An improved optical model for the non-LTE problem for the CO2 molecule in the atmosphere of Mars:: Nighttime populations of vibrational states and the rate of radiative cooling of the atmosphere

The estimates of the population of excited vibrational states of the CO2 molecule and of the rate of radiative cooling of the atmosphere in the 15-mum CO2 band are given for the nighttime mesosphere and thermosphere of Mars. For the first time, these estimates are made (1) with allowance for the overlap of lines in the 15-mum band; (2) for a wide set of vibrational states of seven isotopes of the CO, molecule, which was used earlier in the solution of a similar terrestrial problem; and (3) using the rate constant for quenching of the CO2(01(1)0) state in collisions with oxygen atoms, which has been recently measured for low temperatures by Khvorostovskaya et al. (2002). The main results are as follows. 1. The approximation of isolated lines provides a satisfactory accuracy of determining the radiative cooling rate and overestimates vibrational temperatures of the states of the nu(2) mode by no more than 3 K for the (CO2)-C-12-O-16 molecule and by no more than 2 K for low-abundant isotopes of the CO2 molecule. 2. A reasonably high accuracy of estimating the cooling rate can be achieved by taking into account only fundamental vibrational transitions in (CO2)-C-12-O-16, (CO2)-C-13-O-16, (OCO)-O-16-C-12-O-18, and (OCO)-O-16-C-12-O-17 molecules and the hot transitions 2nu(2) --> nu(2) and 3nu(2) --> 2nu(2) in the (CO2)-C-12-O-16 molecule. 3. The vertical profile of the total rate of radiative cooling displays two peaks. The maximum near a height of 130 km is very sensitive to temperature and to the ratio of the mixture for oxygen in the atmosphere.