Life on Mars: past, present, and future.

Mars appears to be a cold, dry, and dead world. However there is good evidence that early in its history it had liquid water and possibly life. The main question in the future exploration of Mars is the search for an independent origin of life on that planet. Ecosystems in cold, dry locations on Earth, such as the Antarctic dry valleys, provide examples of how life on early Mars might have survived and where to look for fossils. Fossils are not enough. We will want to determine not only if life on Mars, but if that life was a separate genesis from life on Earth. For this determination we need to access intact martian life, possibly frozen in the deep old permafrost. It is possible that at some time in the future we might recreate habitable conditions on Mars, returning it to the life-bearing state it may have enjoyed early in its history. Our studies of Mars are still in a preliminary state but everything we have learned suggests that it may be possible to restore Mars to a habitable climate.

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Mars, and the possibility of life on Mars, has always held a special fascination for both scientists and the general public. This traces back at least to the advent of the telescope. When viewed through even a small telescope Mars shows features reminiscent of Earth; these include polar caps that grow in the winter and recede in the summer, and dark areas in mid latitudes that shift with the seasons. It is not surprising that some early observers assumed that these polar caps were made of ice and that the dark features were due vegetation. The idea of Earth-like conditions and life was proven incorrect by spacecraft data; the winter polar cap is [CO.sub.2] and the dark features are caused by dust. Nonetheless, the space-craft data still show that of all the other planets Mars is the one most likely to have a biological past, present, and future.

A comparison between the conditions on the surface of Mars and Earth is shown in Table 1. The atmosphere of Mars is over 100 times thinner than the atmosphere of the Earth and it is composed predominately of [CO.sub.2]. Because of the thinner atmosphere and the increased distance from the sun, the mean temperature on Mars is --60[degrees]C, compared to +15[degrees]C for Earth. The lower gravity on Mars is due to its lower mass (1/10 the mass of Earth). Another important result of this lower mass is the absence of plate tectonics. The lack of recycling by plate tectonics is thought to be ultimately responsible of the decay of the martian atmosphere from its initially thin, relatively warm, state to the thin desert conditions of today (Kasting et al., 1988; McKay, 1997).

In 1976, the two landers of the Viking Mission reached the surface of Mars to search for evidence of life. Each lander contained three biology experiments and an instrument to detect and characterize organic material (a combination gas chromatograph mass spectrometer).

The three Viking biology experiments were: (1) the pyrolytic release experiment (PR) which sought to detect the ability of microorganisms in the soil to consume [CO.sub.2] using light (Horowitz and Hobby, 1977); (2) the gas exchange experiment (GEx) which searched for gases released by microorganisms when organic nutrients were added to the soil (Oyama and Berdahi, 1977); and (3) the labeled release experiment (LR) which sought to detect the release of [CO.sub.2] from microorganisms when radioactively labeled organic nutrients were added to the sample (Levin and Straat, 1977).

Both the GEx and the LR experiment gave interesting results. When moisture was added to the soil in the GEX experiment, [O.sub.2]was released. The release was rapid and occurred when the soil was exposed just to water vapor. The response persisted even if the soil was heated to sterilization levels (Gyama and Berdahl, 1977). The LR experiment indicated the release of [CO.sub.2] from the added organics. This response was not present when the soil was heated to sterilization levels (Levin and Straat, 1977). The release of [O.sub.2] in the GEX expenment was not indicative of a biological response. On the other hand, the LR results were precisely what would be expected if microorganisms were present in the martian soil.

However a biological interpretation of the LR results is inconsistent with the results of the GCMS. The GCMS did not detect organics in the SOIL samples at the level of one part per billion (Biemann et al., 1977; Biemann, 1979). One ppb of organic material would represent a large number of individual cells existing alone in the soil (Klein, 1978; 1979). However it is not likely that there are microorganisms in the soil on Mars without associated extracellular organic material. The lack of detection of organics is the main reason for the prevailing view that non-biological factors were the cause of the reactivity of the martian soil.

It is perhaps not surprising that Mars lacks life on its surface because there is no liquid water present at any place or at any time. There are regions on Mars where the pressure and temperature is consistent with liquid water stability (Haberle et al., 2001; Lobitz et al., 2001) but at these locations no water or ice is present. At the locations where ice is present the temperature or pressure are too low to allow for liquid. All known life requires liquid water to grow or reproduce and the absence of liquid water on Mars is consistent with the absence of life there... today.

PAST HABITABILITY 

Mars may be cold and dry today but there is compelling evidence that earlier in its history Mars did have liquid water. This evidence comes from the images taken from orbital spacecraft. Figure 1 shows an image of a water carved canyon on Mars and represents probably the best evidence for extended and repeated, if not continuous, flow of liquid water on Mars (Carr, 1996; Carr and Malin, 2000).

Water is the quintessence of life, and the evidence that sometime in its early history Mars had liquid water is the primary motivation for the search for evidence of life (McKay, 1997). Liquid water flowed on the surface of Mars long ago and the most likely place to search for evidence of any life associated with that water is in ancient lakebeds.

