The Possibility of Extant Life on Mars
Although current evidence suggests that the surface of Mars is inimical to life as we know it, there remain plausible scenarios for extant microbial life on Mars—for instance, in possible hydrothermal oases or in subsurface regions.
THE CONTEMPORARY MARTIAN ENVIRONMENT
The surface of Mars today is generally inhospitable to life as we know it. It is cold, dry, and chemically oxidizing and is exposed to an intense flux of solar ultraviolet radiation.
Temperature is of interest, not only because of its controlling influence on metabolic rates but also because of its influence on the stability of liquid water. Although the peak daytime surface temperature near the equator can rise above the freezing point of water during much or all of the year, the average surface temperature is about -55°C, well below the freezing point of water.
Liquid water is essential for life as we know it, as all known terrestrial life is based on aqueous chemistry. Given our current state of knowledge in chemistry and biology, it is hard to imagine the existence of life independent of liquid water.
Water is abundant on Mars but not in liquid form (e.g., Jakosky and Haberle, 1992). Water vapor and ice crystals are present in the atmosphere. In fact, because of the cold temperature of the atmosphere, water is often saturated there or near the surface. Water-ice almost certainly is present in the soil at high latitudes, where the subsurface temperatures are cold enough that atmospheric water vapor can diffuse from the atmosphere into the surface and condense as ice. Ice is present at the surface in the polar regions as well. During the half-year-long north-polar summer, the water-ice residual polar cap heats up enough to allow water to sublime into the atmosphere and be distributed globally. The polar surface temperatures are too low, however, for the ice to melt.
It is possible that liquid water may exist transiently on or near the surface in isolated pockets, although such occurrences probably are very rare. The presence
of salts of the right composition and in sufficient quantity can lower the freezing point enough to allow a liquid solution to exist, although such a liquid is unstable with respect to evaporation (Clark and Van Hart, 1981). Alternatively, ice crystals trapped in closed pores in rocks or regolith grains could melt under certain circumstances, and the resulting liquid water could be prevented from evaporating by virtue of being enclosed.
Analytical experiments carried aboard the Viking landers indicated that the surface environment of Mars is highly oxidizing, although the exact nature of the oxidants was not determined (Hunten, 1979). It is possible that the martian soil contains oxidants, such as hydrogen peroxide, which are postulated to form photochemically from atmospheric water vapor and to diffuse readily into the soil. If present, such oxidants would react with, and destroy, organic molecules or biota and could be effective in sterilizing the surface environment. Their presence may be responsible for the absence of organic molecules in the soil.
The Viking lander experiments found no organic substances in the soil despite the fact that organic molecules are being added continually from meteorite impacts (Biemann et al., 1977).
The atmosphere is relatively thin, averaging about 6 millibar pressure, and consists primarily of carbon dioxide. Owing to the low concentration of atmospheric ozone, ultraviolet light from the sun can reach the surface of Mars almost unattenuated. Winter-hemisphere atmospheric ozone can absorb some of the ultraviolet, but only during a fraction of the year and only over a fraction of the planet. The attenuation is much less than that due to the ozone layer on Earth. Thus, throughout the martian year the entire surface of the planet is subject to an intense flux of ultraviolet radiation.
THE ANCIENT MARTIAN ENVIRONMENT
The surface environment of Mars may not always have been so hostile to life. Early in the planet's history, the average temperature almost certainly was warmer and the atmosphere more dense, and liquid water may have existed at the surface. Evidence for the presence of surface water on early Mars comes from interpretation of the geomorphology of the planet's surface. A substantial fraction of the surface of Mars is older than about 3.5 billion years, based on the number of impact craters, which provide a window into the planet's early history.
Two aspects of these older surfaces suggest that the climate prior to about 3.5 billion years ago was different from the present climate (Squyres and Kasting, 1994). First, impact craters smaller than about 15 kilometers in diameter have been obliterated on these older surfaces, and impact craters larger than this have undergone substantial degradation, whereas younger impact craters have not been altered significantly. This suggests that erosion rates were up to 1,000 times larger early in martian history. The style of erosion that is seen on some of the remaining larger impact craters is indicative of water runoff, and water erosion is
considered to be responsible for removing the smaller craters. Second, many of the same older surfaces contain networks of valleys that form dendritic patterns similar to terrestrial water-carved stream channels. There is continuing debate as to exactly how these valleys were formed—the process may have involved runoff of precipitation, seepage of subsurface water in a process termed "sapping," or erosion by water-rich debris flows. Independent of the exact process, their formation must have involved the presence of liquid water at or very near the surface during these earlier epochs (Carr, 1996).
