Mathematics – Logic
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p51b0353h&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P51B-0353
Mathematics
Logic
5407 Atmospheres: Evolution, 5415 Erosion And Weathering, 5420 Impact Phenomena (Includes Cratering), 6225 Mars
Scientific paper
In a conventional warm and wet model for early Mars, a thick CO2 atmosphere persisted for much of the Noachian, steadily diminishing over a timescale of 100-500 Ma. Slow erosion occurred under a steady-state atmosphere, and ample time was available for the origin and adaptation of life to surface conditions. Large impacts occurred at a slowly decreasing rate, but they are not important to the climatic model and merely provide surface topography for the climate to weather and erode. Expected products of such a history would be extensive carbonate rock layers in crater lake sediments, and possible associated biological fossils. Meanwhile, terrain degradation should record the duration and intensity of fluid activity. In this model, conditions on Mars may even have been Earth-like. However, on Mars we see no surface deposits of carbonate rock, at the ~10% detection limit of TES and preliminary THEMIS data. Furthermore, the degree of erosion and drainage network integration is strictly limited compared to even quite young terrains on Earth. Measuring topographic maturity, we compare Martian and Terrestrial terrains. We find that the Noachian terrains are comparable to modern Terrestrial systems of only 1-20 Ma age. This implies that average Noachian erosion rates were 1-5% those on Earth today, even in arid or frigid environments. Therefore, conditions on Mars cannot have been remotely Earth-like, if they were steady-state. Alternatively, Earth-like conditions could have persisted for only a tiny fraction of the Noachian, yet they appear to extend over the entire Noachian epoch. To resolve this discrepancy, we develop a model where the steady-state of Noachian Mars is an arid, cryogenic, barren surface under the Faint Young Sun, at least 10 K cooler than modern Mars. Permafrost of both water ice and CO2 is stable planetwide, and the volatiles are frozen into the upper crust, or developed as ice sheets on the surface. Major impacts onto this surface can deliver energy equivalent to 10e2 to 10e4 years of sunlight on a timescale of minutes to hours. The primary and secondary impacts lead to instantaneous shock heating of shallow icy layers and slower conductive heating from superposed ejecta blankets. In the aftermath of a large impact, global heating and outgassing of ices will occur, developing a thick CO2 atmosphere with locally abundant liquid water. The atmosphere is essentially boosted into a higher energy state by the impact, but the atmosphere is not permanently self-sustaining. As the residual heat of the impact dies away, and the planet continues its cycle of obliquity changes, the atmosphere collapses on a timescale of 10e3 to 10e6 years. On collapse, the atmosphere freezes out again as surface ices, restoring the status quo ante. In this model, the Noachian erosion is caused by a number (10-100) of short-lived climatic cycles, with an integrated duration of 1-10 Ma, spread over the 100-500 Ma span of the Noachian. This explains the limited degree of fluvial erosion and the lack of carbonate rock at surface. Ephemeral impact-induced atmospheres are responsible for the erosion and deposition of material on Noachian Mars, and the end of large impacts necessarily ends the era of ephemeral atmospheres and river systems on Mars. The implications for astrobiology are obvious - In this model, the surface has essentially always been inhospitable to life, and the brief clement episodes are unlikely to allow surface life to evolve. The search for fossil life on Mars should therefore focus on subsurface niches.
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