Environmental Requirements For Life

Life requires energy sources, the nutrients necessary for building structures and synthesizing catalysts, and access to environments in which biosynthesis and maintenance of biostructures are possible. Potential energy sources for life on Mars include the direct and indirect utilization of solar radiation, lightning, ionizing radiation, geothermal heat, and various redox couples involving carbon or inorganic compounds. As discussed by Price and Sowers,1 the energy requirements for survival are likely a million times less than the energy required for growth. The concept of survival energy, that which is required for repair of macromolecules, maintenance of ion gradients, and so on,2 implies that life could survive, although not multiply, in a much wider range of environments than was formerly thought possible.

The availability of nutrient elements may also impose limits to life. As mentioned in Chapter 1, it is reasonable to assume that martian life, if any, would be based on carbon (C), hydrogen (H), nitrogen (N), oxygen (O), phosphorus (P), and sulfur (S). Mars was built from the same carbonaceous chondritic material as Earth. The same elements would accordingly be available to be utilized by life on Mars as on Earth unless processes on Mars caused their depletion. C, H, N, O, P, and S have all been detected on Mars. But the amount of nitrogen is a potential problem.3 The martian atmosphere contains only 160 microbars (mbar) of N2, and nitrates have not been detected in the soils. Much of the original nitrogen inventory could have been lost by impact erosion of the atmosphere during heavy bombardment, subsequent sputtering by the solar wind, or photochemical processes. Access to organic carbon is another potential problem. While CO2 is the dominant species in the atmosphere, reduced carbon compounds may be rare at the surface, despite continual delivery of organic compounds to the martian surface in meteorites and interplanetary dust particles. Nevertheless, reduced carbon compounds have been detected in the martian meteorite ALH 84001. There is little reason to assume that the availability of other elements on Mars is significantly different from their availability on Earth.

Since no martian life has been detected, it is not possible to determine what its environmental requirements are. However, current understanding of the limits of terran life continue to expand as living forms are found in

FIGURE 2.1 The layers in Candor Chasma mimic those created by sedimentation in underwater environments on Earth. This image from the Mars Orbiter Camera on the Mars Global Surveyor spacecraft is courtesy of NASA/JPL/Malin Space Science Systems. The image covers an area of 1.5 by 2.9 km.

ever greater extremes of temperature, pressure, pH, salinity, and so forth.4 One environmental factor that is almost universally accepted as necessary for life is at least episodic access to liquid water. This observation has implications for temperature but does not necessarily imply temperatures between 273 K and 373 K. Although there is theoretical evidence that metabolism can continue at temperatures at or below 233 K, there is no evidence of active cells below 253 K and no direct observation of cell replication below 248 K.56 At the high-temperature end, life has been cultured in the laboratory at 394 K,7 although evidence from deep-sea hydrothermal vents indicates that the upper temperature limit for life may be much higher than this.8 Apart from temperature, two other parameters critical to the survival of terran life are worthy of note—water activity and radiation resistance. The availability of liquid water to an organism is critical for its survival. Many organisms are resistant to desiccation; however, currently no organisms are known to survive at water activities lower than aW = 0.61.9 Therefore, temperature and water activity have recently been used to constrain regions on Mars associated with special planetary protection considerations.10

DOES MARS MEET HABITABILITY REQUIREMENTS?

The following discussion focuses on the availability of water. The committee assumes that solar energy, geo-thermal energy, and chemical energy, as well as nutrients, are available on Mars. Sources of energy and nutrients are present on Mars as they would be on any geologically active planet, although questions about the availability of nitrogen have not been resolved.11 A major unknown is where and when liquid water was available to enable the assets present to be used for a possible origin or maintenance of life.

Present Environmental Conditions

Conditions on the surface of Mars today are very inhospitable for life, but geological evidence suggests that conditions were more hospitable in the past, particularly the distant past. Liquid water is believed to be essential for life. With mean annual surface temperatures close to 215 K at the equator and 160 K at the poles, the ground is frozen on average to a depth of several kilometers to form a thick cryosphere. Any water present in this zone would be frozen. The cryosphere might be thinner locally in areas of anomalously high heat flow, but no such areas have been identified. The atmosphere is thin, with an average surface pressure of 5.6 mbar, and composed largely of CO2. Because the atmosphere is so thin, the Sun's ultraviolet radiation passes almost unattenuated through it to the surface. Surface temperatures fluctuate widely during the day. On a clear summer day they may exceed 273 K close to noon. However, the fluctuations damp out rapidly at depth to converge on the average daily temperature, which is everywhere well below freezing, so that temperatures above freezing are restricted to the upper few centimeters. The ground is permanently frozen down to a kilometer or so below these depths.

