Terrestrial and (potential) Martian Biospheres--a Contrast

NASA may be poised to announce evidence for unearthly life, perhaps in a meterorite from Mars, or in terrain investigated by the Spirit and Opportunity rovers.

Generally, it may be futile to search for life on the surface of Mars. Such a search reflects the terrestrial biosphere. Because temperature increases rapidly with depth, areas deep underground are too hot for life. Earth's biosphere is essentially limited to its upper crust. The surface is hospitable, and has nurtured abundant life.

On Mars, things are the reverse. The Martian surface is bitterly cold, heavily irradiated with UV and cosmic rays, and loaded with oxidants, which destroy organic compounds. Yet, unlike Earth's interior, that of Mars seems habitable. Because Mars long ago lost most of its internal heat, in the great eruptions of late Hesperian times, areas deep underground are certain to be cooler. Between the uppermost frozen layer, the cryosphere, and the core of the planet, there is probably a wide region where temperatures are between the freezing and boiling points of water. Remarkably, despite the small size of Mars, the habitable subsurface zone may accomodate a biosphere which approaches that of Earth in extent or volume. Conceivably, the Martian biomass may be from 10-50% as great as the terrestrial. This is possible in part because the Martian interior harbors vast reservoirs of liquid water. According to one estimate, only about 10% of the original subsurface H2O was lost due to outflow events such as those that formed the circumchrysean channels. Even if less than half of the water inventory remains underground in liquid form, it is certainly enough to nurture a great biosphere.

If Martian life arose concurrently with life on Earth, it might have evolved well beyond the level of microorganisms. Various plants and invertebrates may lurk in the great hydrothermal systems. Some evidence supports the existence of a hidden biomass. Inasmuch as methane is a byproduct of life, recent release of the gas into the Martian atmosphere suggests an underground, inhabited source.