. Revised and expanded from Boston, 2004.

Figure 1: Mars lavatube, on the East flank of Olympus Mons. A - Context image showing the entire extent of a long lava channel with segments of closed tube. B. High resolution closeup of open and closed channels and the probable drainage point through which lava exited the tube structure. HiRISE Camera System, NASA/JPL/University of Arizona.

Recent image datasets from MOC (the Mars Onboard Camera aboard the Mars Global Surveyor mission, 1996-2006, MGS) were used to claim the presence of lavatubes on Mars (Boston (2004), and the HiRise camera on the Mars Reconnaissance Orbiter (MRO, inserted into Mars orbit in 2006) now show clear evidence of large tubes visible in a number of volcanic regions on Mars (Figure 1). More extensive analyses of Martian tubes have been conducted by Cushing and colleagues (2007) and Wynne et al (2008) and a tube system has even been identified by a middle school student group in June 2010 from HiRise image data! In the lunar case, years of speculation about lunar lavatubes has been confirmed by the finding of a clear lavatube entrance (Figure 2) by the recent SELENE mission (Haruyama et al. 2009). Interestingly, the dimensions of this tube are enormous (130m diam), consistent with the large sizes of Martian tubes, possibly a result of the lower surface gravity on both bodies with respect to Earth (Boston, 2004). Of course, significant differences in lava chemistry, temperature, and resulting rheology may also play a part in the production of such enormous tube structures. A study in 1995 by Keszthelyi has quantified heat loss during lavatube flow and concluded that basaltic volcanic flows that are tube-fed can produce tubes hundreds of kilometers long with effusion rates of only a few tens of cubic meters per second, thus Martian tubes could be produced by low to moderate effusion rates.

Besides lavatube caves, there exists the potential for other cave types to be present on Mars. Abundant evidence of water-created features exists on Mars (e.g. Malin and Edgett, 2000; Carr and Wanke, 1992; McKay et al., 1992) and recent detection of subsurface hydrogen probably associated with water has been reported (Boynton et al., 2002; Feldman et al., 2002). Ice-created features have also been explored (Squyres et al., 1987 and 1992). Where cracks have been formed in the Martian surface, fluid flow may dissolve at least some of the material depending upon chemistries of the solids and liquids involved potentially even produced by a type of catastrophic speleogenesis (cave formation) from impact cratering (Boston et al., 2006). Mechanisms for cave formation on Mars may differ from Earth providing a variety of new features not found here (Grin et al., 1998, 1999; Boston et al., 2004 a, b, 2006).

Figure 2: Lunar lavatube skylight. A - Context image showing the Mare Ingenii (Sea of Cleverness) region of the Moon where a lavatube skylight has been found. B - This lavatube skylight pit is ~130 meters in diameter. This size dwarfs Earth lavatubes and is similar to tube dimensions identified on Mars. The total image width is ~550 meters. Illumination is from the upper right. LROC Frame: NAC M128202846LE. Credit: NASA, Goddard, Arizona State University.

Although the presence of significant carbonate deposits has been the subject of speculation and prediction (Nedell et al., 1987; McKay and Nedell, 1988), presence of carbonate in Martian dust (Hamilton et al., 2005; Bandfield et al., 2006), and recent large scale detection in Nili Fossae from orbit (Ehlman et al., 2008), confirmation of carbonate outcrops has been slow in coming. Finally, an apparent positive detection of magnesium iron carbonates has been claimed for Gusev Crater in the Columbia Hills (Morris et al., 2010). However, the precise geological circumstances of these carbonate detections is unclear, thus, no predictions can be made as to whether carbonate dissolutional caves may be likely on Mars.

Recent mineralogical results from both MER Opportunity at Meridiani Planum (Christensen et al., 2005; Squyres et al., 2005; Yen et al., 2005) and from the Mars Express OMEGA instrument (Arvidson et al., 2005; Gendrin et al., 2005; Bibring et al., 2005) indicate that evaporite basin settings rich in various sulfates may be a major environmental type on Mars. Gypsum caves on Earth occur in arid and even some non-arid environments, some of great size and length (eg. Klimchouk, 2009; Klimchouk & Aksem, 2005). Fracturing and subsequent dissolution by surface or upwelling subsurface waters are the potential cave-forming mechanisms involved on Earth that may also be present at least episodically or in the past on Mars.

