COMPARATIVE THERMOSPHERES - VENUS AND MARS

The thermospheres of Venus and Mars are compared using Venera, Pioneer Venus, Mariner and Viking data. These measurements provide constraints for modeling efforts which probe thermospheric processes, balances, and responses to various forcings and input parameters. In particular, the National Center for Atmospheric Research (NCAR) non-LTE CO2 radiative transfer model and the Thermospheric General Circulation Model (TGCM) have been adapted to the thermospheres of Venus and Mars for standard solar flux conditions. The better constrained Venus TGCM serves as a valuable precursor to the subsequent Mars TGCM, since both planets share many radiative and chemical processes peculiar to CO2 atmospheres. Results indicate that planetary rotation significantly modifies thermospheric structure and winds; no nightside ''cryosphere'' of the type observed for Venus is predicted for Mars. Atomic O is greatly depleted on Mars compared to Venus. Resulting dayside heat balances suggest that CO2 15-mu m cooling strongly regulates Venus temperatures, while the Mars circulation is more effective as a thermostat. Fundamental planetary parameters are shown to be responsible for many of these differences in Venus and Mars thermospheric features.