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D Excerpts from the 1978 Report* *Reprinted from the Space Science Board, National Research Council, 1978. Recommendations on Quarantine Policy for Mars, Jupiter, Saturn, Uranus, Neptune, and Titan. National Academy of Sciences, Washington, DC. 82

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2 Recommendations on Quarantine Policy for Mars Based on the Current Viking Findings The current NASA policy on the likelihood of growth of terrestrial microorganisms on Mars is based on the December 14, 1970, Space Science Board report, Review of Sterilization Parameter Probability of Growth (Pg). The report established the minimum conditions necessary to define a microenvironment on Mars that would support growth of the most "hardy terrestrial organisms." The conditions established were the following: (a) Water activity ( w) 0.95. (b) Temperature 0°C for at least 0.5 h/day. (c) Nutrients: At least small amounts of water-soluble nitrogen, sulfur, phosphorus, carbon (and/or light). pH values between 5 and 8. (d) Attenuation of uv flux by more than 103. (e) Antinutrients-absence of antimetabolites. All the above conditions must occur simultaneously, or nearly so. The report then proceeded to estimate the value of Pg, the "estimated probability that growth and spreading of terrestrial organisms on the planet surface will occur." The estimated value of Pg was 3 x 10-9, with less than one chance in a thousand that it exceeded 1 x 10-4. For the Viking project, NASA adopted a value of Pg = 10-6, some three orders of magnitude more favorable to growth than the best estimate of the review committee, but still two orders of magnitude less than the extreme upper limit. The adoption of this value required terminal heat sterilization of the entire Viking Lander but not of the Orbiter. The value remains NASA policy to date. 83

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I. VIKING FINDINGS PERTINENT TO QUARANTINE Estimating the likelihood of the growth of terrestrial organisms on Mars requires a comparison between the known physical and chemical limits to terrestrial growth and the known and inferred conditions present on or just below the Martian surface. Table 1 makes that comparison in abbreviated form. Appendix A discusses in fuller form the inferences that can be drawn from the Viking findings about those physical and chemical characteristics of the Martian surface that are pertinent to the question of the growth of terrestrial microorganisms. Orbital measurements have covered appreciable fractions of the planet's surface, but the two Landers (VL-1 and VL-2) have sampled only a few square meters of the surface at two subpolar sites. The biologically relevant experiments were conducted on soil samples acquired during the Martian summer and early fall from as deep as 6 cm below the surface. (In March 1977 a sample was acquired from a depth of 20 cm, but as of April 1977 an inorganic analysis is the only experiment that has been performed.) Nevertheless, certain extrapolations relevant to the quarantine question can be made with various degrees of confidence to other regions of the planet, to greater depths, and to other seasons of the year. TABLE 1 Limits for Growth of Terrestrial Organisms 1970 Conditions on Marsa Factor Refs. 2 and 3 Study Water activity ( w) >0.9 0 to 1 0.95 Water (liquid) — Required Not detected Temperature >-15°C +20 to -143°C (see text) 0°C pH 5-8 <11.5 Not known Ultraviolet 0.1 J cm-2 0.04 J cm-2 min-1 radiationb Ionizing radiationb — <500 rad/yrc 2-4 Mrad Nutrients See text and Refs. 2 and 3 Organic compounds ppb; most required elements detected (see text)d Antimetabolites None present Strong oxidants present (see text)d a Cf. Reference 1; uv flux data from Reference 18. b Limit for survival. Limits for growth are not known. c See p. 11. d At VL-1 and VL-2 sites. 84

