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

The study of the micro-meteoroid environment is relevant to planetary science, space weathering of airless bodies, as Mercury, and their upper atmospheric chemistry. In this case, the meteoroids hit directly the surface without any interaction with the atmospheric particles, producing impact debris and contributing to shape its thin exosphere. This work is focused on study and modelling of the Mercury’s exosphere formation through the process of Micro-Meteoroids Impact Vaporization (MMIV) from the planetary surface. The MESSENGER/NASA mission visited Mercury in the period 2008-2015, providing measurements of unprecedented quality of Mercury’s exosphere, which permit the study of the seasonal variations of metals like Calcium. The Ca in Mercury’s exosphere exhibited very high energies, with a scale height consistent with a temperature > 20,000 K, seen mainly on the dawnside of the planet. The origin of this high-energy, asymmetric source is unknown. The generating mechanism is believed to be a combination of different processes including the release of atomic and molecular surface particles and the photodissociation of exospheric molecules. We work on models of Mercury’s impactors: we consider the arrival geometry of the Mercury-intercepting particles and provide a detailed Ca-source extraction model simulating the expected 3-D CaO and Ca density distribution in Mercury’s exosphere due to the MIV mechanism. We simulate the photodissociation of the initially released CaO molecules that populates the exosphere with thermal Ca atoms and energetic Ca components generated from the dissociative ionization and neutralization processes, excluding specific events like comet stream crossing. We study how the impact vapor varies with heliocentric distance and compare the results to the MESSENGER observations. Our results show that the 3-D morphology of the MIV-generated Ca exosphere is consistent with the UVVS observations, these support the idea that the Ca source peaks near the dawn region: CaO exosphere is denser above the dawn hemisphere where the molecules are preferentially ejected into the exosphere by MIV process; Ca is preferentially seen in the midnight-to-dawn quadrant where CaO molecules are released by micrometeroid impacts and dissociated by the sunlight. The results presented in this work will be useful for the exosphere observations planning and for the data interpretation in the frame of the ESA/JAXA BepiColombo mission, that will study Mercury orbiting around the planet from 2025. More specifically, the resulting molecular distributions will be compared to the measurements of the SERENA-STROFIO mass spectrometer that will be the only instrument able to identify the molecular components.