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2 The Potential for Past or Present Habitable Environments on Mars Evaluation of the potential for living entities to be included in samples returned from Mars requires a careful consideration of the nature of past and present habitable conditions on the red planet, both at the surface and in the subsurface. Thus, future investigations that could help to reduce uncertainty in assessments of the potential for living entities to be present in returned martian samples include detailed site investigations prior to Mars sample return to define geological and environmental contexts and assess the potential for past or present habitable conditions. An important focus of the current surface robotic program for Mars is the discovery of past or present habit- able environments as a context for selecting sites for future life detection experiments and Mars sample return. It is important to continue such targeted site investigations in preparation for Mars sample return to provide insight into the kinds of samples that will be returned. It seems logical that to address the question of extant martian life, Mars sample return will be targeted to a Special Region1 where habitable conditions exist today, or may have existed in the recent past. In contrast, to explore for fossil biosignatures in ancient sediments, Mars sample return should be targeted to sites that are likely to have had habitable conditions in the past but that might not support extant life today. In either case, a detailed characterization of the site prior to Mars sample return would enhance understanding of the potential for samples to contain life, whether extant or fossil, and would provide essential context for interpreting sample data, if or when samples are collected, returned, and analyzed from that site. When considering the advances in understanding of Mars that have occurred in the past decade, it is important not to forget that the scientific environment for martian studies has undergone considerable change. A decadeâs worth of successful missions has caused significant growth in the size of the Mars exploration community and the scope of mission activities. The U.S. scientific community has played a highly active role in the definition of future science goals and mission plans through the activities of, for example, the Mars Exploration Program Analysis Group and the initiation of a solar system exploration decadal survey process. The Mars exploration community is now thoroughly international with, for example, the development of an ambitious Mars exploration program by the European Space Agency, a resurgence of Russian interest, and the initiation of exploration activities by new space powers such as China and India. The combined effect of all these trends has been a significant acceleration in the pace of acquisition of new information about the origin and evolution of the martian environment. 22
THE POTENTIAL FOR PAST OR PRESENT HABITABLE ENVIRONMENTS ON MARS 23 FOLLOWING THE WATER ON MARS Water is an essential requirement for terrestrial life. The history of water in all its forms is central to an understanding of the geologic and climatic history of Mars and for assessing the potential for past or present habitable environments on Mars. Water is also an essential resource that will be needed to sustain future human exploration of Mars. Discoveries over the past decade have revealed water to be abundant on Mars today, mostly in the form of surface and subsurface ice and, to a lesser extent, as ephemeral water films in soils, or as atmospheric water vapor. The Gamma Ray Spectrometer investigation onboard the Mars Odyssey orbiter confirmed high concentrations of water ice buried just a few centimeters below the surface in both hemispheres poleward of ~60Â° latitude. 2,3 The MARSIS radar experiment on Mars Express has shown that in many places this buried, ice-rich layer may be on the order of a kilometer thick.4 One of the major scientific results of the Phoenix mission was the in situ confirmation of this high-latitude, ground ice reservoirâsitting literally just beneath the spacecraft (Figure 2.1). 5 FIGURE 2.1â Image acquired by the Surface Stereo Imager on NASAâs Phoenix lander. The view is of a 22-centimeter-wide trench excavated by the landerâs robotic arm on Sol 18. The white material exposed in the floor of the trench is interpreted to be water ice. SOURCE: Courtesy of NASA/Jet Propulsion Laboratory-California Institute of Technology/University of Arizona/Texas A&M University.