Under present conditions, and probably under conditions that have prevailed for the last few billion years, weathering rates have been extremely low. Rocks in the Gusev crater have a millimeter-thick, oxidized rind, rich in volatiles such as sulfur, chlorine, and bromine.1213 Soils have highly variable volatile contents and may contain an oxidizing agent. The fraction of organics in the soil is unknown. Although it was anticipated that the Viking gas chromatograph-mass spectrometer would detect some complex organic compounds, none were detected at the parts per billion level, and the Mars Pathfinder APXS measurements of soil could not detect carbon. The nitrogen content of the soils is also unknown.

Both poles have a residual water ice cap that is exposed when the CO2 cap recedes in the summer (see Figures 3.1 and 6.1), although in the south only small areas are exposed even at midsummer.14 Because of the cold polar temperatures, only minute amounts of water vapor are present in the atmosphere.15 Observations of seasonal frost and water fog in some areas on Mars demonstrate that the water content of the atmosphere varies both spatially and seasonally. However, if all the water vapor were precipitated out, it would form a global layer only about 10 pm thick. Abundant ground ice, however, may be present and available to interact with the atmosphere, and to enhance its water content should conditions change.16 Under present conditions, at depths greater than a few tens of centimeters below the surface at latitudes in excess of 50° north and south, water ice is stable. Consistent with these conditions, large fractions of ice have been detected just below the surface at these latitudes by orbital gamma-ray and neutron spectroscopy.17 At lower latitudes ground ice is unstable at all depths because temperatures exceed the 200 K frost-point temperature. The cause of the several percent water detected at these low latitudes is still being debated. It could be mineralogically bound water or ice inherited from an earlier era when stability relationships were different.

Parts of the surface are at elevations where the atmospheric pressure exceeds 6.1 mbar, the pressure at the triple point of water. At most locations, heating of ice-containing soils or surface frosts would result in sublimation without the intervening liquid phase, but where the pressure is in excess of the triple point, liquid could form transiently. Because of the low diurnal mean temperature, any such liquid would be very short lived. It would rapidly freeze and sublimate.

Although the present-day average climate is not conducive to the occurrence of liquid water, the possibility exists that liquid water can occur at the surface as a transient phase. Gullies appear to have released water to the surface, for example, and recent observations (Figure 2.2) suggest that this is happening in the present epoch (i.e., within the last 5 years or so).18 Similarly, transient melting of snow also can occur under very specific conditions.19 While the ramifications of transient liquid water are very different from those for a steady-state occurrence of liquid water, both have potential implications for possible life.

In the recent geological past, stability relationships may have been somewhat different. Mars undergoes large changes in its obliquity (i.e., the tilt of its polar axis).20 At present the obliquity ranges from 23 to 27° about a mean of 25°, but during the last 10 million years obliquities have been at least as high as 46°. At higher obliquities, the water content of the atmosphere is likely higher, ground ice is stable closer to the equator, and surface ice may be transferred from the poles to lower latitudes. In addition, during the summer at high latitudes, pole-facing slopes are continually illuminated by the Sun. One possibility for the formation of young gullies (see Figures 2.7 and 8.1) on steep, pole-facing slopes is that they form during periods of high obliquity as a result of liquid water produced during the summer by melting of snow that accumulated on the slopes during the long cold winter.21 High obliquities may also be implicated in the formation of some of the younger valley networks.

On present-day Mars, the subsurface may be more hospitable to life than the cold, oxidizing surface with its high ultraviolet (UV)-radiation fluxes. As indicated above, the cryosphere is on average several kilometers thick, and liquid water is unlikely within kilometers of the surface. However, the young crystallization ages of most martian meteorites22 and the apparent youthfulness of some volcanic features suggest that Mars today is volcani-cally active, at least intermittently. Heat flow under volcanic regions such as Tharsis may be significantly larger than the average, and the cryosphere correspondingly thinner. Moreover, given the presence of extensive ground ice, hydrothermal activity is likely in volcanically active areas, although none has been detected, and such activity (or even the background geothermal heat flux) could drive water to the surface.

In summary, present conditions at the surface of Mars are inhospitable to life, mainly because of the high UV flux, the presence of oxidants, and the scarcity of organic compounds, and because the low temperatures inhibit the presence of liquid water. However, liquid water may exist near the surface transiently in anomalous situations, ground water may be present at shallow depths in areas of anomalously high heat flow, and hydrothermal systems may be present in volcanic regions. Furthermore, at depths below a couple of kilometers, temperatures will be warm enough to allow liquid water in the pore space in rock, such that a deep-subsurface biosphere is possible, provided that appropriate nutrients are accessible and water can circulate.

