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Did the Lyot Impact End the Liquid Water Era on Mars?

The impact that created the 220km diameter crater Lyot, in the Deuteronilus region of Mars, has been dated by stratigraphy to have, occurred in the Early Amazonian epoch of .7-1.8 Gyr ago .(I) This epoch in Mars history also features much reduced signs of liquid water erosion compared to the previous epoch, the Late Hepersian (2). In this paper we will examine the hypothesis that Mars suffered profound climatological change because of the Lyot impact, which ended the period when large amounts of liquid w ater could move across its surface. The mechanism for this Lyot impact induced climatic change may have been the collapse of C02 atmospheric pressure, from being approximately lbar of C02 to the 6 mbar value of the present. This could have occurred due to an impact at Lyot creating a massive "chilling event" via dust loading of the atmosphere in the North of Mars leading to massive freezing out of the C02 atmosphere at the poles with attendant loss of both atmospheric pressure and greenhouse warming. S uch catastrophic collapses of coupled temperature and atmospheric pressure regimes on Mars have been seen in models done by Haberle (3). Evidences that Mars sustained temperatures and atmospheric pressures conducive to surface liquid water at least until the Early Amazonian epoch(4) will be reviewed, such as: evidence for preterrestrial water alteration in the SNC meteorites Nahk-la and Lafayette which are 1.3 Gyr old (5) and in soils at the Viking lending sites as well as evidence of massive prolonged f loods in the Ch-rysc region.

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