Physics
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p54a..08p&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #P54A-08
Physics
5210 Planetary Atmospheres, Clouds, And Hazes (0343), 5405 Atmospheres (0343, 1060), 5415 Erosion And Weathering
Scientific paper
The discovery of a "large Earth" in Gliese 581 has sparked considerable interest in the habitability of such planets, and the effect of various hypothetical atmospheres on habitability. A vast range of fundamental climate issues remain to be studied, particularly with regard to the long-term climate evolution and lifetime of biosphere. In this contribution I will discuss the implications of the large surface gravity and tide-locked rotation on the runaway greenhouse phenomenon which would pose a habitability crisis if the planet has a substantial water reservoir. In particular, I will show that high surface gravity increases the Kombayashi-Ingersoll limit and makes it harder for a runaway greenhouse to occur; moreover, the slope of the fundamental energy balance curve makes it easier for such a planet to remain habitable over a wide range of stellar luminosity, even in the absence of a CO2 weathering thermostat. The reasoning is elementary, and relies largely on the fact that the surface pressure of water vapor is determined by surface temperature via Clausius-Clapeyron, while the corresponding mass (which determines infrared opacity) is related to the surface pressure divided by surface gravity, in accord with hydrostatics. I will also make some remarks about the novel aspects of the operation of the greenhouse effect in cases like Gliese 581, resulting from the substantial infrared component of the stellar radiation. Finally, I will point out that the runaway greenhouse threshold is determined by the global energy balance, not the dayside energy balance, owing to the strong heat transports that would result from the pressure gradient implicit in a "dayside only" runaway. Prospects for a nightside glaciation with dayside torrid zone, separated by a habitable zone near the ice margin at the terminator, will be discussed. The relavant physics is demonstrated using a range of simplified climate models.
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