24 ASSESSMENT OF PLANETARY PROTECTION REQUIREMENTS FOR MARS SAMPLE RETURN MISSIONS Investigations made of the residual north polar cap using the Mars Reconnaissance Orbiterâs (MROâs) SHARAD radar have shown it to be composed almost entirely of pure water ice. 6 It has been suggested that sub- limation of some of this water ice during the summertime significantly increases the global abundance of water vapor in the atmosphere.7,8 In addition, investigations from nearly all of the active Mars missions during the past decade have provided complementary evidence for the presence of minerals deposited under past aqueous conditions, some of which still contain chemically bound water. For example, observations with Mars Global Surveyorâs Thermal Emission Spectrometer led to the discovery of coarse-grained, crystalline hematite (typically formed in water) at Sinus Meridiani, the landing site for the Mars Exploration Rover Opportunity.9,10 In combination, Mars Odyssey, the Mars Exploration Rovers, Mars Express OMEGA, and MRO CRISM infrared spectrometer have discovered significant deposits of hydrated ferric oxides,11 hydrated sulfate minerals (Figure 2.2),12,13,14,15,16 hydrated phyllosilicate (clay) minerals,17,18,19 amorphous silica deposits,20 and putative chloride salt deposits.21 These mineralogical discover- FIGURE 2.2 Color image of bedrock outcrop called El Capitan, a finely layered, sulfate-rich deposit exposed in Eagle Crater, landing site of the Mars Exploration Rover Opportunity. The bluish-colored spherical grains are concretions 1 to 2 millimeters in diameter that have been cemented by an iron-oxide (hematite). Both the sulfates and the hematite were deposited from water. SOURCE: Courtesy of NASA/Jet Propulsion Laboratory and Cornell University.
THE POTENTIAL FOR PAST OR PRESENT HABITABLE ENVIRONMENTS ON MARS 25 ies indicate that a broad range of potentially habitable liquid water environments have existed on Mars over the planetâs long history. In addition to mineralogical discoveries made over the past decade of Mars exploration, there have also been important geological discoveries that have further enhanced the potential for the presence and duration of past aqueous environments. This evidence includes the discovery of: â¢ Small, morphologically âfreshâ gullies along the inner walls of many equatorial and midlatitude impact craters;22,23,24 â¢ Valley network and alluvial fan-like features that suggest past rainfall and surface runoff; 25,26 â¢ What appear to be river deltas within closed sedimentary impact basins that might once have held crater lakes;27,28 â¢ Widespread evidence (seen in high-spatial-resolution images) for rhythmically layered sedimentary rocks across much of the planet;29 and â¢ Fine laminations and shallow trough cross-bedding interpreted to have formed by aqueous sedimentation in sulfate-rich outcrops imaged by the Opportunity rover at Meridiani Planum.30,31 MARTIAN METHANE Another significant set of results from Mars that postdates the release of the NRCâs 1997 report Mars Sample Return: Issues and Recommendations 32 concerns the spectroscopic detection of methane in the planetâs atmosphere both by ground-based telescopes33,34 and by the Mars Express spacecraft.35 Although the spacecraft results are still somewhat controversial, the most recent and definitive ground-based measurements point to the presence of methane in the planetâs atmosphere at mixing ratios that vary between <3 parts per billion (by volume) and 60 parts per billion.36 Intriguing aspects of these latest findings include the following: â¢ At some times, the methane mixing ratio correlates with the mixing ratio of water in the martian atmosphere. But at other times it does not. â¢ The methane appears to be localized over certain geographic features including Terra Sabae, Nili Fossae, and the southeastern quadrant of Syrtis Major. â¢ The measured lifetime of the methane in the martian atmosphere is <4 years. This is significantly less than the ~350 years expected if the principal mechanism of loss is photodissociation. Although the origin of the methane has not yet been determined, possible sources include volcanic activity, chemical reactions between water