4. Radiation Environment on Mars

We believe that the radiation environment on Mars is the single most challenging aspect of the planet for future human exploration and even for life detection missions. The radiation environment on Mars has long been thought to be very extreme and antithetical to the survival of both indigenous microbial life and to human life, however, no direct measurements have yet been successfully taken on landed missions. The MARIE instrument aboard the Mars Odyssey Mission (launched 2001) was intended to characterize the radiation environment during transit from Earth to Mars and in Mars orbit. However, the instrument malfunctioned during the cruise phase and was not able to take measurements in Mars orbit. The lack of direct measurements has prompted investigators to try to deal with radiation issues using a combination of calculations and modeling efforts. However, the radiation situation is highly complex with a plethora of relevant particles including gamma radiation, heavy iron nuclei, protons and electrons in the solar wind, neutrons and more! The MARIE instrument did take some data and this was used to create a cosmic ray environment map of Mars (Figure 3).

Figure 3 - Cosmic Ray Environment map -This map was produced based on cosmic radiation data by the Mars radiation environment experiment (MARIE), an instrument on NASA's Mars 2001 Odyssey spacecraft. Those data were combined with Mars altimetry from the MOLA instrument aboard MGS (Mars Global Surveyor. Lowest elevations are anticipated to have lowest radiation levels because more atmosphere exists above them to prevent penetration to the surface. The color scale is 10 rems (dark blue) to 20 rems (dark red). Astronauts on the International Space Station in Earth orbit encounter radiation equivalent to an annual rate of 20-40 rems. JPL Press Release, March 1, 2002. Credit: NASA/JPL/JSC

Local environmental radiation doses in the Martian crustal material are anticipated to be smaller than on Earth due to trace quantities of U, Th and K. Data from Martian meteorites (Shubert et al., 1992) is used to bolster this argument. Thus the extrinsic sources of GCR and solar particle (or proton) events (SPE) are the focus of attention and are both anticipated to be vastly more intense than on Earth.

The sun ejects primarily protons and electrons which are major constituents of the ordinary solar wind. These particles typically have velocities of around 400 km s-1 by the time they get to the Earth (Barnes, 1992). The energy range for these particles ranges from eV to keV (Hundhausen, 1995). According to Skidmore and Ambrosi (2009), the thin Martian atmosphere is capable of attenuating the proton flux below energies of 100 MeV, thus only a few solar wind protons ever get to the Martian surface. However, as a result of these high energy protons, a cascade of neutrons and other high energy particles are created at high altitudes in the Martian atmosphere and unfortunately these secondary products DO reach the surface. As these authors point out, the GCR protons ARE able to directly reach the Martian surface because their energies range above 100 MeV. Furthermore, the protons accelerated by SPE are of energies above this 100MeV threshold and thus, also are expected to directly reach the Martian surface. However, direct information about the interaction of gamma radiation with the Martian surface during SPE is not currently available.

A combination of calculations based on assumptions and modeling has been used to try to approximate both GCR and solar intensities likely from SPE, i.e. solar particle events (Dartnell et al., 2007 a & b; Morthekai et al., 2007; Banerjee & Dewangan, 2008; McKeever et al., 2003). Table 2 compares the results of several of these studies and this has provided at least some constraint for our thinking about what the magnitude of the radiation challenge might be and how deeply it may penetrate the Martian surface. From these authors, we see that attenuation to very low levels has occurred by the 3 to 4 meter depth. These models are mostly using "soil", i.e. unconsolidated regolith, as the crustal material. Rock materials may have greater ability than regolith to attenuate radiation in some cases. Basalt and andesite are likely surface lithologies to be found on Mars and their presence has been confirmed (Mustard et al., 1997; Salvatore et al., 2010). The primary relevant constituent in both rock types appears to be silicon content (Goldstein & Wilkins, 1954).