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II. CONCLUSIONS ON THE LIKELIHOOD OF THE GROWTH OF TERRESTRIAL ORGANISMS ON MARS We turn now to a reassessment of Pg, the likelihood of the growth of terrestrial organisms on Mars. We will consider three regions separately: (1) subpolar areas within a few centimeters of the surface, (2) subpolar regions more than a few centimeters below the surface, and (3) the residual polar caps. Finally, we will discuss briefly, the likelihood that terrestrial organisms could survive transport at or above the surface from one region to another. A. Subpolar Regions within about 6 Centimeters of the Surface Our conclusion is that no terrestrial organisms could grow within a few centimeters of the surface in the regions lying between the two residual polar caps. We base this judgment on the following of the Viking findings: — The presence in VL-1 and VL-2 sample of strong oxidants. — The absence of detectable organic compounds, which (a) attests to the power of the oxidants and (b) renders unlikely the existence of the specific types of organic compounds required for terrestrial heterotrophic organisms. — The inability of physical shielding by a rock to eliminate the oxidants. Our conclusion is reinforced by three additional factors that were well known before the mission: — The unlikelihood of organisms being deposited in regions that receive sufficient visible light to support photosynthetic autotrophy without at the same time receiving lethal fluxes of ultraviolet radiation. * — The exceedingly low probability for the existence of liquid water with activity ( w) high enough to support terrestrial growth. — The fact that, even if liquid water were present, vegetative cells would be subjected to daily cycles of injurious freezing; and only vegetative cells can grow. * Although unlikely, the probability is not zero. Sagan and Pollack10 have calculated that, although the uv flux is attenuated several millionfold at 0.8 cm below the Martian surface, the flux of visible light would still be 3.8 x 102 erg cm-2 sec-1 at that depth. 85

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It is highly likely that the surface conditions enumerated above at the VL-1 and VL-2 sites prevail over the subpolar regions of the planet. This conclusion is based on 1. The similarity in the findings at two widely separated points for the elemental composition of the regolith and for the results of the organic analysis and the gas-exchange experiments. 2. The strong probability that the oxidants are derived from atmospheric reactions or atmosphere-regolith reactions. Accordingly, it is difficult to conceive of regions that would be accessible to terrestrial microorganisms and at the same time be capable of excluding the atmosphere. 3. The fact that the Infrared Thermal Mapper (IRTM) has mapped a sizable fraction of the Martian surface without detecting thermal heterogeneities significantly more favorable to terrestrial growth than those that we have reviewed in Appendix A. Viking has provided much information that was either not known beforehand or was known only with considerable uncertainty. None of this new information suggests that the Martian surface is less harsh to terrestrial microorganisms than was thought prior to Viking. * On the other hand, two pieces of information indicate that it is harsher than was thought previously: the lack of detectable organic compounds and the presence of strong oxidants even in regions physically shielded from uv. Our conclusion is that no terrestrial organism could grow under the conditions found by Viking to prevail on subpolar surfaces at the landing sites and none could grow under the conditions that are highly likely to prevail throughout the entire subpolar region. Few if any terrestrial organisms could grow in contact with even one of the adverse conditions cited, much less grow when exposed to all of them simultaneously. Although we cannot absolutely rule out the existence of oases capable of supporting terrestrial life, we believe, for the reasons cited, that the likelihood of their existence is extremely low. Unfortunately, we know of no quantitative basis for assigning a numerical probability to "extremely low" when no oasis has been detected and when the weight of evidence is that none can exist. And yet a numerical value for Pg is required in order to determine what * The demonstration by Viking that the atmosphere contains nitrogen answers an important question that was unknown previously. However, the ignorance prior to Viking of the existence of nitrogen was not a significant factor in prior estimates of the probability of growth of terrestrial organisms. 86

