Analysis and modeling of remote observations of the martian hydrogen exosphere

International audiencePast observations of the martian exosphere have given a wide range of values for the mean temperature and number density of the hydrogen population that occupies this uppermost layer. More recently, observations by HST and MEX have found large variations over short timescales exhibited by this layer, which have been attributed to seasonal effects. Here we present an analysis of the modeling techniques used to study the martian exosphere and their related uncertainties, and discuss the sensitivity of various modeling parameters for any remote observations of the martian hydrogen exosphere. Degeneracy between the two free parameters in the model, the exobase temperature and density of hydrogen at Mars introduces difficulty in accurately characterizing the properties of the martian exosphere. An independent measurement of at least one parameter is required in order to positively identify the other. The likely presence of a superthermal component of H adds another uncertainty to the modeling process, with large changes in the resulting escape flux. A study of the latitudinal symmetry of the martian exosphere found the radial emission profiles to be asymmetric below 2.5 martian radii, and then more uniform at high altitudes. Comparisons between simulated spacecraft and HST intensity profiles with altitude suggest that a larger coverage of intensity profiles is important to better determine the characteristics of the martian exosphere