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3 Surface Environment of Mars Despite an incomplete understanding of the Mars surface environment, it is generally agreed that conditions are extremely inhospitable to life from Earth. This chapter reviews various aspects of the surface environment, focusing on several that may have relevance to the issue of forward contamination, including both the growth of organisms from Earth on Mars and the lifetime of bioorganic matter deposited on the martian surface. SURFACE CHEMISTRY Our understanding of the chemistry and mineralogy of the martian surface is incomplete and is based primarily on (1) the Viking lander experiments; (2) evidence from the shergottite, nakhlite, and chassignite (SNC) meteorites; and (3) remote sensing data.l-4 Results from the gas chromatograph-mass spectrometer (GC-MS) have indicated the presence of 0.1 to 1.0 percent bound water in the soil. The Viking lander inorganic experiment detected most major elements heavier than magnesium and a number of minor elements. Several chemical species of biologic significance (C, N, H2O, P) were left undetermined and had to be inferred. Several analyses were obtained at each of two sites, and the remarkable similarity in composition of all the materials suggested that the material had been homogenized over the whole planet by repeated dust storms. Viking carried no mineralogy experiment, and so mineralogy had to be inferred. Two competing models for the mineralogy of the soil are that it (1) consists largely of ironrich clays or (2) resembles an amorphous, partly hydrated volcanic ash called palagonite. Although faint traces of 23
secondary minerals, such as carbonates, have been detected by spectroscopic analyses, the general lack of absorption features suggests that the soil is poorly crystalline. Part of our current understanding of Mars comes from the analyses of the SNC meteorites,5,6 which are composed of basalts that have crystallized from melts within the past 1.3 billion years. These meteorites were originally suspected to be of martian origin because there was no other plausible parent body that could have erupted basalts so recently. A martian origin appears to have been confirmed from analyses of gases trapped within the meteorites. The isotopic ratios of nitrogen, argon, and xenon are identical, within analytical error, to the ratios found in the martian atmosphere, as determined by Viking, and are distinctively different from those of any other known source in the solar system, including Earth. The SNC meteorites contain a variety of secondary minerals such as illite and smectite clays and water-precipitated salts such as calcium and magnesium carbonate, calcium sulfate, magnesium phosphate, and hematite (Table 3.1). Migration of these water soluble salts within the soil is suggested by the presence of cemented soil at the Viking sites. The chemistry of the soil as determined at the Viking sites is consistent with the mixture of the minerals found in the SNC meteorites, possibly with the addition of palagonite. Because of the mafic nature of the soil, and the basaltic composition of the SNC meteorites, the dominant rocks exposed at the surface are thought to be basaltic. TABLE 3.1 Mars Biogeochemistry from SNC Meteorites Water-Precipitated Shergottite Minerals Confirmed EETA79001 Nakhla Chassigny CaCO3 X X X Mg-bearing CaCO3 X MgCO3 X (Fe,Mn)CO3 X CaSO4·nH2O X X X (Mg)x(P04)y·nH2O X (Mg)x(SO4)y·nH2O X (Na,K)Cl X "Illite" X (K,Na,Ca0.5,H3O)(Al,Mg,Fe)2 O10[(OH)2,H2O] S,Cl-bearing micabole X Smectite X (Na,Ca0.5)0.3(Al,Mg,Fe)2-3 (Si,Al)4O10(OH)2·H2O Fe2O3·nH2O X SOURCE: James L. Gooding, Johnson Space Center, NASA. The chemistry of the soil is of particular biologic interest. One of the major surprises of the Viking missions was the failure of the GC-MS 24
to detect organics in samples to depths of about 10 centimeters, despite the expectation of finding at least some organics of meteoritic origin. The soils were also found to be oxidizing: 70 to 800 nanomoles of O2 were released upon humidification of the soil, and nutrients added to the soil were oxidized. Although the exact nature of the oxidants is unknown, they probably form as a result of (1) condensation on the surface of OH, HO2, and superoxides formed by ultraviolet (UV)-induced photolysis of water in the atmosphere and/or (2) UV-induced photolysis of water absorbed on soil particles.7 The depth to which the soil is oxidizing and is devoid of organics is not known, but much of the loose material near the surface is likely to be episodically turned over, exposed to the surface, and blown around the planet as a result of wind action. The expectation is, therefore, that the Viking results are applicable, in general, to loose, wind-deposited materials at the surface. ULTRAVIOLET AND IONIZING RADIATION Although on Mars no radiation with a wavelength of less than 1900 angstroms (Ã ) reaches the surface because of strong adsorption by CO2, in comparison to Earth the martian surface is only minimally shielded from longer-wavelength UV radiation.8 On Earth a deep ozone absorption band at 2550 Ã prevents most UV from reaching the surface. In contrast, ozone is present only at high latitudes in the martian winter hemisphere and in amounts typically in the