Astrobiology Workshop, Macquarie University July 12-13 2001

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Life on a CO2-dominated Mars

Nick Hoffman (La Trobe University)

In the rush to explore the biological implications of a wet Mars, one small fact seems to have been neglected. Modern Mars is a cold and dry planet at the present day and it is hard to see how it could have been different in the past.

Two lines of evidence are often quoted as "compelling" evidence for water on Mars. These are the "Outburst Flood" channels that date back to the Hesperian epoch (at least 3 Ga), and the recent gullies observed by Malin and Edgett and interpreted as groundwater seepage.

Models of Mars that fit these observations incorporate abundant water at various times in Mars' history and are strongly incompatible with the mineralogy and atmospheric composition of Mars at the present day. The many paradoxes of Mars (such as the lack of surface carbonates) can be resolved by exploring the behaviour of the other volatile - Carbon Dioxide.

New observations show that modern gullies on Mars "flow" during the springtime thaw of CO2 snowpack at temperatures <150K when no aqueous fluid would exist. Instead, a gas-supported flow is required based on subliming CO2 snowpack.

The older outburst floods can be best understood as violent gas-supported density flows akin to the largest terrestrial pyroclastic or turbidite flows, fuelled by outbursts of subsurface liquid CO2.

Mars is, and probably always has been, a cryogenic planet dominated by the phase behaviour of CO2, not water. Any liquid water is confined to many kilometres depth whilst liquid CO2 will be a common subsurface phase. The exobiological implications of this new view of a frozen planet - "White Mars" - are considerable. Any life on Mars will be confined to the "deep dark biosphere" and probably always has been.