Table 2 -Comparison of radiation values from several prior modeling studies. Environmental Radiation pertains to the radiation emitted from radioactive elements in the Martian crustal material.

In a study of the radiation attenuation properties and scattering coefficients of andesite (a volcanic rock) and diorite (a comparable igneous intrusive rock) with respect to gamma radiation (El Taher et al. 2007), these authors show that small differences in precise chemical compositions of the rock types (especially of heavier minerals, e.g. iron and manganese oxides) have consequences for their attenuation behaviors. Penetration and diffusion of gamma radiation in material is expressed by the attenuation coefficient μ. In turn, μ is a function of photon energy (E) and atomic number of the impacted material (z). The density (rho) of target materials is critical. The sidescattering coefficient (phi) which captures the magnitude of the secondary daughter product production, is of particular interest for our purposes. These authors show that the higher density materials contain a greater number of scattering centers which causes more elimination of photons, thus attenuate better in direct proportion to density. They tested rock materials with densities ranging from ~ 2.4 to 2.7 gm cm-3. Their explanation for this effect relies on the decrease of intramolecular voids and hence the potential for increased multiple scattering events. The side scattering photons, phi, also attenuate in direct proportion to increasing density as well. So the message for our purposes is that these rock types can significantly attenuate gamma radiation especially when endowed with enough heavy materials, including silicon and iron. Note that basalt has an even higher density (3 gm cm-3) than andesite and diorite, but its silica content is somewhat lower by percentage (45-52 wt%) compared to 57-63 wt% (andesite) and 55- 65 wt%(diorite) (Le Maitre et al., 2002).

The results of models like those presented in Table 2 are assumption dependent and the Martian radiation problem is still not well constrained. Because of this situation, a major study of astronaut safety issues vis a vis Mars has strongly advocated a radiation measurement precursor mission to definitively measure radiation at the Martian surface, characterize some relevant aspects of that surface, and then calculate the human dose that would be experienced at the measured sites (NRC, 2002). Further, they recommend a mission that will "measure the absorbed dose in a tissue-equivalent material on Mars at a location representative of the expected landing site..." Such information will be critical in formulating an optimal strategy for future human missions because there are significant trade-offs that have to be made. For example, Cuccinotta and colleagues (2001) have pointed out that a long-duration stay on the surface that is coupled with short transit time launch opportunities may actually be better from the radiation exposure standpoint than a short-duration surface stay coupled with longer interplanetary transit times because the planet itself and even its tenuous atmosphere provide better radiation shielding than the interplanetary exposure.

The effective radiation dose is particularly sensitive to the quantity of secondary neutrons given off when the primary radiation influx interacts with matter (NRC 2002), in our case, a particular lithology like basalt or carbonates or evaporites. Neutrons are absorbed better when there is hydrogen present in the material. When a material contains heavy nuclei, this results in secondary neutron generation (daughter products) and this means that attenuation of both primary and secondary radiation must be accounted for in any assessment of radiation attenuation by rock material. Depending on the precise composition of the rock, e.g. basalt, there may or may not be hydrogen containing minerals in the mix. For example, on Earth basalts may contain mica or amphibole and both of those are hydrated minerals. We know from the complex sulfate composition of Mars (e.g. Chipera & Vaniman, 2007; Bonello et al., 2005; Clark et al., 2005; Squyres et al., 2004; Larsen et al., 2000) that at least those mineral suites are capable of being highly hydrated on the Martian surface and we speculate that the basalts may also have avoided the dewatering that may or may not have been the fate of lunar basalts (see recent debate on the hydration state of the moon in McCubbin et al., 2010 and Sharp et al., 2010).

Amidst all this complexity and uncertainty what can be said at present about the radiation environment on Mars? In a study modeling the effects of hydrogen and iron concentrations on absorbed dose, the investigators varied amounts of hydrogen and iron (Clowdsley et al. 2001, cited in NRC 2002). This preliminary study concluded that the current understanding of the elemental composition of Martian soil is adequate for radiation transport calculations through bulk Martian regolith. The authors even suggest that significant variations in the concentration of hydrogen and iron have little effect on the absorbed dose. Thus, even our relatively crude approximation of radiation conditions on Mars is adequate to state that radiation will be one of the primary environmental conditions of concern for humans and other organisms on the Martian surface. Further, the attenuation properties of rock materials are adequate to provide significant shielding with a few meters of overlying rock, precisely the condition found in most caves at least here on Earth.