range of 30 to 60 micrometer- atmospheres, an amount much smaller than that shielding Earth. These amounts of ozone on Mars can attenuate the UV to 10-3 as compared with 10-30 for Earth. At low latitudes and during the summer at high latitudes, there is essentially no attenuation, and the full solar flux at wavelengths greater than 1900 Ã falls on the martian surface unless attenuated by aerosols in the atmosphere. Significant reduction by scattering is expected only in the dust storm season, which lasts roughly one-quarter of the year. Thus, during the entire martian year, the UV flux is sufficient to sterilize the surface environment. Mars is less protected than Earth from ionizing radiation because Mars has no magnetic field and only a thin atmosphere. The main concern is with galactic cosmic rays (GCRs) and occasional solar flare particle fluxes. OCR heavy ions, although significantly less abundant than OCR protons, contribute most of the annual biological dose-equivalent of OCRs at the martian surface. Doses from secondary radiation also accrue within the upper 50 centimeters of the regolith. At low elevations, where the atmosphere provides maximum protection, the OCR doses approach annual limits allowed for humans but fall far short of values commonly certified for sterilization of food. Ionizing radiation does not appear, therefore, to be sterilizing for the short term, although the effects of such exposures over many years are unclear. 25
TEMPERATURE Temperatures are of particular biological interest because of their influence on the stability of water. Surface temperatures are determined mainly by latitude and season and by the properties of the surface, especially thermal inertia and albedo.9 Mean daily temperatures range from 215 K at the equator to 150 K at the poles. Daily excursions from the mean are controlled largely by the thermal inertia of the soil. Martian soils have very low thermal inertias compared with those of typical terrestrial soils, which, together with the thin atmosphere, cause the near surface to heat rapidly during the day and cool rapidly at night. As a result, equatorial temperatures can range from as low as 180 K at night to 290 K at noon. However, these daily fluctuations damp out rapidly at depth, such that at a few centimeters depth the temperatures remain close to the diurnal mean of 215 K. Temperatures at the poles remain close to 150 K, the condensation temperature of CO2, for most of the year. For a short period in midsummer, CO2 completely sublimes at the north pole, exposing a water-ice cap and allowing the surface temperature to rise to 200 to 210 K on the water-ice and possibly to 230 K on dark ground. At the south pole, only incomplete sublimation of the CO2 was observed during the year that Viking viewed Mars; even then, however, within the seasonal cap some ground was exposed, which rose to higher temperatures. Because the CO2 cap disappears for only a short period of time, the mean annual temperature at both poles is close to 150 K, and at depths greater than 1 to 2 meters, the ground remains permanently at this temperature. Any summer increase in ground temperature is restricted to shallower depths. WATER Estimates of the amount of water present at the martian surface have ranged widely in recent years.10-13 However, recent recognition of the efficacy of gas-dynamic escape and impact erosion in removing volatiles from the planet early in its history has undermined geochemical arguments for low water abundances and has led to greater credence of the higher geologic estimates based on the observed effects of water on the surface. Recent estimates suggest that if all the water that flowed across the surface during the last 3.8 billion years were spread evenly over the planet, it would form a layer tens to hundreds of meters deep. For comparison, all the water present at the surface of Earth would form a layer 2.7 kilometers deep. The total crustal inventory of water on Mars is difficult to assess, but it could be considerably larger than that which flowed across the surface. Identifiable near-surface reservoirs are the residual north polar ice cap, the polar layered terrains, and water absorbed in the regolith minerals. All these reservoirs are probably small in comparison to the total inventory. Most of the water is thought to occur as ground ice and, at depths greater 26
than 1 kilometer, as ground water. The atmosphere contains very little water (10-3 M), but it is close to saturation for nighttime conditions. For the average amount of water present in the atmosphere of 10 precipitable microns of water, the frost-point temperature is 200 K (corresponding to a vapor pressure of water of 0.1 pascal). Any part of the near surface of the planet where the temperature exceeds 200 K should be ice-free because of the slow sublimation of the ice over geologic time. At low latitudes, where mean annual temperatures exceed 200 K, the ground is generally expected to be ice-free to depths of a few hundred meters. However, anomalous combinations of albedo, thermal inertia, and porosity could result in nearsurface ice locally. At latitudes in excess of 30 to 40°, ice may be present at depths greater than 1 to 2 meters, the depth of penetration of the annual wave, because mean annual temperatures