The comparative approach to planetary problems is becoming increasingly fruitful as new information from various planet atmospheres is assimilated. Recent studies have shown that substantial advances in understanding can be realized by investigating common processes as they apply in different planetary settings. The long-term objective of our research program in comparative terrestrial planetary thermospheres is to contrast and compare the physical and chemical processes responsible for the structure and dynamics of the thermospheres of Venus, Earth and Mars. This program of study is important to our overall understanding of how these atmospheres are driven and how they change over time, both naturally and in the case of the Earth as a result of human influence. Our strategy involves modeling their characteristics and responses to different forcings by first using 1-D global mean heat balance and composition models [Bougher and Roble, 1991]. The present maturity of available planetary databases and modeling capabilities now permits us to extend this comparison of the thermospheres of Venus, Earth, and Mars using well tested and individually validated 3-D general circulation models of these three planets [e.g. Bougher and Roble, 1997; Bougher et al. 1998a,b].