5. Caves as Science Sites

Although the search for life or life's past traces on Mars is a high priority within the astrobiology community, we have argued that any present Martian life is more likely to be analogous to Earth's subsurface biosphere than anything currently found on Earth's surface (Boston et al., 1992, 2001; Boston, 1999, 2000b). Even traces of ancient life will have been preserved more successfully in the subsurface than the surface. In this capacity, we have been strong advocates of science missions to detect and then investigate extraterrestrial caves, especially those on Mars, and with particular emphasis on life detection missions (see Figure 4 for examples of potential Martian subsurface habitats). However, the high premium placed on Mars life detection missions and the extreme delicacy of the balance between investigation and potential damage or contamination of such life requires extraordinary protocols analogous to those of biohazard containment of the most virulent Earth viruses (Rummel, 2001). This requirement for complete containment must be coupled with the demands of field science, active exploration, robotic functionality, and ultimately astronaut survival. Nothing of this magnitude has ever been attempted in human history. Pristine Earth caves that contain numerous novel species of microorganisms provide genuine biologically sensitive sites for developing and practicing the operations required for application to Mars life detection sites and ultimately extraterrestrial human habitat. In this capacity, the work that our team and other investigators do in the subsurface is of great importance in informing us about the potential for such future missions.

Figure 4: Artist's conception of an astrobiologically hectic corner of Mars. This illustration demonstrates a plethora of possible extremophile habitats on Mars for which there are analogs on Earth that have been studied by the astrobiological community. Included are three subsurface examples: lavatube caves, caves in soluble rock, and the rock fracture habitat itself. Lavatube caves, visible on the surface, have been confirmed for Mars (see text). Dissolutional caves and a potential rock fracture environment have yet to be confirmed and are more challenging to discover using orbital data. Art courtesy of R.D. Frederick.

Caves of scientific interest have a high probability of interesting geological and biological features, e.g. extreme age, isolation from the surface, evidence of gases or water. They must be big enough for instrumentation, microrobotic access, or drilling into but not necessarily for humans. Caves of all depths may be scientifically interesting, but very deep caves may be the most interesting for stratigraphy, mineralogy, geomorphology, and life detection. For scientific purposes caves without natural openings or with very limited openings are highly desirable because of superior preservation of the contents. Certainly caves in or near geologically or hydrothermally active areas would be of great interest to many scientific disciplines.

6. Earth Cave Analog Value

Caves on Earth can provide significant value as analogs of some aspects of the Martian environment. The unusual gases (H2S, CO2, CO, NH3, and others) contained in the air of some Earth caves present the opportunity to practice protection from and management of poisonous or deleterious atmospheres. The condition of no or little atmosphere is not easily simulated in Earth caves, of course. Conceivably, any caves existing at extremely high altitudes in the Andes or the Himalayas could provide a partial simulation but other logistics probably make it too difficult to be worth the effort involved. However, some caves are depleted in oxygen or heavily laden with toxic gases, thus requiring full breathing gear for investigators (e.g. Hose et al., 2000).

The greatest similarity between terrestrial caves and those of other planets exists in the realm of operational considerations within the confines and topography of caves. That is, the very experience of living, working, doing science, and extracting resources in the lightless and potentially hazardous cave environment is the primary value of the Earth cave analog for human exploration. This particularly extends to matters of planetary protection. The minute a human investigator enters a cave on Earth, the potential exists for deleterious impact on the indigenous biota (Moser et al., 2001; Boston et al., 2006b). But unlike the way that we deal with extreme biocontainment in the laboratory, in the cave environment investigators must deal with a difficult and dangerous environment and still accomplish their goals while simultaneously protecting the biota.