and iron-bearing minerals in hydrothermal systems, and biological activity. Confirmation of the methane observations will be an important goal for future Mars orbiters and landers. Implications for Habitability Discoveries made during the past decade of Mars exploration hold profound implications for the past and present habitability of Mars and the potential that returned samples from Mars might include living entities, or their fossilized remains. For example, during the past decade, the presence of ground ice has gone from a hypothetical construct37 to an actual, measured realityâat least in the near surface. During this same period, there have been significant advances in understanding of the environmental limits of habitability on our own planet (see Chapter 3). Conditions for the origin and persistence of life are at present unknown. However, it is presumed that basic requirements for life include the presence of liquid water. Conservative estimates constrain lifeâs propagation to T > â25Â°C and a thermodynamic water activity 38 of aw > 0.5,39,40 although there is some limited evidence for the maintenance of metabolic activity at aw = 0.3.41 Surface environments in equilibrium with the current atmosphere of Mars do not appear to meet these basic requirements for habitability. And although liquid water could exist transiently as thin films at or near the surface, such circumstances are likely to be both rare and short-lived. However, habitable zones of liquid water could be present deeper in the subsurface,
26 ASSESSMENT OF PLANETARY PROTECTION REQUIREMENTS FOR MARS SAMPLE RETURN MISSIONS where pressures and temperatures provide conditions that favor the stability of liquid water. Some have modeled the presence of a martian deep hydrosphere,42 and others have suggested that if present, a martian subsurface hydrosphere should be dominated by salt-rich brines.43 Dissolved salts lower the freezing point of water, allowing it to remain liquid at temperatures below â25Â°C. However, the activity of water in concentrated brines is low, which limits the potential for life. The effects of salin- ity and water activity on the habitability of cold, hypersaline environments on Earth are still poorly constrained, 44 and the salinity of a putative martian subsurface hydrosphere is unknown. An improved understanding of both of these research areas will be crucial for refining exploration for habitable zones on Mars and in defining Special Regions as potential targets for Mars sample return. Insights Gained from the Study of Martian Meteorites Understanding of past and present environmental conditions on Mars has also been advanced through stud- ies of martian meteorites that have been found on Earth. The SNC meteorites (named for the Shergotty, Nakhla, and Chassigny meteorites that are representative of this class) are believed to have been ejected from Mars into heliocentric orbits by large impacts and subsequently captured by Earth. 45 The evidence for a martian origin is compelling, and a broad consensus now exists in the scientific community that this class of meteorites indeed came from Mars. To date, more than 30 martian meteorites have been found on Earth. This number has continued to increase each year through sustained, international discovery efforts supported by NASA, the National Science Foundation, and other agencies. During the past decade, significant advances in measurement capabilities have enabled the identification of accessory mineral assemblages in martian meteorites. This progress has yielded new insights into the nature of martian crustal environments and the role that water has played in the alteration of rocks and soils. Specifically, the identification of accessory mineral phases in SNC meteorites and precise measurements of isotopes for hydro- gen, carbon, oxygen, and sulfur for aqueous phases have provided important information about interactions in the atmosphere-regolith-water system. These observations have supported the development of models to specify the mechanisms by which surface sulfur is admixed into martian subsurface reservoirs. Results hold important implications for the history of water on Mars and the nature of past habitable environments and life. Similar mea- surements for other (nonmartian) meteorites have also extended the discussion of habitability in the solar system by showing that hydrothermal conditions once existed on some asteroidal bodies. 