are below 200 K. At shallow depths small amounts of ice could be present only transiently as water vapor moves in and out of the soil in response to the seasonal temperature cycle. Although there are no direct measurements of ground ice at these high latitudes, there is abundant geologic evidence that, in contrast to low latitudes, ice is indeed present. The stability conditions just described are equilibrium conditions, and various events could result in the presence of ice in disequilibrium. If ice were buried beneath a few centimeters in equatorial soil, sublimation rates would be very low (about 10-5 gm cm-2 yen-1). If water were supplied at a rate greater than this, such as by volcanic action, then ice could accumulate near the surface, despite being in disequilibrium with the atmosphere. VOLCANISM Volcanism is of biologic interest because of the possibility of hydrothermal activity and because of its potential effects on the distribution of water. Evidence from both counts of impact craters and chemical analyses of SNC meteorites suggest that Mars has been volcanically active in the recent past and thus could still be volcanically active today. Crater counts suggest that parts of the surface could be as young as 108 years, a number consistent with the estimated ages of the SNC meteorites, which suggest that volcanic activity could have occurred as recently as 108 years ago; geologically this is very recent. However, even if the planet is currently volcanically active, the rates of volcanic activity must be orders of magnitude lower than those found on Earth, because young surfaces are so restricted in area. There is no direct evidence of current volcanic activity, such as thermal anomalies or volcanism, which might be accompanied by hydrothermal activity. Sites of such activity may be identified by venting of steam and/or local concentrations of hydrothermal minerals. Alternatively, all of the 27
hydrothermal activity might be associated with subsurface environments without any surface manifestations. FORMER CLIMATIC CONDITIONS ON MARS Although present-day Mars is very hostile to life, there are good reasons to believe that Mars has experienced more hospitable conditions in the past. The evidence is particularly strong for the very early history of the planet, during the times that life first started on Earth. For most of Mars' history, erosion rates would have been extremely low. However, terrains that date from the early period are highly degraded and commonly dissected by branching valley networks.14,11 The networks resemble dry terrestrial river valleys and are thought to have been formed by slow erosion owing to running water. Despite uncertainty about the precise conditions required for these valleys to form, it is probable that some combination of high heat flow and high surface temperatures is required. For small streams to flow any appreciable distance, the surface temperature must be at or above 0°C. To maintain such a temperature, an atmosphere of at least 1 to 2 bars of CO2 was probably required. It has accordingly been suggested that about 3.5 billion to 3.8 billion years ago the surface of Mars, being warm and wet, was hospitable to life.16 However, after this time most of the CO2 was permanently removed to form carbonates, and the surface of the planet evolved to its present cold, dry conditions. If life started during the early era, it might have survived at least for a time, either intact or as biochemical remnants in isolated niches such as in hydrothermal systems, subsurface brines, or endolithic environments. REFERENCES 1. Margulis, L., P. Mazur, E. Barghoorn, H. Halvorson, T. Jukes, and I. Kaplan. 1979. "The Viking Mission: Implications for Life on Mars." J. Mol. Evol. 14:223-232. 2. Arvidson, R.E., J. Gooding, and H. Moore. 1989. "The Martian Surface as Imaged, Sampled, and Analyzed by the Viking Landers." Rev. Geophys. 27:3960. 3. Gooding, J.R. 1992. "Soil Mineralogy and Chemistry on Mars: Possible Clues from Salts and Clays in SNC Meteorites." Icarus (in press). 4. Kieffer, H.H., T. Martin, A. Peterfreund, B. Jakosky, E. Miner, and F. Palluconi. 1977. "Thermal and Albedo Mapping of Mars During Viking Primary Mission." J. Geophys. Res. 82:424-429. 5. Pepin, R.O., and M.H. Carr. 1992. "Major Issues and Outstanding Questions." Pp. 120-143 in Mars. H.H. Kieffer, B.M. Jakosky, and M.S. Matthew (eds.). University of Arizona Press, Tucson (in press). 6. See Gooding, J.R., 1992. 28
7. Hunten, D.M. 1979. "Possible Oxidant Sources in the Atmosphere and Surface of Mars." J. Mol. Evol. 14:71-78. 8. Kuhn, W.R., and S. Atreya. 1979. "Solar Radiation Incident on the Martian Surface." J. Mol. Evol. 14:57-64. 9. See Kieffer, H.H., et al., 1977. 10. Baker, V.R. 1982. The Channels of Mars. University of Texas Press, Austin, 198 pp. 11. Carr, M.H. 1981. The Surface on Mars. Yale University Press, New Haven, Conn., 232 pp. 12. Pollack, J.B., J.F. Kasting, S.M. Richardson, and K. Poliakoff. 1982. "The Case for a Warm, Wet Climate on Early Mars." Icarus 71:203-224. 13. Baker V., R. Strom, V. Gulick, J. Kargel, G. Komatsu, and V. Kale. 1991. "Ancient Oceans, Ice Sheets and the Hydrological Cycle on Mars." Nature 351:589-594. 14. See Baker, V.R., 1982. 15. See Carr, M.H., 1981. 16. See Pollack, J.B., et al., 1982. 29