The range of observed Mars exospheric temperatures obtained by Mariner, Mars, and Viking instruments is not highly correlated with solar EUV input. Various physical processes that influence Mars temperatures are now examined using parameterizations previously employed for Venus in order to investigate the plausibility of solar EUV as the most important thermospheric heat source. We demonstrate, using a one-dimensional NLTE radiative transfer code, that observed long-term variations in Mars exospheric temperatures can be largely reproduced (~210 to 360 K) using strong 15-&mgr;m cooling and moderate eddy conduction to balance 16% efficient EUV heating. A wider range of calculated exospheric temperatures (150 to 410 K) is acheived under the influence of variable eddy conduction and uncertain thermospheric atomic O. We observe that CO2 cooling is relatively less important for Mars thermal balance in comparison with Venus owing to the larger Mars scale heights which shift maximum heating to lower pressures where molecular thermal conduction is dominant, and because Mars atomic O is depleted. As a result, our calculated Mars global mean energy balance more closely resembles that of the earth than that of Venus. We also find that it is not possible to derive realistic heat balance models for both Venus and Mars using identical parameters. Venus's closer proximity to the sun requires additional cooling (possibly very efficient eddy conduction) if ≥16% efficient EUV heating is to be used to obtain observed temperatures. However, strong eddy conduction may be inconsistent with accompanying Venus model eddy effects on composition. These Mars calculations of reasonable global average temperatures provide the first step in the simulation of the dynamics and observed characteristics of the Mars upper atmosphere. ¿ American Geophysical Union 1988