PLANETARY ATMOSPHERES: THE ROLE OF COLLISION-INDUCED ABSORPTION
Collision-induced absorption plays several roles in planetary atmospheres. It provides the thermal opacity of the giant planets whereby H2 and He radiate most of the absorbed sunlight and internal heat to space. This process governs the vertical temperature structure in their upper atmospheres. CIA also is expected to have played a role in the formation and early evolution of the giant planets by governing the rate of cooling of the protoplanetary material and the resultant contraction to their present sizes. It may also have played a role in an early massive atmosphere of Triton if this satellite was captured. CIA from H2, N2, and CH4 in Titan's atmosphere is primarily responsible for the warm surface and the increase of atmospheric temperature with depth. On Venus, CIA of CO2 is thought to contribute to the continuum spectrum of the nightside thermal emission in its atmospheric spectral windows. On Triton and Pluto, the spectral profile of the 2.15 pm band of N2 ice provides the temperature of the ice which constrains the ice vapor pressure and atmospheric density. Since these atmospheres are supported entirely by the equilibrium vapor pressure, their mass varies sensitively with seasonal changes in the insolation. Finally, CIA may play a significant role in the thermal opacity of Jupiter-sized extrasolar planets. However, the thermal opacity of brown dwarfs and hot super-Jupiter sized extrasolar planets is likely to be dominated by H2O and CH4 absorption.
