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 develop ed. The model runs from the planet's surface up to 180 km and has been especially elaborated to study non-local thermodynamic equilibrium (n on-LTE) situations. It includes the most important energy levels and v ibration-rotation bands able to give a significant atmospheric emissio n or produce a significant cooling/heating rate. Exchanges of energy i n 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 proces ses have been treated by using a modified Curtis matrix formulation. T he populations of the excited vibrational levels for nighttime conditi ons axe presented along with a sensitivity study of their variations t o the kinetic temperature profile and to collisional rate constants. T he 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 i n maintaining this situation above the tropopause. This result is prac tically insensitive to plausible variations in the kinetic temperature of the troposphere. The uncertainties in the rate constants play an i mportant role in determining the populations of the levels at thermosp heric altitudes, but they are of little significance for the heights w here they start departing from LTE. The CO2(0,0(0),1) level breaks dow n from LTE at about 60 km, the laser bands at 10 Am giving a significa nt 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.