Cabrol et al. (1998) have shown that Gusev Crater (Figure 2) was probably an ice-covered lake and that the material in the crater represents sediments deposited in this lake. Kempe and Kazmierczak (1997) have argued that the delta deposits which can be seen near the mouth of the river (Ma'adim Vallis) as it enters Gusev Crater may be carbonates. This large flat crater makes a promising target for a search for microfossils on Mars.

However Gusev Crater poses an apparent paradox. As seen in Figure 1, Gusev Crater was a large lake (160 km diameter) fed by a large river but the surrounding terrain appears heavily cratered and unaltered. The preservation of this cratered terrain is inconsistent with the rain that would be expected to be associated with such a large river and lake. The paradox of rivers and lakes without rain can be resolved by considering the only place on Earth where rivers and lakes exist without rain; the dry valleys of the Antarctic.

The dry valleys of Antarctica comprise the largest ice-free region on that continent. The valleys are a cold desert environment with mean annual temperatures of -20[degrees]C (Doran et al., 2002). Figure 3 shows a view of upper Wright Valley with Lake Vanda and the Onxy River on the valley floor. Precipitation in the dry valleys occurs only as snow and the amount is equivalent to about 1-2 cm of water. The hydrological cycle in these valleys is as follows: (1) Snow falls. (2) In the lower regions of the valleys the snow evaporates with little liquid formation or erosion. In the higher elevations snow accumulates and forms glaciers. (3)The glacial ice flows downward in the valley (as can be seen in Figure 3). (4) The lower parts of the glaciers melt in the summer providing a source of meltwater that then flows in the lakes (for Lake Vanda this is via the Onyx River). (5) The liquid water freezes to the underside of the ice cover. (6) Ablation and evaporation from the top of the ice cover returns moisture to the atmosphere and completes the cycle. This hydrological cycle has rivers in summer and lakes all year round but no rain. The minimum requirements for such a cycle are atmospheric pressures well above the triple point of water (610 Pa) and summer temperatures above freezing. McKay and Davis (1991) have shown that such conditions could have prevailed on Mars for many hundreds of millions of years early in its history.

The lakes in the dry valleys of Antarctica provide an example of the physical processes that can maintain large bodies of liquid water under mean annual temperatures well below freezing. Biologically these lakes are also important analogs because of the plankton and benthic communities of microorganisms that thrive there (Parker et al., 1982; Wharton et al., 1982). Life could have existed in lakes on Mars in similar ecological conditions.

One interesting feature of the dry valley lakes visible in Figure 3 is the evidence of ancient shore levels. In these ancient lake deposits evidence of life is present and provide a basis for considering how remnants of life in martian lakes might be preserved (Wharton et al., 1995; Doran et al., 1998).

THE SEARCH FOR A SECOND GENESIS 

The discovery of fossilized evidence for life in a dry lakebed on Mars would be of great interest. However it would not directly address the fundamental question of life on Mars, that is: was there a separate genesis of life on Mars. It is now known that rocks can be ejected from Mars and carried to Earth and it is assumed that the reverse is possible as well. Work on the preservation of magnetic signatures shows that the temperature in these rocks never reaches temperatures high enough to sterilize them (Weiss et al., 2000). Thus it is possible that Mars and Earth are not biologically isolated and share a common origin of life.

To determine if life on Mars is a second genesis requires more than fossils, it requires access to the organic remains of an actual martian organism (McKay, 2001). Life on Earth has a distinctive and universal signature both in its "hardware" and in its "software." The hardware of Earth-life is composed of 20 L amino acids, 5 nucleotide bases that appear in DNA and RNA, and a few D sugars the compose the polysaccharides. The common software of Earth-life is most clearly seen in the conserved sequences in the ribosomal RNA that demonstrate the phylogenetic connectivity of all life on Earth on the so-called "free of life" (Woess, 1987). To determine if martian organism represent a second genesis of life, the hardware and software of martian life must be compared to that of Earth life.

It is unlikely that preserved organic remains of martian organisms will be found in paleo lake sediments in the equatorial regions. These locations may hold only fossils. A possible site for finding preserved organic remains of past martian life is in the polar regions. The most promising site may be at 80[degrees]S, 180[degrees]W located in the heavily cratered highlands. Locations closer to the polar cap are covered with the relatively young polar layered deposits. At this location there is also the presence of strong crustal magnetism (Connemey et al., 1999). These features were presumed to be caused by an early martian magnetic field. The apparent erasure of these magnetic features in the vicinity of the Halles and Argyre impacts suggests that such features are old, possible the oldest phenomenon detected on Mars. Feldman et al. (2002) have reported ice rich ground in the southern polar regions. Thus the ice and sediments at 80[degrees]S, 180[degrees]W may represent the oldest, coldest, undisturbed ice-ri ch permafrost on Mars. Here we might find remnants of martian microorganisms frozen since the early water-rich period of martian history.

