Thermal Emission from Atmospheres and Surfaces

The amount of thermal radiation expected from a given planet depends in detail on the physical characteristics of the planet s atmosphere and surface. A starting point for understanding the observed flux densities of the planets is to assume that the planets are blackbodies in equilibrium with the energy they receive from the Sun and that which is radiated into free space. The radiation energy incident 

If the planet rotates rapidly, equilibrium will be reached between the insulation and the radiation from the entire planetary surface area, AnR. This leads to the estimate 

If a planet did not rotate and its emitted radiation came only from the sunlit hemisphere, the effective temperature of the suniit hemisphere would increase by the factor because of the reduction in the emission surface area. The equilibrium temperature for this case would become 

Thus far, we have discussed the ideal model in which the planets behave like blackbody radiators. This gives planetary astronomers a crude model from which they can estimate flux densities and search for departures. The planets would not be very interesting to study if they behaved like blackbodies since a single parameter, namely the temperature, could be used to define their radiation properties. More important, it is the departures from the simple model that allows radio astronomers to deduce the physical properties of the planets. 

The observed temperatures of the planets depart markedly from this ideal model for a number of different reasons. The presence of atmospheres on the planets produces strong perturbations from the ideal model. 

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