Thus, geological evidence suggests that the martian climate prior to about 3.5 billion years ago was somehow warmer than the present climate and that liquid water flowed on the surface in a way that is not observed today. Unfortunately, the observations do not allow a unique determination of what the temperature, atmospheric pressure, or partitioning of liquid water between the subsurface, surface, and atmosphere were at that time. Evidence from measurements of martian stable isotopes suggests that a large fraction of the volatiles from early Mars may have been lost to space, causing the surface environment to become cooler and drier and to evolve into the state observed today (Jakosky et al., 1994).
COULD LIFE HAVE ARISEN ON MARS?
Life on Earth appeared sometime prior to 3.5 billion years ago, although the details of its origin are unknown (e.g., Chyba and McDonald, 1995). The origin of life is believed to require a source of organic molecules, a source of energy that can drive disequilibrium processes, and access to the biogenic elements (such as carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus) (Chang, 1988). The source of organic molecules could be external—for example, organic molecules formed in interplanetary space and supplied to Earth along with meteoritic dust and debris that accreted onto Earth, or terrestrial, formed by chemical reactions in Earth's environment. Transient evaporating ponds, hydrothermal vents where water circulates beneath the surface near volcanic intrusions, and the surfaces of clay minerals that could provide stability and order to long chains of molecules have all been postulated as candidate environments where prebiotic chemistry may have undergone a transition leading to self-replicating entities.
Significantly, molecular phylogeny techniques that allow determination of the genetic distances between modern-day terrestrial species (Woese, 1987) suggest that their most recent common ancestor may have been hyperthermophilic, existing in water heated by near-surface volcanic magma. This indicates either that life first arose in a hydrothermal (hot-spring-like) environment or that it passed through some hydrothermal bottleneck event, such as heating of the early oceans by an energetic asteroid impact, during which nonhyperthermophilic organisms were exterminated.
Hot-spring environments may have been widespread on early Mars (Brakenridge et al., 1985). Hot springs or hydrothermal systems require water in
the crust and substantial sources of heat. Local heating of the crust can result from meteorite impacts. Such impacts were a common occurrence during the tail end of the heavy bombardment, as recorded in the impact craters on the oldest surfaces; thus local thermal anomalies that could have driven hydrothermal systems were probably common at the time. Isotopic evidence from martian meteorites indicates that the planet melted globally and differentiated shortly after accretion. The hot initial conditions imply extensive early volcanism. The rate of volcanic activity probably declined with time, but numerous volcanic landforms indicate that Mars has remained volcanically active throughout its history. Clearly, there has been sufficient heat to drive hydrothermal circulation throughout the history of the planet, although such activity was more common early in its history.
Geological evidence also suggests that abundant water has been present in the crust (Carr, 1996). The evidence is derived from the form of the valley networks that involved liquid water, as discussed above; catastrophic flood channels that indicate the presence of water reservoirs in the crust; morphologies such as rampart crater ejecta and lobate debris aprons that might be indicative of near-surface ice; and, of course, the polar caps, which contain substantial quantities of water. Given the extensive evidence for both heat sources and accessible water, it is likely that hydrothermal systems have been present throughout martian history.
The climate on early Mars may have been similar to the climate on Earth at that time. Although martian erosion rates undoubtedly were substantially lower than terrestrial erosion rates, suggesting less widespread water, liquid water certainly was present on both planets. Both planets probably had a mildly reducing atmosphere, containing substantial quantities of carbon dioxide. Given that life arose on Earth, it seems possible and even plausible that life could have arisen on Mars under similar conditions and at roughly the same time. If such were the case, a significant community of microorganisms may have existed on early Mars (McKay et al., 1992a,b; Boston et al., 1992).
Interestingly, an alternative source for life on Mars may have been Earth itself. Asteroid impacts are capable of ejecting rocky material from planets into space (see Chapter 3). Once in space, close encounters with their planet of origin would alter the orbits of such material. The orbits of material ejected from Mars could evolve to the point that they would cross the orbit of Earth; similarly, ejecta from Earth could evolve to the point that their orbits would cross the orbit of Mars (Melosh, 1988; Gladman et al., 1996). At that point, collisions could occur, providing a mechanism for transferring mass from one planet to the other. Meteorites have been discovered on Earth that are identified as having come from Mars, indicating that this process actually does occur. A martian origin for these meteorites is indicated by their young age, by the presence of oxygen isotopes that rule out an origin on Earth or the moon, and by gases trapped within them that are identical in composition to the martian atmosphere and distinct from any
other known source of gas in the solar system (Bogard and Johnson, 1993; McSween, 1994). Some of the material ejected by an impact is not heated or shocked substantially, and bacteria or bacterial spores may be able to survive the ejection event. If organisms or spores could survive within a rock during interplanetary transit and find a satisfactory environment on a new planet, they could possibly survive and multiply. This would allow living organisms on one planet to be transferred to another. Indeed, one can ask the following questions: On which planet did life originate? Could life have originated on Mars and been transferred to Earth or vice versa?