FUTURE HABITABILITY

The evidence that Mars had a more clement environment early in its history has lead to the suggestion that it might be possible to restore that planet to habitable conditions (McKay et al., 1991; McKay and Marmnova, 2001). Climate models for warming Mars suggest that a thick atmosphere composed primarily of carbon dioxide could be recreated on Mars is a hundred years or so (McKay and Marinova, 2001). Creating an oxygen rich atmosphere is not possible with foreseeable technologies in time scales that are comparable to human lifetimes (McKay and Marmnova, 2001).

To make a biosphere on Mars requires three basic ingredients in planetary amounts: water, [CO.sub.2] and [N.sub.2]. The total inventory of these key compounds on Mars is uncertain (McKay et al., 1991) but the range of plausible values include the amounts needed for terraforming. A key science goal for the exploration of Mars will be to determine the location and extent of subsurface reservoirs of these compounds.

The first step in terraforming Mars would be to simply warm its surface. Several methods have been suggested for warming Mars. Perhaps the most practical is the production of compounds that are extremely efficient greenhouse gases (Marmnova et al., 2000). These include PFCs, nitrous oxide, sulfur hexafluoride and methane. When combined with water and carbon dioxide these gases can effectively block infrared radiation from leaving the surface, creating a strong greenhouse warming. Calculations suggest (Marinova et al., 2000) that total concentrations of a few ppm or less could warm Mars to Earth-like conditions. Even at the ppm level the total mass of gases produced is much too large to transport from Earth and would have to be produced in situ. The essential elements are known to be present on the surface of Mars. As greenhouse gases are produced on Mars and the surface warms, any carbon dioxide adsorbed into the regolith or frozen onto the polar caps would evaporate into the atmosphere. This positive feedback would accelerate and amplify the warming from the greenhouse gases.

The warming of Mars due to the greenhouse effect could proceed quite rapidly. If there was enough carbon dioxide to provide for a thick atmosphere and if the super greenhouse gases were produced on Mars it would not take very long for Mars to warm appreciably. If every single photon reaching Mars from the sun were used to warm Mars it would have a warm surface in only 10 years. Another 50 years would be enough to melt a layer of water 500 meters thick over the entire planet--a very suitable ocean. However, it is not possible to trap solar energy with 100% efficiency. Even with a strong greenhouse effect a value near 10% is more probable. With 10% efficiency it would take 100 years to warm the surface of Mars and an additional 500 years to melt its ocean. The melting of the martian permafrost to release an ocean of water may take longer than the calculation above suggests. The surface warmth would have to diffuse down into the soil. To warm the subsurface to a kilometer depth simply by diffusion would take abo ut 100,000 years. However, passive diffusion of heat would be augmented by active transport due to the flowing of the resulting liquid water. Thus Mars could have a thick warm atmosphere in about 100 years and a fully formed ocean-biosphere in 600 years. Not a long time even on human time scales.

The production of an oxygen-rich atmosphere suitable for humans to breathe is much more energy intensive than just warming the planet. The conversion of an atmosphere of carbon dioxide to organic material and the concomitant production of oxygen required billions of years on the Earth and was due to photosynthesis by primitive algae. On Mars, plants would also provide a global-scale, self-replicating mechanism for the production of oxygen. Although, plants have perfected their biochemical technique over billions of years of evolution, their efficiency at incorporating the global average solar radiation into biomass is still only 0.01%. Even optimistically assuming that this can be improved by an order of magnitude by selective breeding and genetic engineering, it will take 10,000 years for an oxygen rich atmosphere to be produced. Thus, humans on Mars may doff their space suits in the relatively near future but they will require their oxygen supplies for many generations.

CONCLUSIONS

Mars presents a challenge and an opportunity. The challenge is to explore a distant planet with a complex history, first with robotic probes and eventually with human explorers. The opportunity is to learn about the nature of life, to search for a possible second type of life in our own solar system and thereby begin to understand the profound philosophical and scientific issues related to life in the universe. The ultimate challenge on Mars will be the reconstruction of a biosphere on that world. A task of restoration ecology worthy of the best efforts of a space-faring humanity.

able 1

Comparison of Mars and Earth Surface Conditions.

Parameter            Mars               Earth

Surface pressure     0.5 to 1 kPa       101.3 kPa
Temperature range    -130[degrees]C to  -60[degrees]C to
                      +15[degrees]C      +50[degrees]C
Temperature average  -60[degrees]C      +15[degrees]C
Composition          95% [CO.sub.2]     78% [N.sub.2]
                     2.7% [N.sub.2]     21% [O.sub.2]
                     1.6% Ar            1%Ar
Gravity              0.38 g             1 g
Distance from Sun    1.52 A.U.          1 A.U.
Tilt of axis         25[degrees]        23.5[degrees]
Length of mean       24 hr 39.6 mm      24 hr
 solar day
Length of year       1.88 yr            1 yr

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