Surface life

There are two possible occurrences of surface life on Mars. The first is based on the biological interpretation of the Viking LR experiment. If the LR results are assumed to be due to biology then dormant Martian microbes are widespread in the surface soils of Mars. If the LR results are due to chemical agents, as is widely believed, then no dormant microbes are present.

A more plausible model for surface life on Mars is related to episodic liquid water formed in the north polar regions during favourable orbital conditions. As first pointed out by Murray et al. (1973), the conditions in the polar regions of Mars change dramatically in response to changes in the parameters of Mars' orbit. An analysis by Laskar et al. (2002) shows that of particular interest over the past 10 My are periodic changes in the obliquity and timing of perihelion. Figure 12.2 shows the obliquity, eccentricity, and summer equinox insulation at the north pole over the past 10 My. It is useful to consider three epochs in this time history; the last 0.5 My, from 0.5 to 6 My ago, and from 6 to 10 My ago.

In the first epoch (the last 0.5 My) the obliquity varied only slightly and changes in polar conditions were dominated by the relative phase of perihelion and equinox. During this epoch, Mars' eccentricity remained high (~0.1) and therefore the solar flux at perihelion was 49% more than at aphelion (see Table 12.2). Today perihelion (longitude of the sun on Mars, Ls = 251) almost coincides with summer solstice (Ls = 270) and the southern summer sun is therefore stronger than the northern summer sun. Procession of the orbit reverses this situation in ~50 000 y. The effect is not symmetrical due to the fact that the north polar regions are at low elevation and higher pressure and therefore, as discussed below, the formation of liquid water by the melting of ground ice is possible. This is not possible in the southern polar regions which are at much higher elevation. Thus ~50 000 y ago conditions in the north polar region of Mars were different than today, the sunlight reaching the polar regions was about 49% stronger.

The second epoch to consider in the orbital history of Mars is the period from about 0.5 My ago to 6 My ago. Laskar et al. (2002) showed that during this period the obliquity of Mars varied considerably over the range 15-30° with the average value approximately equal to the value today of 25°. During this epoch there were even larger changes in polar summer sunlight. At the highest obliquity and at high eccentricity, the summer sun could be twice as bright as the value at the present time.

The third epoch to consider began about 6 My ago when the obliquity assumed a larger average value (35°) with excursions as high as 45° (Laskar et al., 2002).

Fig. 12.2. Obliquity cycles and insolation at the north pole of Mars over the past 10 My (adapted from Laskar et al. (2002)).

Fig. 12.2. Obliquity cycles and insolation at the north pole of Mars over the past 10 My (adapted from Laskar et al. (2002)).

Table 12.2. Northern polar insolation and depth of ice exchange

Time period Summer insolation Depth of ice exchange present 200 Wm-2 0.1m

During this epoch the maximum summer sun in the north polar regions could be 2.5 times brighter than its present value.

Jakosky et al. (2003) discussed the potential habitability of Mars' polar regions as a function of obliquity. They concluded that temperatures of ice covered by a dust layer can become high enough (-20 °C) that liquid brine solutions form and microbial activity is possible. Rivkina etal. (2000) have shown that microorganisms can function in ice-soil mixtures at temperatures as low as -20 °C.

Costard et al. (2002) computed peak temperatures for different obliquities for varying surface properties and slopes. They found that peak temperatures are >0 °C at the highest obliquities, and that temperatures above -20 °C occur for an obliquity as low as 45°. They suggested this as a possible cause of the gullies observed by Malin and Edgett (2000).

Environmental conditions on the surface of Mars today are inauspicious for the survival and growth of even the hardiest terrestrial life forms. The Antarctic cryptoendolithic microbial ecosystems and snow algae found in alpine and polar snowpacks are probably the best candidates for Martian surface life (McKay, 1993). Both ecosystems can grow in environments where the mean air temperatures are below freezing but the temperatures in the substrate (the sandstone rock and the snowpack, respectively) must be at or about melting and liquid water must be present for growth. There are no plausible models for the growth of these systems anywhere on Mars at the present time.

