A long-term perspective on Mars' exosphere variability at 405 km is provided by merging together density data derived from precise orbit determination of the Mars Global Surveyor and Mars Odyssey (MO) satellites extending from 2001 to 2010. These data are heavily weighted toward afternoon local times at high latitudes in the Southern Hemisphere. Clear long-term solar and annual variations are well captured by empirical formulas. Residuals from the empirical fit show evidence for relative depletions in exosphere density around Mars' closest approach to Earth, which would be consistent with a scavenging mechanism that is dependent on solar wind dynamic pressure. Superimposed on this variation with Mars-Sun distance are positive density residuals during Mars year (MY)25, MY27, and MY29 that are apparently due to elevated dust levels in Mars' middle atmosphere. However, during MY24, MY26, and MY28 there are dust level increases without any corresponding increase in exosphere density. We suspect that this inconsistency is related to a variable ability to sense the response to dust-related effects, imposed by the high-latitude limitations of our measurements combined with interference between the mechanisms that translate middle atmosphere heating to an exosphere response. Evidence also supports the hypothesis that winter helium bulge effects contributed to the inferred interannual density variability during the 2007–2009 solar minimum period, when the O-He transition height likely resided near the ~405 km orbit of MO.
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