Other
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p51b0345k&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P51B-0345
Other
5407 Atmospheres: Evolution, 6225 Mars
Scientific paper
Nearly all authors agree that Mars early (pre-3.8 Ga) surface was wet, as evidenced by many signs of flowing water on its heavily cratered southern highlands. Exactly what this implies about the early martian climate is a topic of ongoing debate. Some authors (1) have argued for a warm, nearly Earth-like climate; others (2) have suggested that the mean surface temperature could have been significantly below freezing. Here, I argue that the wetter early Mars was, the higher its mean surface temperature must have been in order to create the observed fluvial features. A planet with a large ocean, like Earth, would need to have a mean surface temperature at or above 0oC in order to avoid freezing over entirely. For a low-obliquity planet like Earth, ice albedo feedback causes the climate to become unstable when the polar caps extend equatorward of 30o (3). Mars has a highly variable obliquity, but the same reasoning is still likely to apply. If, indeed, Mars had an Earth-like early climate, then it must have had a substantial atmospheric greenhouse effect. Solar luminosity increased from about 70 percent of its present value at 4.6 Ga to 0.75 times present at 3.8 Ga (4). Gaseous CO2-H2O atmospheres can produce surface temperatures no higher than 225 K during this time (5). Radiative heating by CO2 ice clouds might raise this temperature somewhat (6). Obtaining surface temperatures above freezing with this mechanism, though, requires nearly 100 percent cloud cover, which is highly unlikely for condensation clouds. A methane greenhouse is a more attractive mechanism. The combination of a few bars of CO2, along with a few tenths of a percent CH4, could have kept early martian surface temperatures above freezing. This would probably have required a biogenic source for methane, however, as abiogenic sources are unlikely to have been strong enough to maintain such concentrations in the face of photolysis by solar UV radiation. Although admittedly speculative, such a mechanism could explain the fluvial features that we see today. References: 1) Pollack, J. B., Kasting, J. F., Richardson, S. M. and Poliakoff, K. Icarus 71, 203-224 (1987). 2) McKay, C. P. & Stoker, C. R. Rev. Geophys. 27, 189-214 (1989). 3) Caldeira, K. and Kasting, J. F. Nature 359, 226-228 (1992). 4) Gough, D. O. Solar Phys. 74, 21-34 (1981). 5) Kasting, J. F. Icarus 94, 1-13 (1991). 6) Forget, F. and Pierrehumbert, R. T. Science 278, 1273-1276 (1997).
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