However, conditions at the northern polar regions are the closest to habitability in several respects (Table 12.3). First the low elevation of the northern plains results in atmospheric pressures that are above the triple point of liquid water. Indeed the pressure measured by the Viking 2 lander at 48° N never fell below 0.7 kPa. As Lobitz et al. (2001) and Haberle et al. (2001) showed, the northern plains of Mars are the main location on the planet where liquid water could be present and stable against boiling due to low pressure. If surface insolation increased in the northern polar regions the surface ice would melt to form liquid. In contrast in the southern polar regions the warmed ice would sublime due to the low pressure. The second factor that favours habitability in the polar regions is the presence of ice near the surface. The third factor arises from the nature of the polar seasons. While orbital average conditions at the polar regions can be quite cold, the summer sun never sets. Indeed, for the range of obliquities considered here the polar regions receive more sunlight per day at solstice than anywhere else on the planet. The polar summer solstice is an energy-rich period and can cause strong seasonally dependent melting. This is observed in the polar regions of Earth. The effect is even stronger as the obliquity increases from its present value of 25° to 45°. Although at the present time liquid water is not expected in the northern latitudes on average, Hecht (2002)

Table 12.3. Favourable conditions for liquid water at the northern polar region

1. pressure above triple point (610 Pa)

2. ice near the surface

3. high insolation during summer has shown that under favourable conditions of solar exposure melting of ice can form liquid. This is consistent with the results of Clow (1987) for the melting of a dusty snowpack. In both cases increased solar heating compared to the present average conditions is required. As the obliquity increases, the average conditions are closer to melting and the variations required to produce liquid water are reduced. Thus liquid water should become more plentiful at the surface, although remaining frozen at depth.

12.3.2 Subsurface life

Although current conditions on Mars suggest there is little chance for life on the surface, there is interest in the possibility of subsurface life on Mars (Boston et al., 1992). Liquid water could be provided by the heat of geothermal or volcanic activity melting permafrost or other subsurface water sources. Gases from volcanic activity deep in the planet could provide reducing power (as CH4, H2, or H2S) percolating up from below and enabling the development of a microbial community based upon chemolithoautotrophy, especially methanogens that use H2 and CO2 in the production of CH4. Stevens and McKinley (1995) and Chapelle et al. (2002) have reported on microbial ecosystems deep within basalt rocks on Earth that are based on methanogens and are completely independent of the surface biosphere. In this terrestrial system, H2 comes from weathering reactions between water and basaltic rocks. With a source of hot water, all the ingredients for this subsurface habitat are present on Mars; CO2 makes up the bulk of the Martian atmosphere and basaltic rocks are abundant. Lin et al. (2005) have reported on a subsurface microbial ecosystem also based on methanogens but with the H2 produced by radioactive decay.

The possibility of subsurface life on Mars today depends on the existence of hydrothermal systems. While it certainly seems that volcanic activity on Mars has diminished over geological time, crater ages (Hartmann et al., 1999) and the age of the youngest Martian meteorite, 180 My (McSween, 1994), indicates volcanism on Mars as recently as that time. Volcanic activity by itself does not provide a suitable habitat for life - liquid water presumably derived from the melting of ground ice is also required. It is likely that any volcanic source in the equatorial region would have depleted any initial reservoir of ground ice and there would be no mechanism for renewal. Closer to the poles ground ice is stable (Fanale and Cannon, 1974; Squyres and Carr, 1986; Feldman et al., 2002). It is conceivable that a geothermal heat source could result in cycling of water through the cryosphere (Squyres et al., 1987; Clifford, 1993). The heat source would be melting and drawing in water from any underlying reservoir of groundwater or ice that might exist.

The isotopic measurements of water in the SNC meteorites (a class of meteorites thought to have originated on Mars) show an enrichment of D, the heavy isotope of hydrogen, about equal to that in the present Martian atmosphere (Watson et al., 1994). Assuming that this enhancement is due to atmospheric escape then this similarity suggests that there was an exchange between that atmosphere and the rocks from which the SNC meteorites derived. Probably this exchange involved hydrothermal groundwater systems driven by volcanism or impact events (Gulick and Baker, 1989).

Such hypothetical ecosystems are neither supported, nor excluded, by current observations of Mars. Tests for such a subsurface system involve locating active geothermal areas associated with ground ice or detecting trace quantities of reduced atmospheric gases that would leak from such a system. The reports of possible CH4 in the Martian atmosphere could be an indication of such subsurface hydrothermal activity and possibly biology (Formisano et al., 2004; Krasnopolsky et al., 2004).

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