7. Planetary Protection

Planetary protection considerations emerging from recent studies (e.g. MEPAG, 2006; Criswell et al., 2005; Hogan et al., 2006) advocate a localization and zoning of degrees of human impact, much like that being implemented in the Antarctic as Special Regions.

The July 2008 COSPAR document (COSPAR Panel on Planetary Protection, 2008). attempted to define ‘‘Special Regions'' by way of measurable parameters so that such metrics can be applied to specific areas on Mars that may need particular protection. Such an idea has already been put forward by MEPAG (2006), especially for temperature and water activity (biological availability of water to enter into biochemical reactions). Although Conley and Rummel (2010) point out that there is a consensus that future human exploration of Mars will inevitably result in some forward contamination, they further advocate monitoring and minimization as essential steps in controlling such contamination. These authors specifically advocate "selecting landing sites so that release of contamination will remain local". We suggest that containment of the primary human habitation and work activities within the confines of a subsurface habitat are highly consistent with this specific recommendation and with the new approaches to Planetary Protection forward contamination discussed here. Further, we have suggested that Earth caves with sensitive biological (especially microbiological) contents can serve as a particularly cogent testbed for the development of minimal impact or non-impact methodologies and protocols for extraterrestrial use because the same operating principles are also essential to our work with indigenous cave and subsurface microorganisms (Boston et al., 2006; Moser et al. 2001).

8. Caves as Potential Martian Habitat

We have argued that the intense radiation environment on the surface of Mars makes the subsurface the best place to look for any extant Martian life (Boston et al., 1992). In our view, it is also clearly the only currently practicable human habitat choice. Whether natural caves, artificial tunnels, or bermed structures are used, the problems of living in a subsurface environment may be more easily overcome than developing methods to ameliorate the effects of radiation as experienced on the surface. The problem of protecting humans on the surface in suits and transportation devices even for limited duration forays is intractable enough (e.g. Carr & Newman, 2008; Zeitlin et al., 2006; Cowing, 2004; Hodgson, 2001).

To explore the idea of the Martian subsurface as habitat, we have studied a number of aspects of the problem (Boston et al., 2004 a & b). This section provides a brief synopsis of the main points of those studies. The primary objective of these studies was to show that relatively simple, easily deployable subsurface habitats are constructible in caves, lavatubes, and other subsurface voids. Further, we wished to demonstrate that they are suitable to sustain small animals, plants, and ultimately humans in an otherwise hostile environment. The secondary objective was to show that humans can do useful work and scientific exploration in a subsurface environment facing some of the constraints that they will meet in the Martian environment including potential biologically sensitive sites. The third objective was to separate those features of a Mars or lunar subsurface mission that can be simulated in an Earth cave (e.g. Mars-derived breathing mixtures) from those that cannot (e.g. lunar or Martian gravity) to provide the basis for future work by other investigators.

We focused particularly on two aspects of the habitat issue: 1) the radiation protection provided by the rock overburden of caves, and 2) the ability of caves to provide a natural "pressure vessel" for the construction of subsurface habitats and their ability to hold breathable air. Secondary considerations included insulation from thermal oscillations, protection from impacting objects, sealability to contain a higher than ambient atmospheric pressure, and possibly access to potentially important subsurface resources, e.g. geothermal energy sources, water, reduced gases, and minerals.

The caves most suitable for human habitation purposes will be shallow, easily accessible, and relatively horizontal. Obviously, large spacious rooms and passages with smooth walls and floors are highly desirable. There are countless such examples on Earth in both lavatube and solutional caves (e.g. see Figure 5 for one that we have used in robotic access simulations, Dubowsky et al., 2004).

Figure 5: Hibashi Cave, Saudi Arabia. This lavatube cave has a smooth floor courtesy of blown in sand. We believe that the frequent dust storms on Mars probably have deposited a similar blanket of material on the floors of open lavatubes that may be there. Such a flat floor would be a valuable asset for a potential Martian habitat cave. Image courtesy of J. Pint.

