Mars today is a cold, dry desert world with a thin atmosphere. There is no firm evidence for liquid water on its surface at any place or any season. There has been recent evidence of activity on Mars, notably gullies on the side of crater walls; however, while these features might be explained by liquid water, they can also be explained by the movement of dry materials.

Although there is no evidence for liquid water presently there is extensive evidence for liquid water on the surface of Mars in the past. Figure 1 shows a sinuous canyon on Mars. This canyon, Nanedi Vallis, is the best evidence we have that some of the fluvial features on Mars were carved by liquid water in stable flow on the surface for an extended interval. Note in particular the presence of a channel on the bottom of the canyon which presumably reflects the flow path of a liquid. Explanations other than liquid water have been suggested for the fluvial features on Mars, including ice flow, lava flow, wind, and carbon dioxide flow. None of these can explain the morphology of Nanedi Vallis.

The low pressure on Mars today is inconsistent with the stable flow of liquid water on the surface. Thus, the water-carved features on Mars attest to an early climate with a thicker atmosphere and at least slightly warmer conditions. We do not know how long this cold thick atmosphere persisted, but climate models suggest that liquid water habitats would have been present on Mars for longer than the time associated with the earliest evidence for microscopic life on Earth.

If there had been a time on Mars when liquid water was widespread, we can expect that life, if present, would have also been widespread. Even if life had been global, we would only expect evidence of this early biosphere to be preserved until today in the polar permafrost. Organisms preserved for billions of years in the martian permafrost would probably be dead due to accumulated radiation from crustal levels of radioactive elements. However, these dead microbes would retain the biomolecules of martian life. Thus, unlike fossils, these frozen dead remains could be used to determine if martian life shared a common origin with life on Earth or represents a second genesis. It is possible that life on Earth and Mars shared a common origin through the exchange of meteorites.

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