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. 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.

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.

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.

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. 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. 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.

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 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.

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. 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.

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.

The 2009 Mars Science Laboratory is planned as the first set of biological experiments in the current exploration strategy. As the NASA Office of Space Science noted however, there has been considerable debate about when to time a sample return: "We note with concern that there appears to be a growing division within the Mars community between scientists seeking early Mars Sample Return and those who believe it is best to delay it."

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