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| Early Mars: Mostly Cold and Dry |
| Presentation Time: Tuesday, 9:40 a.m. - 9:50 a.m. |
Pascal Lee1, C. P. McKay2
1Mars Institute, SETI Institute, NASA Ames Research Center, 2NASA Ames Research Center. |
| Presentation Number: 14.08 |
The two fundamental geologic observations on Mars at the basis of the classical notion that Early Mars was climatically wet and warm are: a) the relatively degraded morphology of most ancient impact structures in the martian highlands compared to fresh-looking impact craters on airless planetary bodies, and b) the existence and quasi ubiquitous distribution of small valley networks in the ancient martian highlands. Classical interpretations of these two observations imply that the atmosphere on Early Mars was substantially thicker and warmer than at present, sufficiently so to allow liquid H2O precipitation and/or flow under open air over distances of hundreds of kilometers. Meanwhile, even a thick (1-2 bar CO2) atmosphere could produce only marginally adequate greenhouse warming due to the faint early Sun, while the sustainability of a thick atmosphere is problematic in itself due to impact erosion.
Based on our investigations of terrestrial cold-climate and polar features and processes, in particular at the 40 Ma-old Haughton impact structure and surrounding terrain on Devon Island, High Arctic, we show that: 1) time-averaged denudation rates implied for craters on Noachian Mars are substantially (2-3 orders of magnitude) lower than classically estimated, inconsistent with rain or any substantial episode of climatic warmth; 2) the small valley networks on Noachian Mars do not imply a warm climate and are more likely to have originated, like their unique morphologic analogs on Devon Island, by localized basal melting of otherwise cold-based surface H2O ice deposits under a predominantly frigid climate. The invoked surface ice deposits were sustained by a dynamic hydrologic cycle powered by the higher geothermal heat flux, volcanic activity, and impact cratering rate expected on Early Mars. We conclude that Early Mars must have been mostly cold and dry (climatically), with liquid H2O occurring only transiently, mostly in subsurface (including sub-ice) environments. |
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