Modeling Results on the Seasonal Influence at the Martian Exosphere

International audienceAnalysis of HST ACS/SBC images of Mars in the far-ultraviolet taken in Oct-Nov 2007 and May-November 2014 indicate seasonal influence as a driving factor of the hydrogen corona, as has been reported earlier. To derive the changes in number density and temperature, the data must be compared to a radiative transfer model to simulate the resonantly scattered optically thick Lyman α emission from the exosphere of Mars. This work presents details on the modeling process used to analyze the data and the corresponding uncertainties. The escape flux is highly dependent on the characteristics of the martian exosphere like the exobase temperature and number density of H atoms as well as the presence or absence of a superthermal population of hydrogen atoms. Detailed studies on the workings of the radiative transfer model indicate degeneracy between temperature and number density values that can fit the data. Therefore it is difficult to accurately determine the characteristics of the martian hydrogen exosphere without independent measurement of at least one of the variables. However, HST observations have the advantage of observing a large portion of the dayside exosphere with intensity profiles extending from altitudes of 700 - 30,000 km giving a better estimate of the best-fit temperature and density values which characterize the martian exosphere under different seasonal conditions. Comparisons of the latitudinal symmetry from the HST images indicate the exosphere to be symmetric beyond 2.5 martian radii due to the broad trajectories of atoms at high altitudes