Micro-meteoroids impact vaporization as source for Ca and CaO exosphere along Mercury’s orbit

The study of the micro-meteoroid environment is relevant to planetary science, upper atmospheric chemistry, and space weathering of airless bodies, as Mercury. In that case, the meteoroids hit directly the surfaces producing impact debris and vapor, thus contributing to shape thin exosphere of the planet. This work is focused on the study and modelling of the Mercury’s exosphere formation through the process of Micro-Meteoroids Impact Vaporization (MMIV). The MESSENGER/NASA mission provided measurements of Mercury’s Ca exosphere, allowing the study of its configuration and its seasonal variations. The observed Ca exhibited very high energies, with a scale height consistent with a temperature > 50,000 K, measured mainly on the dawn-side of the planet. It was suggested that the originating process is due to MMIV, but previous estimations were not able to justify the observed intensity and energy. We investigate the possible pathways to produce the high energy observed in the Ca exosphere and discuss about the generating mechanism that produces the required abundance of this element. The most likely origin may be a combination of different processes involving the release of atomic and molecular surface particles. We use the exospheric Monte Carlo model by Mura et al. (2007) in order to simulate the 3-D spatial distribution of the CaO and Ca exospheres generated through the MMIV process, and we show that their morphology and intensity are consistent with the available MESSENGER observations. The results presented in this paper can be useful in exospheric studies and interpretation of the active surface release processes, as well as the exosphere observations planning for the ESA-JAXA BepiColombo mission that will start its nominal mission phase in 2026.