Biology
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.b13b0516a&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #B13B-0516
Biology
[0343] Atmospheric Composition And Structure / Planetary Atmospheres, [5210] Planetary Sciences: Astrobiology / Planetary Atmospheres, Clouds, And Hazes
Scientific paper
Most of the previous studies on the habitable zone implicitly assume an ocean-covered 'aqua' planet that has a large amount of liquid water like the present Earth. However, there is a possibility of a habitable 'land' planet that is covered by vast dry desert but has locally abundant water. The land planet state is expected when the fraction of water covered areas is less than about half of surface. Ancient Mars might be in such a state. The conditions for the existence of liquid water can be different for a less water land planet from that of an aqua planet, because both the ice-albedo feedback, which causes the complete freezing, and the runaway greenhouse, which causes the complete evaporation, are enhanced by abundant water. Here, we investigated the condition for the solar flux that cause the complete freezing or evaporation of liquid water on a land planet using a general circulation model. We use a general circulation model, CCSR/NIES AGCM5.4g, which have been developed for the Earth’s climate modeling by the Centre for Climate System Research, University of Tokyo and the National Institute for Environmental Research. To compare a land planet with an aqua planet, we consider an Earth-sized planet without topography with 1 bar air atmosphere on a circular orbit. The distribution of water on a land planet is completely determined by the atmospheric circulation. On a land planet, complete freezing and complete evaporation of water occurred at the 77% and 170% of the present Earth's solar flux, respectively. On the other hand, complete freezing and evaporation of an aqua planet occurs at 90% and about 130%, respectively. Thus, a land planet shows stronger resistance to both the complete freezing and evaporation of liquid water than an aqua planet. The stability field of liquid water is quite wide on a land planet compared with that of an aqua planet. In addition, escape flux of water from a land planet is very small because of dry upper atmosphere. It suggests that a water rich aqua planet may not be the best place for life, and a land planet with some water may be better than an aqua planet. If an ancient Mars had a big ocean it was easily covered by permanent snow compared to the case with a limited region of wet surface.
Abe Yasuhiro
Abe-Ouchi Ayako
Zahnle Kevin J.
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