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 and their upper atmospheric chemistry. In the case of airless bodies as Mercury, the exobase, namely the limit under which the collisions start to be important, is actually represented by its surface and meteoroids hit its surface directly, producing impact debris and contributing to shape its thin exosphere. The study of the generation mechanisms, the compositions and the configuration of the Hermean exosphere will provide crucial insight in the planet status and evolution. 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 almost exclusively 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. In this paper we work on models of Mercury’s impactors: we provide a detailed Ca-source extraction model simulating the expected 3-D Ca density distribution in Mercury’s exosphere due to the MIV mechanism. A prototype of the Virtual Activity (VA) SPIDER (Sun-Planet Interactions Digital Environment on Request) services is used as a Monte Carlo three-dimensional model of the Hermean exosphere to simulate the bombardment of Mercury’s surface by micrometeorites and to analyze particles ejected. The assumed physical parameters of these Mercury-impacting grains are examined to be consistent or not with the observations data. We study how the impact vapor varies with heliocentric distance and reproduce the morphology of the Mercury exosphere, demonstrating a persistent enhancement of the dust/meteoroid at dawn, which should be responsible of the dawn–dusk asymmetry in Mercury’s Ca exosphere. The results presented in this work can be useful in the exosphere observations planning for ESA BepiColombo mission, that will study Mercury orbiting around the planet from 2025, it is important to have a modelling tool ready for interpreting observational data and the results presented in this paper can be useful in the exosphere observations planning for the mission.