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procedures are needed to reduce the microbial burden on future spacer rift to Mars to levels that fulfill current COSPAR quarantine policy. Reluctantly, then, we recommend for these purposes, and these purposes alone, that NASA adopt a value of Pg < 10-10 for the subpolar regions of the planet within 6 cm of the surface. ** This number, which is more than four orders of magnitude below the current value of Pg, reflects the fact that Viking has found the conditions to be considerably harsher to terrestrial life than was heretofore assumed and has obtained evidence that renders the existence of oases far less likely than was heretofore assumed. B. Regions More than 6 Centimeters below the Surface of Subpolar Regions As mentioned, Viking conducted biology experiments and organic analysis on samples obtained from depths of 4-6 cm. Greater depths would be required. to reduce or eliminate the lethal surface conditions. The depths required are unknown chiefly because the relation between depth and the presence of oxidants is unknown. However, the maximum temperature falls rapidly with depth. In the northern hemisphere, even at a depth of 4 cm, the maximum temperature is estimated to be 20° below the minimum confirmed growth temperatures (-15°) observed for terrestrial organisms (Appendix B). By a depth of 24 cm, the maximum temperature is estimated to be -50°C, some 35° below the minimum confirmed terrestrial growth temperature. In the southern hemisphere, the maximum temperature at a depth of 24 cm is estimated to be -35°C, still 20° below the minimum terrestrial growth temperature.4,8 At increased depths there is an increased likelihood of encountering ice, the existence of which would enhance the possibility of liquid water. But water that is liquid below -20°C and is in equilibrium with ice has an activity ( w) below that which will support the growth of any known terrestrial organism capable of growing under the partial pressure of oxygen on Mars (Appendix B, Figure B.2).9 Thus, temperature alone seems an absolute barrier to the growth of any terrestrial organisms at depths below a few tenths of a meter. But again, sufficient uncertainties exist to preclude an absolute statement to this effect; viz., **We obtain this value by estimating probabilities of < 10-2 for the presence of liquid water of high enough aw, < 10-1 for the ability to survive multiple freezing and thawing, < 10-1 for the avoidance of lethal uv <<10-2 for the presence of organic compounds of appropriate types in appropriate concentrations, <<10-3 for the absence of powerful oxidants, and 0.1 that the deposited microorganism is an anaerobe. 87

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— Although the surface temperatures are derived directly from the orbital infrared measurements and are consistent with the direct meteorological measurements at the landing sites 1.5 m above the surface, the estimates of subsurface temperatures require assumptions about the thermal diffusivity of the soil. The range of error is estimated by Kieffer8 to be 5°C. This error would not be sufficient to change our conclusions, but larger errors are conceivable. — There could exist heterogeneities below the resolving power of the IRTM (a minimum of 2 km) that have higher temperatures. — Although there is extensive information on the minimum growth temperatures of terrestrial microorganisms, the remote possibility exists that some unknown organism has a growth minimum below -15°C. We view this as extremely remote because, as indicated in Appendix B, the number of species capable of growth diminishes drastically as the temperature is lowered below 0°C. Furthermore, growth below -15°C is tantamount to growth in 8 osmolal solute, conditions that even at ordinary temperatures preclude the growth of all except halophiles and osmophiles. — There is the remote possibility that there exists somewhere a narrow zone of subsurface that is deep enough to preclude oxidants and shallow enough to have temperatures high enough to support growth. Although these uncertainties prevent us from concluding that the possibility for growth is zero, we are still forced to conclude that subsurfaces of Mars are exceedingly harsh for terrestrial life. Accordingly, for the specific purpose of determining quarantine requirements for future Martian missions, we recommend that NASA adopt a value of Pg < 10-8 for subsurfaces in the subpolar regions of the planet. C. The Residual Polar Caps The arguments just presented for subsurface regions generally apply to the residual polar caps as well. As in the subsurface regions, the temperatures mapped by the IRTM are too low to permit the growth of known terrestrial organisms. However, thermal heterogeneities h have been detected. The maximum temperatures observed (237 K) are not high enough to permit the growth of earth organisms, but their presence raises the remote possibility that there exist .other undetected heterogeneities for which the temperature does rise high enough. But 88

