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

Introduction The study of the micro-meteoroid environment is rele-vant to planetary science, space weathering of airless bodies, as Mercury, and their upper atmospheric chem-istry. In this case, the meteoroids hit directly the sur-face without any interaction with the atmospheric par-ticles, 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 un-precedented quality of Mercury’s exosphere, which permit the study of the seasonal variations of metals like Calcium [2]. The Ca in Mercury’s exosphere ex-hibited very high energies, with a scale height con-sistent with a temperature > 20,000 K, seen mainly on the dawnside of the planet. The origin of this high-energy, asymmetric source is unknown [1]. The gener-ating 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. Methods In this paper we work on models of Mercury’s im-pactors: using as an input the information on the arri-val geometry of the Mercury-intercepting particles [5], we modify the exosphere generation model by Mura et al. 2009 [3] in order to 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 photodissocia-tion of the initially released CaO molecules that popu-lates the exosphere with thermal Ca atoms (Fig. 1) and energetic Ca components (Fig.2) generated from the dissociative ionization and neutralization processes, excluding specific events like comet stream crossing [4]. We study the different contributions of Ca exosphere due to each process (Fig.3), how the impact vapor varies with heliocentric distance compare the results to the MESSENGER observations (Fig.4). Results Our results show that the 3-D morphology of the MIV-generated Ca exosphere, thermal and energetic, is con-sistent with the UVVS observations, these support the idea that the Ca source peaks near the dawn region. In the figures we ca see that the CaO exosphere is denser above the dawn hemisphere where the molecules are preferen-tially ejected into the exosphere by MIV process; Ca is preferentially seen in the midnight-to-dawn quadrant where CaO molecules are released by micrometeroid im-pacts and dissociated by the sunlight. We can also ob-serve that the exosphere due by energetic Ca generated by the dissociative ionization and neutralization processes is denser that the thermal Ca, produced by the photodis-sociation of CaO molecules. So the first process seems to be more significant that the second one. Conclusions The results presented in this work will be useful for the exosphere observations planning and for the data inter-pretation in the frame of the ESA/JAXA BepiColombo mission, that will study Mercury orbiting around the planet from 2025. More specifically, the resulting molecu-lar distributions will be compared to the measurements of the SERENA-STROFIO mass spectrometer that will be the only instrument able to identify the molecular com-ponents. Acknowledgments: the study is supported by ASI-SERENA contract no. 2018-8-HH.O Partecipazione scientifica alla missione BEPICOLOMBO SERENA Fase E