Yearly variability of Mercury's exosphere: comparison of the Na and Ca cases.

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Abstract

The surface of Mercury is exposed to several processes that induce and/or modulate the release of gas into its thin atmosphere (exosphere): solar radiation, solar wind plasma and meteoroids precipitation. All these phenomena have a strong temporal and spatial variability; in some cases, there is a clear link with the true anomaly angle of Mercury, so that there is, in general, a yearly periodicity in the exosphere variability for almost all species. Here we focus on two different and quite opposite cases, the Calcium and Sodium components of the exosphere. The first is a paradigm for an exosphere that is strongly linked with the meteoroid and micrometeoroid precipitation, so that there is little need to consider or model the uppermost surface layer properties. The second, Sodium, is much more complex. For example, there is general agreement that a model of Sodium abundance is needed to explain the Na temporal variability, and that possibly more all surface processes should be accounted in a combined way. Still, recent analysis of the NASA MESSENGER mission and ground-based dataset revealed that the observed cycle of sodium has periodic afternoon enhancements that current models cannot explain.  Hence, we focus this study on the improvements that an updated Na composition and surface thermal model could possibly bring to the agreement between models and data.