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warmer regions will also be drier regions, because the increased vapor pressure associated with higher temperatures would cause water to distill rapidly from these regions and freeze out at the cold trap furnished by the remainder of the residual cap. The water ice itself in the residual caps constitutes a possible source of liquid water, provided that special conditions were present to permit that ice to liquefy rather than to sublime (e.g., freezing point depression by electrolytes). But even then, as in the case of subsurfaces, the temperatures would be too low to permit the growth of terrestrial organisms. The polar regions would not be immune from the atmospheric oxidants, but chemical interactions between atmosphere and ice might be different from chemical interactions between atmosphere and regolith. Our conclusions about the likelihood of growth in the residual polar caps are similar to those reached in Section B above for subsurface subpolar regions—it is extremely low. Nevertheless, because there is more uncertainty about the physical and chemical conditions at the residual polar caps, we believe that these regions should be handled with prudence and recommend that they be assigned a value of Pg <10-7. D. Transport from Subpolar Regions into the Residual Polar Caps or into Putative Oases There is little likelihood that any terrestrial organism could survive a voyage on or above the surface requiring more than a few minutes. First, the uv flux on the surface of Mars is 4 X 10-2 J cm-2 min- 1 , and that flux would kill the most resistant of terrestrial microorganisms in a few minutes (upper terrestrial limit 0.1 J/cm2) (Table 1). Second, organisms protected from the direct exposure to the uv by a layer of soil particles would nevertheless be in contact with the oxidants in those soil particles. One consequence of these lethal conditions is that our recommended value of <10-7 for Pg in the residual polar caps applies only to terrestrial organisms that are released directly in that region. The Pg for organisms transported into the polar caps from the subpolar regions would be orders of magnitude lower. Similarly, even if Mars were to possess oases that were hospitable to terrestrial life, few if any terrestrial organisms would survive a surface or aerial trip to the oasis and few if any would ever survive an escape from the oasis. 89

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III. LIMITS TO THE GROWTH OF TERRESTRIAL LIFE VERSUS THE QUESTION OF INDIGENOUS LIFE ON MARS The evidence that leads us to the conclusion that terrestrial microorganisms have little and in most regions of the planet no probability of growth does not rule out the possibility that indigenous life forms may exist currently on Mars or may have existed sometime in the past. The limiting conditions listed in Table 2 for terrestrial life are not the limits for conceivable life elsewhere. There is fairly wide agreement that life, if it exists elsewhere, is based on carbon chemistry and that it requires nitrogen; organic compounds of high information content, energy, and substrates to permit the synthesis of the organic compounds; and liquid water. Although, as discussed, organic compounds and liquid water have not been detected on Mars, there is no basis for precluding their existence. There is, moreover, strong evidence that liquid water in large quantities existed in the Martian past. It might be argued that, if indigenous life forms do exist, they themselves could constitute micro-oases for the growth of terrestrial organisms. We consider this unlikely. For example, a Martian organism growing in thermal equilibrium with its surroundings at -40°C is conceivable. However, to do so, a spherical organism 2 x 10-4 cm in diameter, for example, encased in efficient insulation 1 mm thick would have to assimilate and burn about 1000 times its steady-state concentration or organic compounds per second to maintain the 40- degree differential. The probelm would be only slightly less serious in a macroscopic Martian organism. Analogous difficulties arise in postulating that the organic compounds in putative Martian biota would be compatible with and utilizable by the enzyme systems of terrestrial microorganisms. 90