Geological stability (very likely in the long history of relatively tectonically quiet Mars), and formation in relatively impermeable materials will provide safety and a degree of sealing against gas losses from habitat leaks. However, all rock (including human concrete structures!) fractures over time and we anticipate that such fractures must be sealed. We developed the idea of an inflatable liner that can conform to the natural shape of the cave. After inflation, a secondary step to rigidify the structure with foamed in place insulation, much as is done in conventional construction, is then conducted. An interior finish can be applied over these structural elements to provide a more human acceptable living space. Compatible with this construction methodology is the idea of foamed in place airlocks (Figure 6). We envision standardized rigid airlock assemblies that can be custom tailored to the precise topology of a cave cross section by a combination of rough cutting a skirting area surrounding the airlock, combined with another use for foamed in place technology to support and seal the airlock assembly.

Figure 6: Artist's conception of a foamed in place airlock assembly. A rigid airlock mechanism can be mounted in a cuttable frame. This can be trimmed to the approximate shape of a cave cross section, then filled in with rigidifying foam to seal the unit. Art courtesy of R.D. Frederick.

Natural openings are convenient but not essential as drilling can provide access to cavities through a few meters of rock. Location within a lower area like a canyon or a crater could provide additional protection from some surface conditions like large dust storms and a slightly higher ambient atmospheric pressure. Natural gas blowout features of lavatubes, known as hornitos, occur in Earth caves and may be present in Martian caves. Such natural entry holes are convenient and relatively easily modifiable. Resource-providing caves obviously must contain minerals, volatiles, or other assets. To be useful, they must provide nearer proximity to the resources than surface drilling and mining would afford. Additionally, they could be used as storage space for geothermal fluids, volatiles, and raw materials. Caves that contain volatiles will likely be naturally sealed from the outside low Martian air pressure.

Any residual geothermal activity within caves could be a potential source of power as can solar arrays deployed on the surface. Geothermal activity occurs in some caves on Earth, but on Mars we have little grasp of the planet's geothermal gradient, nor any current knowledge about active geothermal hotspots. For lighting, a combination of light piping of natural surface light with optical fibers from the exterior, and interior artificially powered lighting and temperature control will probably be necessary due to atmospheric opacity during dust storm events. Optical fiber or mirror light piping is a very convenient methodology pioneered in the mid 1990's (e.g. Swift & Smith, 1995). When fibers are used, they are now available for a multitude of applications and come in flexible or rigid form. Many of them can select specific wavelengths, and be designed to deliver all of their light to the end point or to attenuate the light along a length.

Lastly, not all of the cave interior must be given over to containment of breathable air and interior habitat. We also envision that low permeability sealed parent material on a cave interior would allow for filling parts of a cave with inert gases compressed (but not separated out) from the Mars atmosphere, thus allowing a "shirt-sleeve" environment in terms of pressure, but requiring thermal protection and oxygen breathing gear much as we currently use to work in Earth caves that contain poisonous gases like CO2, H2S, and others.

9. Conclusion

Mission planners cannot take particular options seriously unless the necessary background work is in place. While we have simply scratched the surface on many of the issues that we have consider in our studies (Boston et al., 2003, 2004a,b) , we hope that we have helped to lay a solid foundation that can be built upon further by ourselves and other interested colleagues. The Solar System is an amazing place filled with many spectacular natural features. We believe that it is time that caves took their place alongside the other natural phenomena worthy of exploration, study, and human utilization.

Acknowledgements: Thanks go to Gus (R.D.) Frederick for Figures 4 and 6. Appreciation to long time collaborators and cave partners Mike Spilde, Diana Northup, Kenneth Ingham, Jim Werker, and Val Hildreth-Werker. The author thanks NCKRI (the National Cave and Karst Research Institute) for support of various relevant field activities. Part of the work reported here was supported by the NASA Institute for Advanced Concepts (NIAC) grants #CP-01-01, and #CP-99-01 to the author, #CP 02- 02 to S. Dubowsky, and Subcontract 07605-003-020 to P. Todd. Fundamental geomicrobial studies of cave and rock varnish organisms and how we deal with them with minimally contaminating methodologies have been funded by the National Science Foundation EAR 0719669 and EAR 0311990 (P. Boston, PI), and LExEn DEB 9809096 (C. Dahm, PI).

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