46 While water vapor was detected in Marsâs atmosphere via telescopic measurements and water-ice was unam- biguously detected at the martian north pole by Viking,47,48 the first direct measurement of the isotopic compo- sition of water in a martian sample was obtained by the stepwise thermal decomposition and release of water from the SNC meteorites Nakhla and Chassigny.49 That study revealed several important features, including the following: â¢ Water on Mars is not in equilibrium with the host rock, presumably due to the absence of plate-tectonic recycling of the crust; â¢ The composition of water in the martian regolith has evolved over time through groundwater circulation and precipitation of secondary mineral phases; and â¢ The carbonates observed in SNC meteorites were formed on Mars and precipitated by circulating fluids that constitute the subsurface water reservoir. Other measurements of the isotopic systems for hydrogen and carbon have added information about the precipitation of secondary alteration minerals, further refining the understanding of fluid compositions and their evolution.50,51,52,53 The carbon dioxide in the martian atmosphere possesses a highly specific isotopic signature, owing to its interaction with electronically excited atomic oxygen (O 1D), a product of the ultraviolet photolysis of ozone. The first direct evidence for interactions between the martian atmosphere and surface water reservoirs was provided by
THE POTENTIAL FOR PAST OR PRESENT HABITABLE ENVIRONMENTS ON MARS 27 Farquhar and colleagues.54,55 These studies demonstrated that SNC carbonates show a mass-independent, oxygen- isotope anomaly that cannot be completely ascribed to equilibrium exchange between martian surficial water and carbon dioxide. Rather, the isotopic signature of oxygen in carbonates derives from water in the surface reservoir of Mars that has interacted with isotopically anomalous atmospheric carbon dioxide (Figure 2.3). The development of secondary ion mass spectrometry (SIMS) has provided a means for measuring the isotopic composition of individual crystals within samples. The enhanced resolution possible with this method has signifi- cantly advanced the ability of researchers to discriminate between martian hydrological reservoirs and processes. For example, using this approach, Valley and colleagues observed the same isotopically anomalous oxygen isoto- pic signature in martian SNC carbonates observed previously by conventional bulk analysis methods. 56,57 These discoveries have provided additional support for the argument that the SNC carbonates record exchanges between the surface regolith and water reservoirs, through subsurface groundwater transport. This model also provides a mechanism for the active transport of oxidation products from surface to subsurface reservoirs, via circulating groundwater. This opens possibilities for redox-based energy sources essential for life, such as the oxidation of ferrous iron and reduced sulfur compounds. Other details have emerged regarding the martian atmosphere-regolith system with the measurement of all three oxygen isotopes in SNC sulfates.58 The measurement of the isotopic partitioning between silicates, carbon- ates, and sulfates in SNC meteorites has so far provided the best record of martian water-mediated geochemical FIGURE 2.3â Photomicrograph of a thin section of the martian meteorite ALH 84001 showing white-rimmed orange blebs of Fe-rich carbonate (siderite), a secondary mineral deposited from water. The view is about 0.5 millimeters across. SOURCE: Courtesy of Allan Treiman and the Lunar and Planetary Institute.