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TABLE 2 Estimated Contributions to Pg for Jupiter and Saturn 1974 1976 Jupiter Uranus Factor Report Report Comments Temperature 1 1 Assumed between -20 to 100°C Pressure 1 1 Not a critical parameter for microbiology Radiation 1 Not Deleterious specified but <1 Liquid H2O 1 1 Assumed 10-1 <10-3a Nutrients Organics, ions - aqueous solution 10-1 10-1 Anaerobiosis About 0.10 of the earth's microbes are anaerobes, but these are unlikely to be spacecraft contaminants 10-2 <10-4b NH3 <10-3a Growth in Not Completion of life cycle in the aerosols specified atmosphere has never been reported for any earth organisms 10-3 <10-3 Convection to All models predict that organisms lethal will be carried from water levels to temperatures lethal depths; the times required are somewhat model dependent 10-7 <10-14 TOTALS a Based on more detailed analyses.2,3 b New information, e.g., Reference 22. IV. CONCLUSIONS PERTINENT TO THE CURRENT VIKING ORBITERS As of August 1977, two years have elapsed since the unsterilized Orbiters were launched from earth. Any organisms on the outer surface of the Orbiter have surely been killed by uv irradiation. Most organisms in the interior of the Orbiter have been subjected to moderate temperatures (10 to 38°C), high vacuum, and some ionizing radiation.11 Although the cell dehydration associated with the high vacuum would be lethal to a fraction of the microbial population, many (perhaps 1 to 10 percent) would likely survive.6,12,13 Some protons from galactic cosmic rays and solar flares would strike organisms in the interior, but the dose would be 91

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appreciably less than 500 rad/year,11,14 and many microorganisms can survive such doses. (The flux of solar protons far exceeds that from galactic source, but the great bulk of the solar protons have energies of MeV,11 and such protons are only capable of penetrating 0.1 mm of material with a density of 1, e.g., water.14) Conservatively, then, one cannot assume that the microbial burden within the Orbiter has decreased by more than 1 or 2 orders of magnitude since launch. In spite of the expected survival of a fraction of the original burden of terrestrial microorganisms, our new estimates of the values of Pg lead to the conclusion that COSPAR requirements for planetary quarantine will not be compromised by lowering the periapsis of the Orbiters to 300 km. Indeed, with the new values for Pg, still lower periapses for unsterilized Martian orbiters may well be compatible with COSPAR requirements. NASA will probably wish to determine these minimum orbital altitudes before assessing and designing Mars follow-on missions in detail. V. QUARANTINE STRATEGY FOR FUTURE MISSIONS TO THE MARTIAN SURFACE Our Committee has recommended that the next phase in the biological exploration of Mars should be to acquire and characterize soil samples from areas likely to contain sediments and ice-regolith interfaces.1 Locating these areas and locating sites that are shielded from the powerful atmospheric ultraviolet radiation and the powerful surface oxidants will require subsurface sampling by a soft lander, by penetrators, or by both. The samples acquired from the subsurface of Mars should be characterized with respect to organic compounds, carbon and sulfur isotope ratios, the amount and state of water, the presence of water-soluble electrolytes, and the existence of nonequilibrium gas compositions. The greater the extent to which samples possess these characteristics the greater the priority for the initiation of a second phase of post-Viking biological exploration of Mars—a detailed search for evidence of present or past life on Martian samples returned to earth. With respect to quarantine considerations for the mission that conducts the first exploratory phase, our estimates for the values of Pg lead to the conclusion that terminal heat sterilization would not be required in the case of a nominal soft landing in the subpolar regions (Section A) and possibly in other cases as well. However, we would have no objections to sterilization provided that it has no impact on the scientific payload of the landers and that it does not increase the mission cost. (We have been informed by representatives of NASA that this may be the case.) Decisions on scientific payloads for the missions should be based on their scientific quality and cost effectiveness. We would object to the elimination of an experiment or the degradation of its performance because of the imposition of unessential sterilization requirements. 92

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In the report Post-Viking Biological Investigations of Mars,1 we stated that we consider metabolic-type life-detection experiments on the surface of Mars to be of low priority scientifically. Nevertheless, NASA may decide to include them. If so, a limiting factor with respect to the allowable microbial burden on a soft lander would likely become the avoidance of contaminating the metabolic experiment by terrestrial microorganisms. 93

