A NONLOCAL THERMODYNAMIC-EQUILIBRIUM RADIATIVE-TRANSFER MODEL FOR INFRARED EMISSIONS IN THE ATMOSPHERE OF MARS .1. THEORETICAL BASIS AND NIGHTTIME POPULATIONS OF VIBRATIONAL LEVELS

A radiative transfer model to study the infrared (1-20 mum) emissions of the CO and CO2 molecules in the atmosphere of Mars has been developed. The model runs from the planet's surface up to 180 km and has been especially elaborated to study non-local thermodynamic equilibrium (non-LTE) situations. It includes the most important energy levels and vibration-rotation bands able to give a significant atmospheric emission or produce a significant cooling/heating rate. Exchanges of energy in thermal and nonthermal (vibrational-vibrational) collisions as well as by radiative processes have been included. An exhaustive review of the rate constants for vibrational-thermal and vibrational-vibrational collisional exchanges has been carried out. Radiative transfer processes have been treated by using a modified Curtis matrix formulation. The populations of the excited vibrational levels for nighttime conditions axe presented along with a sensitivity study of their variations to the kinetic temperature profile and to collisional rate constants. The populations of the CO2(0,nu2,0) levels follow LTE up to about 85 km with the radiative transfer processes playing a very important role in maintaining this situation above the tropopause. This result is practically insensitive to plausible variations in the kinetic temperature of the troposphere. The uncertainties in the rate constants play an important role in determining the populations of the levels at thermospheric altitudes, but they are of little significance for the heights where they start departing from LTE. The CO2(0,0(0),1) level breaks down from LTE at about 60 km, the laser bands at 10 Am giving a significant contribution to its population in the Martian mesosphere. The CO(1) level starts departing around 50 km and is noticeably enhanced in the upper thermosphere by absorption of upwelling flux from the planet's surface.