Abstract
A model is developed for the NLTE infrared radiative transfer in a CO2 atmosphere at low pressure. This model is used to determine the source function for CO2 15 µm vibrational levels and the radiative equilibrium temperature for the atmosphere of Venus above cloud level. The most significant improvement in the radiative transfer model over previous efforts is the inclusion of vibrational-vibrational (V-V) exchange between 15 µm levels. The exchange is especially important for determining isotopic and hot band source functions. This rate is introduced parametrically by assuming it is the same for all transitions, and 10, 100 or 1000 times as rapid as the relaxation rate for the 15 µm fundamental level. We analyze in detail the rates of photon escape to space in the various CO2 bands. Over much of the Venusian mesosphere, significantly more photons escape to space in the hot and isotopic bands than do in the C12O216 fundamental band. However, the latter is essentially in LTE to significantly lower pressure than the other bands and is insensitive to the V-V transition rates. Hence remote sensing of temperature from the infrared radiances would require smallest corrections for NLTE and be least sensitive to V-V rates if only radiances from the latter band were measured. With this band, the assumption of LTE is valid up to the 1 µb level. Sufficient thermal excitation exists up to 0.01 µb for a NLTE temperature inversion to be practical. Detection of hot bands or isotopic band emission from around 0.1 µb in conjunction with a known temperature would allow deduction of the band source functions and hence inference as to the V-V rates.