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(Appendix D—continued) Appendix A: Findings from Viking Pertinent to the Possible Growth of Terrestrial Microorganisms on Mars* I. DEFINITIVE FINDINGS FROM VIKING A. Water The gas chromatograph-mass spectrometer (GCMS) has detected less than 0.1 percent water in soil samples (several tenths of a percent in one sample collected from beneath a rock). The current belief is that this water represents mineral hydrate water of moderate or low thermal stability. Neither this instrument nor the others on the lander were designed to detect free liquid water, nor have they done so. Unfortunately, Viking carried no instrument to measure relative humidity. However, indirect evidence (e.g., cloud formation) indicates that saturation does occur in the atmosphere. The probability for the existence of liquid water anywhere on the planet remains low. The surface temperatures and atmospheric pressures preclude the existence of pure bulk liquid water under equilibrium conditions. However, there continue to be three remote possibilities for the existence of liquid water: (1) water adsorbed to subsoil, (2) water that is liquid by virtue of kinetic factors slowing * Pertinent references not cited here will be found in Reference 1. 94

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the approach to equilibrium (i.e., conditions under which diffusion of water is slower than diffusion of heat), and (3) water that has its chemical potential (and hence freezing point) lowered by the presence of dissolved solutes. The solutes could be one or more of the several salts that are almost certainly present. The eutectic points of salts like CaCl2 MgCl2, and K2CO3 are below -30°C; hence their presence would permit stable liquid water down to these temperatures. The electrolyte concentrations, however, would be multimolar. Another argument against the existence of liquid water at the landing sites is the findings of the Labelled Release and Gas Exchange biology experiments. In both cases, the initial addition of water vapor or liquid water to the soil samples dissipated the reactants so that further additions produced no further reactions (release of 14CO2 and release of oxygen in the two experiments, respectively). Presumably, therefore, no reactions at all would have been observed if the soil itself had been exposed to high-activity liquid water just prior to the acquisition of samples by the Landers. B. Temperature The maximum temperatures observed at the surface of the landing sites during the summer-autumn observation period were -2 to -3°C. This is below the minimum growth temperature of most terrestrial microorganisms, although, as discussed later and in Appendix B, a few terrestrial organisms can grow at temperatures as low as -14°C. In the southern hemisphere of Mars, the maximum summer surface temperatures may reach 20°C.4 At night, even in summer, the temperature drops to -83°C. Dry bacterial and fungal spores could survive many cycles of such freezing, but hydrated and germinated spores or vegetative cells of most terrestrial species could not.5-7 And any terrestrial microorganism that is to grow on Mars must by definition be in the vegetative state to carry out such growth. C. Lack of Detected Organic Compounds No organic compounds other than traces attributable to terrestrial contaminants have been detected in regolith samples analyzed by the GCMS. If volatizable organic compounds were present in the samples, they were either present in concentrations below the parts per billion range (the detection limit of the instrument) or they were totally restricted to substances like methane 95

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with molecular weights of less than 18, which were undetectable or detectable only at reduced sensitivities. The inability to detect organic compounds does not, of course, prove that none was present. But even if trace amounts of organic compounds are in fact present in the soil of the landing sites, the probability is remote that these would provide a nutrient medium that could be used by terrestrial microorganisms (see Reference 3 for further discussion). D. Elemental The biologically vital element nitrogen has now been shown to be in the Martian atmosphere. Calcium, sulfur, magnesium, chlorine, and probably potassium and phosphorus have also been detected in soil samples. All six are essential to terrestrial living systems. Instrument limitations precluded the detection of sodium, but there is no reason to believe that it is not present, although probably only in low concentrations. One striking finding is that the elemental composition of the samples was nearly identical at the two widely separated landing sites.26 This similarity indicates that the fine-grained material in at least the upper surfaces of the regolith has been thoroughly mixed over large regions of the planet-presumably as the result of wind action.27 E. Oxidants Two lines of evidence indicate that strong oxidants are present in at least the top few centimeters of the regolith at the landing sites. The first line of evidence comes from orbital measurements of the atmosphere and from modeling. One model. predicts the existence of active strongly oxidizing species, especially hydrogen peroxide: Second, the gas-exchange experiment (GEX) on the Viking landers showed the release of up to nearly a micromole of oxygen when samples were humidified with water and warmed to ~10°C. The GEX experiment suggests that oxidants are present to at least the 4-6-cm depth from which samples were acquired. The experiment also showed that the oxidants were present in samples collected from beneath a rock, a rock that presumably had laid undisturbed for many years. Finally, the experiment showed that the oxidants were present at both landing sites.28 The oxidants are believed to be responsible for the lack of detectable organic compounds, i.e., they have decomposed them. 96

