Solar flux variability of Mars' exosphere densities and temperatures

Using densities derived from precise orbit determination of the Mars Global Surveyor (MGS) spacecraft from 1999 to mid‐2005, the response of Mars' exosphere to long‐term solar change is established and compared to that of Earth and Venus. At Mars, exosphere temperatures (weighted towards high‐latitude Southern Hemisphere daytime conditions) change only 36–50% as much as those at Earth as solar activity increases from solar minimum to solar maximum, whereas the response at Venus is one‐fifth that at Mars. General circulation models suggest that this difference may be strongly influenced by adiabatic cooling associated with the thermosphere general circulation. However, other processes such as differences in CO 2 cooling rates may also be playing a role.