A NONLOCAL THERMODYNAMIC-EQUILIBRIUM RADIATIVE-TRANSFER MODEL FOR INFRARED EMISSIONS IN THE ATMOSPHERE OF MARS .2. DAYTIME POPULATIONS OF VIBRATIONAL LEVELS

A non-local thermodynamic equilibrium (LTE) radiative transfer model has been applied to study the CO2 and CO emissions in the infrared (1-20 mum) in the atmosphere of Mars during daytime conditions. An extensive set of vibrational-translational (V-T) and vibrational-vibrational (V-V) collisional exchanges among the vibrational levels responsible for these emissions has been considered. Radiative transfer has been included for most of the transitions and its importance illustrated for some of them. The populations of the most important vibrational levels of CO and of the nu2 and nu3 modes of CO2 axe presented. The CO2(0,nu2,0) levels follow LTE up to about 80 km at daytime, some 5 km lower than at nighttime conditions. The absorption of solar radiation at 1.6, 2.0, and 2.7 mum, and subsequent relaxation by V-V and radiative processes, significantly populates these levels in the lower thermosphere, increasing all their vibrational temperatures with respect to nighttime conditions. Solar excitation and radiative transfer in 4.3 mum constitute the main sources of excitation of the (0,0(0),1) level in the thermosphere, where this level shows a very large vibrational temperature. The V-V transfer from highly excited CO2 levels is even larger than the direct radiative excitation of the (0,0(0),1) level in the mesosphere. The model predicts that the known inversion population between this vibrational level and the lower (0,2(0),0) and (1,0(0),0) levels will occur in the high mesosphere and above. The CO(1) level also shows much larger populations than during nighttime conditions, due to direct solar absorption at 4.7 mum and the role played by radiative transfer. A sensitivity study of the effect of current uncertainties in rate constants on the level populations is also presented. The uncertainties in the rate for nu3 quanta exchange among CO2 levels have significant effects on the deactivation of high energy states, leading to changes of importance in the daytime populations of the 2.7-mum states in the mesosphere and in the (0,0(0),1) level in the lower thermosphere.