Nir J. Shaviv, Giora Shaviv, Rainer Wehrse
We generalize the problem of the semi-gray model to cases in which a
non-negligible fraction of the stellar radiation falls on the long-wavelength
range, and/or that the planetary long-wavelength emission penetrates into the
transparent short wavelength domain of the absorption.
Second, applying the most general assumptions and independently of any
particular properties of an absorber, we show that the greenhouse effect
saturates and any Earth-like planet has a maximal temperature which depends on
the type of and distance to its main-sequence star, its albedo and the primary
atmospheric components which determine the cutoff frequency below which the
atmosphere is optically thick. For example, a hypothetical convection-less
planet similar to Venus, that is optically thin in the visible, could have at
most a surface temperature of 1200-1300K irrespective of the nature of the
greenhouse gas.
We show that two primary mechanisms are responsible for the saturation of the
runaway greenhouse effect, depending on the value of the wavelength above which
the atmosphere becomes optically thick. Unless this wavelength is small and
resides in the optical region, saturation is achieved by radiating the thermal
flux of the planet through the short wavelength tail of the thermal
distribution. This has the observational implication, the radiation from such a
planet should be skewed towards the NIR. Otherwise, saturation takes place by
radiating through windows in the FIR.
View original:
http://arxiv.org/abs/1202.2644
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