Past Environmental Conditions

Noachian

Conditions in the geological past, particularly the distant past, were likely, at least at times, to have been very different from present-day conditions. The best evidence for different conditions is for the Noachian, the period of heavy meteorite bombardment that ended around 3.8 billion years ago (Box 2.1). Most surfaces that date from this era are heavily dissected by networks of valleys a few kilometers wide but up to a few thousand kilometers long. Relatively high drainage densities suggest surface runoff that would require either rainfall or melting snow, which in turn implies significantly warmer and wetter conditions than those that prevail today (Figure 2.3). The

FIGURE 2.2 A gully (see arrow) in the wall of an unnamed crater (A) in Terra Sirenum (36.6° S, 161.8° W) provides tantalizing evidence that water might have flowed on the martian surface in recent times. Close-up images (B) clearly indicate that sometime between December 2001 (left) and August/September 2005 (right) a new, light-toned deposit filled the gully. The thinness of the deposit and its multilobed appearance at its downhill end (C) suggest that material of some sort flowed in a fluid-like manner down the crater wall and then splayed out when it reached the relatively flat crater floor. These characteristics can be interpreted as suggesting that a mixture of sediment and a fluid with the properties of liquid water emerged from the crater wall and ran down through a preexisting gully channel within the last 5 years. These images were taken with the Mars Orbiter Camera on the Mars Global Surveyor spacecraft and are courtesy of NASA/JPL/Malin Space Science Systems.

belief that Mars was warmer in the Noachian than it is today is also supported by the finding of evaporites at the Mars Exploration Rover landing site on Meridiani Planum and elsewhere, by evidence for fluctuation of the water table at Meridiani, by detection of clay minerals from orbit in Noachian terrains, by the presence of hydrothermally altered rocks in the Columbia Hills at the Mars Exploration Rover landing site in Gusev crater, and by surface erosion rates that were 4 to 5 orders of magnitude higher than they were subsequently.23 These observations, in combination, suggest an Earth-like, active hydrological cycle with large lakes or oceans that acted as evaporative sources, sinks, and base levels for erosion. Given the likely large inventory of water at the surface, if Mars did have periods with an active hydrological cycle, oceans could have been present in lows such as Hellas and the northern plains (see Figure 7.1). However, although shorelines have been tentatively identified around both these lows, the observational evidence for the postulated oceans is weak.24 In contrast, there is abundant evidence, such as deltas, of lakes in local lows within the uplands (Figure 2.4).

Although dissection of Noachian terrain is widespread, several morphological characteristics of the drainage basins suggest that, compared with those of Earth, the drainage system is immature.25 The morphology of the Noachian terrains is dominated by primary terrain-building processes, such as impacts, volcanism, and deformation, rather than by fluvial processes. Even for the Noachian, for which researchers have the best evidence of abundant liquid water at the surface, the conditions necessary for fluvial erosion may have been achieved only episodically.

The Noachian was also characterized by high rates of volcanism and high rates of impact. The formation of large impact basins such as Hellas and Argyre would have had devastating effects on any emerging life.26 Large fractions of any oceans present would have boiled away, and the planet would have been enveloped in hot-rock vapor that would have condensed and rained hot rock back onto the surface. Such global catastrophes would, however, have been separated by millions of years of relative quiet even in the era of heavy bombardment.

Although Mars appears to have had benign periods during the Noachian, when water flowed freely across the surface, the cause of the warmer conditions remains unknown. A thick CO2-H2O atmosphere may be incapable of warming the surface to above freezing without additional forms of heating such as infrared absorption by dust in the atmosphere.27 Moreover, thick carbonate deposits that would contain CO2 from a thick, early atmosphere have not been found despite intensive searches using orbital spectroscopy, although the CO2 may have been lost to space instead. Another possibility is that large impacts or large volcanic eruptions episodically altered surface conditions temporarily,28 thereby briefly stabilizing liquid water at the surface. At the end of the Noachian, the rate of formation of valley networks declined rapidly, although not to zero, erosion rates fell precipitously, and clay minerals appear to have stopped forming.29 There can be little doubt that a major change in surface conditions occurred at the end of the Noachian.

Post-Noachian

The post-Noachian period, which encompasses roughly the last 3.8 billion years, is characterized by very low rates of weathering and erosion. The most characteristic fluvial feature of the post-Noachian era is the outflow channel, formed by large floods, rather than the valley networks that characterize the preceding era (Figure 2.5). Nevertheless, young valley networks are found in places, such as on young volcanoes, indicating that conditions necessary for slow erosion by running water were occasionally and locally met.

Large flood channels are readily recognizable by the scoured floors, streamlined walls, and tear-drop-shaped islands.30 The largest are around the Chryse basin, into which several enormous channels converge. Peak discharges may have ranged as high as 108 m3s-1, as compared with 104 m3s-1 for the Mississippi River. The Chryse channels emerge from local rubble-filled depressions, or from the Valles Marineris. Elsewhere channels may start at faults. The Chryse channels are mostly Hesperian in age (some 3.0 to 3.7 billion years ago), but crater dating of some channels elsewhere suggests that they can be as young as a few tens of millions of years.31 If so, then floods could form today. The flood channels appear to have formed by eruptions of groundwater from below a thick cryosphere, or in the case of those adjacent to Valles Marineris, by the drainage of large lakes. Eruptions may have been triggered by a variety of causes such as large impacts, tectonic forces, or dike injection.

The best morphological evidence for volcano-ice interactions, and hence hydrothermal systems, is in the

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