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II. EXTRAPOLATION FROM THE VIKING FINDINGS TO THE PLANET'S SURFACE AS A WHOLE, TO REGIONS BELOW THE SURFACE, AND TO OTHER SEASONS OF THE YEAR Surface temperatures in the Martian winter will drop far lower than those experienced during the Lander experiments. Estimates from the infrared thermal mapper (IRTM) indicate that the maximum surface temperature will fall below -15°C (the minimal terrestrial growth temperature—see Appendix B) for more than half the Martian year at the VL-2 site (48° N) and further north. At VL-1 (22° N) the maximum surface temperature will just about reach - 15°C in the winter.8 (Orbital IRTM measurements during winter will become available during the ensuing months.) In considering extrapolations from the findings of VL-1 and VL-2 on surface chemistry, we note that, although the two landing sites (22° N and 48° N) are separated by some 1500 km in latitude and 176 degrees-in longitude, the results of the gas-exchange (GEX) and labeled release (LR) biology experiments and of the organic and inorganic analyses at the two sites were either similar or essentially identical*. Strong similarities were evident as well from the imaging experiment and from the atmospheric analyses. As noted, the results of the GEX, LR, and GCMS experiments are consistent with the presence of powerful oxidants in the surface samples. Since these oxidants are almost certainly derived from atmospheric photochemical reactions or from chemical reactions between atmospheric species and the regolith, there is every reason to expect that they will be globally distributed in the Martian surface, except possibly in the residual polar caps. Certain extrapolation can also be made to depths below the 4-6 cm sampled by the Landers. A. Water Several Viking experiments have confirmed or strengthened the inference that large amounts of water are locked beneath the surface in the form of ice. Subsurface liquid, water is conceivable; however, because of the low temperatures at subsurfaces (see below), the existence of liquid water in an equilibrium state would * Samples from the two sites in the Pyrolytic Release (PR) experiment, however, responded differently to the addition of water vapor.25 The experimenters suggest that this reflects differences in the properties of the soil at the two sites, but they draw no inferences as to the nature and degree of the differences. 97

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require multimolar concentrations of electrolytes (see Appendix B, Table B.1). B. Temperature The maximum summer temperatures some 6 cm below the surface at the VL-1 and VL-2 sites are estimated from the IRTM measurements to be -35°C.8 This temperature is 20° below the minimum confirmed growth temperature for terrestrial microorganisms. It is even below the lowest growth temperature ever claimed in published reports. At a depth of 24 cm, the maximum summer temperatures at the VL-1 and VL-2 sites are estimated to be -50°C, or 35° below the minimum confirmed terrestrial growth temperature. In the southern hemisphere as a result of the eccentricity of the Martian orbit, the maximum surface-.temperatures between latitudes 5° and 45° are about 15° warmer than at the present landing sites. As a result, at subsurface depths sufficient to damp out diurnal variations, the maximum summer temperature is calculated to be about -35°C, still some 20° below the minimum confirmed terrestrial growth temperature.4 C. Ultraviolet Light As shown in Table 1, the flux of ultraviolet radiation impinging on the Martian surface would be rapidly lethal to any terrestrial organism. However, the uv flux is sharply attenuated below the surface. For example, Sagan and Pollack10 estimate an attenuation of several millionfold at a depth of 0.8 cm. D. Oxidants and Organic Compounds Since the oxidants in the regolith are almost certainly derived from atmospheric processes, their concentrations ought to diminish with depth below the surface. But the relationship between depth and concentration is unknown. Presumably at least some of the oxidant species are diffusable, for they were present in the soil samples collected from beneath the rock at the VL-2 site. Since the lack of detectable organic compounds within 4-6 cm of the surface seems due to the presence of the oxidants, the likelihood of organic compounds ought to increase with depth. (Organic matter must be present at least transiently on the Martian surface, if from no other source than the infall of carbonaceous chondrites.) Although the Martian surface is strikingly similar at two widely separated points when viewed close-up from the two Landers, the surface 98