IF LIFE DID ARISE, COULD IT SURVIVE UP TO THE PRESENT TIME?
If life forms ever existed on Mars, either by having been formed in an independent origin or by having been transferred there from Earth, it is possible that they have continued to exist up to the present time. Such life forms could survive in occasional localized ecological niches. Such niches could be liquid water or hot springs associated with extrusive and intrusive volcanism or liquid water buried deep beneath the surface where it is stable. It is important to note, however, that biological material may not stay confined in such locations; organisms conceivably might produce dormant propagules (spores) that could be dispersed more widely.
Although volcanism has been declining in intensity throughout the latter half of martian history, it has occurred up to recent times and possibly to the present (Greeley and Schneid, 1991). Certainly, volcanism has occurred in the most recent recognizable geological epoch. This epoch, known as the late Amazonian, occupies approximately the last half billion years of martian history. Evidence for recent volcanism also comes from the martian meteorites. Many of these are basaltic rocks formed by volcanism more recently than 200 million years ago, and so it seems unlikely that volcanic activity just recently ceased. The abundance of water in the martian crust suggests that recent surface or near-surface volcanism might involve associated hot springs or near-surface hydrothermal systems where life could thrive. In addition, life could exist deep in the crust, where liquid water could occur. The geothermal temperature gradient is such that Mars is likely to have liquid water near the equator at depths as shallow as only about 2 kilometers (Carr, 1996). The presence of water is suggested by large flood channels that appear to have been caused by the occasional sudden release of large quantities of water from deep below the surface. The recent discovery of terrestrial organisms living deep within the Columbia River basalts in the Pacific Northwest (Stevens and McKinley, 1995), and elsewhere on Earth as deep as 3 kilometers below the surface, bolsters the possibility of organisms living under similar conditions on Mars. These organisms survive by metabolizing hydrogen that has been produced by chemical interactions between pore water and the ba-
salt; they are thought to be completely independent of any input of chemical energy from the surface, and to survive completely isolated from it. Presumably, these organisms did not originate in the basalt but migrated there from elsewhere. Similar migration to the deep subsurface could have occurred on Mars as surface temperatures declined from early higher values to their present cold level.
Did results from the Viking mission in the late 1970s not suggest that Mars was probably devoid of life? That was the accepted interpretation at the time, based on the results of three experiments that tested for biological activity and the absence of organic molecules in the surface materials (Klein, 1979). However, this conclusion may be open to some debate based on recent advances in our understanding of biology. The Viking experiments were able to test for only a couple of the possible mechanisms by which putative martian organisms might obtain energy; these involved the utilization of either carbon dioxide or extant organic molecules as a source of carbon in the production of organic molecules. Putative martian biota might employ other mechanisms to obtain energy and might do so under physical conditions quite different from those of the Viking biology experiments. Martian life also might reside in the interior of rocks (which were not sampled by Viking), where liquid water might occur. Finally, if life exists only in isolated oases where liquid water exists, such as recent volcanic vents or fumaroles, the Viking experiments might have been the right ones but carried out at the wrong location.
Furthermore, recent analyses of one of the martian meteorites, ALH84001, suggest that it contains possible indicators of ancient biological activity (McKay et al., 1996). This meteorite crystallized 4.5 billion years ago and contains abundant carbonate veins that appear to have been deposited in water through aqueous or hydrothermal activity. The possible indicators include carbonate mineral zonation and the presence of mineral grains similar to those found in terrestrial mineral deposits of biological origin, the presence of polycyclic aromatic hydrocarbons (PAHs) that may be remnants of decayed organic matter (although PAHs can also be formed by inorganic processes), and the presence of features that some researchers have interpreted as bacteria-like fossil biota. However, despite the occurrence of several intriguing indicators, the biological origin of these features has not yet been demonstrated with a high degree of certainty.
In summary, the surface of Mars is inhospitable to life as we know it, although there may be localized environments where life could exist. Conditions on Mars may have been conducive to the formation of life, either during an earlier epoch when the climate was likely more clement or in hydrothermal systems and hot springs that may have existed on Mars throughout geological time. Therefore, it is possible that life arose on Mars. It is also possible that living organisms from Earth could have been delivered to Mars by impact transfer, and, if so, such
organisms might have chanced upon the occasional oasis in which they could survive and multiply. If life arose on Mars or was delivered to Mars from Earth, it is possible that it has survived in localized environments that may be more hospitable than the general surface. Thus, there are plausible scenarios in which a sample returned from Mars could contain living organisms, either active or dormant.