28 ASSESSMENT OF PLANETARY PROTECTION REQUIREMENTS FOR MARS SAMPLE RETURN MISSIONS processes. Laboratory investigations of the sulfur oxidative process and the concomitant isotopic partitioning 59 suggest that the predominant oxidants were hydrogen peroxide and/or ozone. This indicates that the primary oxi- dants had sufficient electro-negativity to impart their isotopic composition to any secondary minerals formed. If so, then subsurface water circulation would have been restricted, with water/rock ratios remaining low. Although the significance of these observations for the origin and persistence of putative subsurface martian life forms is still unclear, the isotopic record of secondary aqueous minerals (carbonates and sulfates) in the SNC meteorites provides a direct record of hydrological processes of great importance for assessing the long-term habitability of the martian subsurface. In summary, the application of new high-resolution isotopic methods to the study of martian meteorites sug- gests the following: â¢ Liquid water has existed in the martian subsurface over prolonged periods of geological time; â¢ Active exchanges between surface and subsurface water reservoirs maintained by groundwater circulation provided a means for the active transport of oxidants needed to maintain subsurface redox gradients; and â¢ The abundance of water was sufficient for authigenic mineral precipitation, but relative to the host rock, water volumes have remained low. CONCLUSIONS AND RECOMMENDATIONS The assessment of martian habitability made in the NRCâs 1997 report Mars Sample Return: Issues and Rec- ommendations led to its recommendation that: âSamples returned from Mars by spacecraft should be contained and treated as though potentially hazardous until proven otherwise. No uncontained martian materials, including spacecraft surfaces that have been exposed to the martian environment, should be returned to Earth unless sterilizedâ (p. 3). The present committee found that the knowledge gained from both orbital and landed missions conducted over the past decade, combined with findings from studies of martian meteorites, has enhanced the prospect that habitable environments were once widespread over the surface of Mars. In addition, the potential for modern hab- itable environments, both as transient surface environments and as stable habitats in the deep subsurface, is much better understood. This understanding has, in turn, enhanced the possibility that living entities could be present in samples returned from Mars. Therefore, the committee concurs with and expands on the 1997 recommendation that no uncontained martian materials should be returned to Earth unless sterilized. Recommendation: Based on current knowledge of past and present habitability of Mars, NASA should continue to maintain a strong and conservative program of planetary protection for Mars sample return. That is, samples returned from Mars by spacecraft should be contained and treated as though potentially hazardous until proven otherwise. No uncontained martian materials, including spacecraft surfaces that have been exposed to the martian environment, should be returned to Earth unless sterilized. The committee found that uncertainties in the current assessment of martian habitability and the potential for the inclusion of living entities in samples returned from Mars might be reduced by continuing research in the following areas: â¢ A vigorous program of remote-sensing and in situ exploration of Mars with the goal of answering questions relating to martian habitability, including those concerned with the presence of water in surface and subsurface environments through time, the distribution of biogenic elements, and the availability of redox-based energy sources (e.g., those based on the oxidation of ferrous iron and reduced sulfur compounds); and â¢ Continued studies of martian meteorites to help refine understanding of the history of interactions of Marsâs rock-water-atmosphere system throughout the planetâs history.