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is strikingly heterogeneous when viewed from orbit. Still, there is no evidence that any of the heterogeneities represent oases that possess characteristics more favorable to terrestrial life than those already enumerated. One dramatic class of heterogeneities, for example, is the huge channels that were almost certainly formed by flowing liquid water. But these channels are too old (probably 109 years) to have much bearing on their current suitability for the growth of terrestrial organisms, except that they might possibly contain concentrated deposits of electrolytes and organic compounds. The orbital infrared temperature and water-vapor measurements also show heterogeneities, but again none of those detected have properties significantly more favorable to terrestrial life than do the larger- scale features. The resolving power of the IRTM is 0.3°, which translates to 8 km at the normal periapsis of 1500 km and 1.6 km for the now lowered periapsis of Orbiter 1 (300 km). Smaller oases with respect to some of the biologically relevant factors are conceivable (e.g., higher temperatures on south-facing slopes in the northern hemisphere; higher temperatures because of heat absorbed by dark objects). It is difficult, however, to conceive of any oasis on the surface of subpolar regions that would be accessible to terrestrial organisms and yet not contain the atmospherically induced oxidants. As mentioned, the subrock sample at VL-2 indicates that some of the oxidants can diffuse in the regolith. 99

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(Appendix D—continued) Appendix B Minimum Temperature for Terrestrial Microbial Growth The most thorough review known to us of the minimum growth temperatures of terrestrial microorganisms is that of Michener and Elliott.15 A histogram summarizing their findings on reports of growth below 0°C is shown in Figure B.1. Many of these reports are based on incubation times of over a year. We separate bacteria from fungi because the latter are nearly all aerobic and would be incapable of growing at Martian partial pressures of oxygen. The single case of a bacterium growing below -12°C was a report of growth at -20°C. Neither it nor the three reports of fungal growth below -12°C have been confirmed. Michener and Elliott point out that "The best evidence that growth does not generally occur in foods in this temperature range [i.e., <-17°C] is that billions of cartons of frozen food have been stored at or near -18°C without reported microbial spoilage." A more recent study by Fennema et al.16 confirms Michener and Elliott's conclusion that microbial growth in foods does not occur at -18°C. This inability of organisms to grow below about -15°C is consistent with the known physical state of aqueous solutions at these temperatures. As Table B.1 shows, when solutions of sodium chloride in water, for example, are equilibrated at various subzero temperatures, the concentrations in the unfrozen portions exceed 4 molal below -15°C. For solutes in general, the concentrations of solutes in the unfrozen portions of solutions are given by vm = T/ 1.86 where is the osmotic coefficient, v the number of species into which the solute dissociates, and m is the molality.17 Aside from the toxic effects to nearly all microorganisms of such high 100

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FIGURE B.1 Reported cases of microbial growth below 0°C. (Adapted from Reference 15.) 101

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FIGURE B.2 Water activity w = Pice/PH2O of a solution in equilibrium with ice as a function of temperature. concentrations of electrolytes, the high concentrations also depress the water activity (αw) below the value permitting the growth even at optimal temperatures of all microorganisms save halophilic and osmophilic forms. As shown in Table B-1 and Figure B-2, the values of aw at -14, -16, -18, and -20°C are 0.87, 0.85, 0.84, and 0.82, respectively. 102