CO2 COOLING IN TERRESTRIAL PLANET THERMOSPHERES

The comparative approach to planetary problems is becoming increasingl y fruitful as new information from various planet atmospheres is assim ilated. In particular, it is clear that the important problem of CO2 c ooling in the Earth's lower thermosphere is closely tied to the thermo spheric heat budgets of Venus and Mars. CO2 cooling in each of these t hermospheres is strongly impacted by collisions of CO2 and O, yielding vibrationally excited CO2 and enhanced 15-mum emissions in regions wh ere non-local thermodynamic equilibrium conditions prevail. Both the r elative abundance of atomic O and the CO2-O relaxation rate affect the magnitude of this enhanced cooling process. We examine the recent pro gress in the debate on the CO2-O relaxation rate, its temperature depe ndence, and its corresponding impact on the thermospheric heat budgets of Venus, Earth, and Mars. This comparative approach provides the bro adest range of conditions under which a common CO2-O relaxation mte sh ould provide consistent results. New global mean calculations are pres ented for the heat budgets of these three planets using large CO2-O re laxation rates that have been inferred recently from Earth CO2 radianc e measurements and laboratory studies. Results indicate that available Venus and Mars data constrain the CO2-O relaxation rate to be 2-4 x 1 0(-12) cm3/s at 300 K. For Venus, this strong cooling serves as an eff ective thermostat that gives rise to a small variation of thermospheri c temperatures over the solar cycle, just as observed. Conversely, CO2 cooling does not appear to be dominant in the dayside heat budget of the Mars thermosphere over most of the solar cycle. For the Earth, thi s strong cooling implies that the lower thermosphere does not typicall y require significant eddy diffusion or heat conduction. However, glob al-scale dynamics or an additional heating mechanism may be needed to restore calculated temperatures to observed values when relaxation rat es exceeding 2 x 10(-12) cm3/s are employed.