Seasonal variation of Ca and Ca-bearing molecules in Mercury's exosphere as a product of micro-meteoroids and comet stream particles impact

The NASA/MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission provided measurements of Mercury’s Ca exosphere, allowing the study of its configuration and its seasonal variations. The observed Ca column densities exhibit a scale height consistent with a temperature > 50,000 K, and with a source located mainly on the dawn-side of the planet. It was suggested that the originating process is due to MMIV (Micro-Meteoroids Impact Vaporization), but previous estimations were not able to justify the observed intensity and energy. The most likely origin of this exospheric element is very probably a combination of different processes involving the release of atomic and molecular surface particles. We use an exospheric Monte Carlo model (Mura et al., 2007) to simulate the 3-D spatial distribution of the Ca-bearing molecule and atomic Ca in the exosphere of Mercury generated by the MMIV process. We investigate the possible pathways to produce the observed Ca exosphere and we discuss about the generating mechanism. Following previous studies, we consider that the atomic Ca in Mercury’s exosphere may be produced in a sequence of different processes: the exospheric energetic Ca component derives from the shock-induced non-equilibrium dissociative ionization and neutralization of Ca+ during the vapor cloud expansion, while a low energy Ca component is generated later by the photo-dissociation of the CaO molecules released by micro-meteoroid impact vaporization. Since the exact temperature, the photolysis lifetimes of the produced molecules and the excess energy during photolysis processes are still not well constrained by observations, we investigate different model assumptions. The theoretical calculations better agree with observations at shorter photolysis lifetimes and higher excess energy of Ca atoms obtained during photolysis of Ca-bearing species. In that case we show the presence of two Ca components: energetic Ca component more intense at high altitudes, and a low energy component in the post-dawn region at low altitudes. The total Ca content obtained through a best fit to the observations shows excess emission near TAA ∼ 25° and TAA ∼150°, which was attributed to the vaporization of surface material induced by the impact of a meteor stream. We investigate the possible contribution due to the comet 2P/Encke for explaining the excess Ca emission at specific orbit positions; the simulation results show some discrepancy when compared to the observations.