THE POTENTIAL FOR PAST OR PRESENT HABITABLE ENVIRONMENTS ON MARS 29 NOTES â 1â Special Regions are places where liquid water is present, or where the presence of a spacecraft could cause liquid . water to be present. â 2â W.V. Boynton, G.J. Taylor, S. Karunatillake, R.C. Reedy, and J.M. Keller, âElemental Abundances Determined via . the Mars Odyssey GRS,â Chapter 5, pp. 105-124, in The Martian Surface: Composition, Mineralogy, and Physical Properties (J.F. Bell III, ed.), Cambridge University Press, Cambridge, U.K., 2008. â 3â W.C. Feldman, M.C. Mellon, O. Gasnault, S. Maurice, and T.H. Prettyman, âVolatiles on Mars: Scientific Results . from the Mars Odyssey Neutron Spectrometer,â Chapter 6, pp. 125-152, in The Martian Surface: Composition, Mineralogy, and Physical Properties (J.F. Bell III, ed.), Cambridge University Press, Cambridge, U.K., 2008. â 4â G. Picardi, J.J. Plaut, D. Biccari, O. Bombaci, D. Calabrese, M. Cartacci, A. Cicchetti, S.M. Clifford, P. Edenhofer, . W.M. Farrell, C. Federico, A. Frigeri, D.A. Gurnett, T. Hagfors, E. Heggy, A. Herique, R.L. Huff, A.B. Ivanov, W.T.K. Johnson, R.L. Jordan, D.L. Kirchner, W. Kofman, C.J. Leuschen, E. Nielsen, R. Orosei, E. Pettinelli, R.J. Phillips, D. Plettemeier, A. Safaeinili, R. Seu, E.R. Stofan, G. Vannaroni, T.R. Watters, and E. Zampolini, âRadar Soundings of the Subsurface of Mars,â Science 310:1925-1928, 2005. â 5â P. Smith and the Phoenix Science Team, in preparation, 2008. . â 6â R.J. Phillips, M.T. Zuber, S.E. Smrekar, M.T. Mellon, J.W. Head III, K.L. Tanaka, N.E. Putzig, S.M. Milkovich, B.A. . Campbell, J.J. Plaut, A. Safaeinili, R. Seu, D. Biccari, L.M. Carter, G. Picardi, R. Orosei, P. Surdas Mohit, E. Heggy, R.W. Zurek, A.F. Egan, E. Giacomoni, F. Russo, M. Cutigni, E. Pettinelli, J.W. Holt, C.J. Leuschen, and L. Marinangeli, âMars North Polar Deposits: Stratigraphy, Age, and Geodynamical Response,â Science 320:1182-1185, 2008. â 7â B.M. Jakosky and C.B. Farmer, âThe Seasonal and Global Behavior of Water Vapor in the Mars Atmosphere: Com- . plete Global Results of the Viking Atmospheric Water Detector Experiment,â Journal of Geophysical Research 87:2999-3019, 1982. â 8â B.M. Jakosky and R.M. Haberle, âThe Seasonal Behavior of Water on Mars,â pp. 969-1016 in Mars (H.H. Kieffer, . B.M. Jakosky, C.W. Snyder, and M.S. Matthews, eds.), University of Arizona Press, Tucson, Ariz., 1992. â 9â P.R. Christensen, J.L. Bandfield, R.N. Clark, K.S. Edgett, V.E. Hamilton, T. Hoefen, H.H. Kieffer, R.O. Kuzmin, . M.D. Lane, M.C. Malin, R.V. Morris, J.C. Pearl, R. Pearson, T.L. Roush, S.W. Ruff, and M.D. Smith, âDetection of Crystal- line Hematite Mineralization on Mars by the Thermal Emission Spectrometer: Evidence for Near-surface Water,â Journal of Geophysical Research 105:9623-9642, 2000. 10â P.R. Christensen, J.L. Bandfield, A.D. Rogers, T.D. Glotch, V.E. Hamilton, S.W. Ruff, and M.B. Wyatt, âGlobal . Mineralogy Mapped from the Mars Global Surveyor Thermal Emission Spectrometer,â Chapter 9, pp. 195-220, in The Martian Surface: Composition, Mineralogy, and Physical Properties (J.F. Bell III, ed.), Cambridge University Press, Cambridge, U.K., 2008. 11â R.V. Morris, G. KlingelhÃ¶fer, C. SchrÃ¶der, D.S. Rodionov, A. Yen, D.W. Ming, P.A. de Souza Jr., I. Fleischer, T. . Wdowiak, R. Gellert, B. Bernhardt, E.N. Evlanov, B. Zubkov, J. Foh,, U. Bonnes, E. Kankeleit, P. GÃ¼tlich, F. Renz, S.W. Squyres, and R.E. Arvidson, âMÃ¶ssbauer Mineralogy of Rock, Soil, and Dust at Gusev Crater, Mars: Spiritâs Journey Through Weakly Altered Olivine Basalt on the Plains and Pervasively Altered Basalt in the Columbia Hills,â Journal of Geophysical Research 111:E02S13, doi:10.1029/2005JE002584, 2006. 12â S.W. Squyres, R.E. Arvidson, J.F. Bell III, J. Bruckner, N.A. Cabrol, W. Calvin, M.H. Carr, P.R. Christensen, B.C. . Clark, L. Crumpler, D.J. Des Marais, C. dâHuston, T. Economou, J. Farmer, W. Farrand, W. Folkner, M. Golombek, S. Gorevan, J.A. Grant, R. Greeley, J. Grotzinger, L. Haskin, K.E. Herkenhoff, S. Hviid, J. Johnson, G. KlingelhÃ¶fer, A.H. Knoll, G. Landis, M. Lemmon, R. Li, M.B. Madsen, M.C. Malin, S.M. McLennan, H. McSween, D.W. Ming, J. Moersch, R.V. Morris, T. Parker, J.W. Rice, Jr., L. Richter, R. Rieder, M. Sims, M. Smith, P. Smith, L.A. Soderblom, R. Sullivan, H. WÃ¤nke, T. Wdowiak, M. Wolff, and A. Yen, âThe Opportunity Roverâs Athena Science Investigation at Meridiani Planum, Mars,â Science 306:1698- 1703, 2004. 13â J.P. Grotzinger, R.E. Arvidson, J.F. Bell III, W. Calvin, B.C. Clark, D.A. Fike, M. Golombek, R. Greeley, A. . Haldemann, K.E. Herkenhoff, B.L. Jolliff, A.H. Knoll, M. Malin, S.M. McLennan, T. Parker, L. Soderblom, J.N. Sohl-Dickstein, S.W. Squyres, N.J. Tosca, and W.A. 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