Long-Term Variability of Mars' Exosphere Density Based on Precise Orbital Analysis of Mars Reconnaissance Orbiter and Mars Odyssey

The variability of Mars exosphere over monthly to solar-cycle scales at 251 and 412 km altitude is quantified by analysis of 41-Ls mean densities derived from precise orbit determination of the Mars Reconnaissance Orbiter (MRO) and Mars Odyssey (MO) satellites, respectively. The data encompass 2006-2020 (MRO) and 2002-2020 (MO). At both altitudes, most of the variance is captured by cos(Ls-phi), where phi approximate to 258 degrees. This term represents the effects of solar heating changes due to the eccentricity of Mars orbit around the Sun, and climatological changes in heating due to lower-atmosphere dust loading, which does not play a significant role. The remaining variability is connected with the "irregular" variability of solar flux over monthly time scales. For MO, the presence of Helium disrupts a clean correlation with these sources. Plain Language Summary The force of continuous bombardment of atmospheric particles on a satellite slowly changes its orbit. By tracking the satellite, changes in its orbit can in turn be used to infer atmospheric particle densities. In this paper, we use tracking from NASA's Deep Space Network, an international array of giant antennas, to measure minute orbital changes of the Mars Reconnaissance Orbiter and Mars Odyssey satellite orbits (and thereby atmospheric densities) around Mars during the 2002-2020 time frame. These data are used to study how the outer regions of Mars atmosphere respond to changes in heating due to solar radation absorption by various chemical species, and by dust that is elevated into the atmosphere by strong winds near the surface. The major finding of this paper is that the variability of Mars outer atmosphere is mainly controlled by solar heating changes due to the eccentricity of Mars orbit around the Sun.