Meteoroids as a Source for Mercury’s Calcium exosphere

The study of the meteoroid environment for particles with masses in the 1 μg - 10 g range is relevant to planetary science, space weathering of airless bodies and their upper atmospheric chemistry. For the case of airless bodies as Mercury, meteoroids hit their surfaces directly, producing impact debris and contributing to shape their thin exospheres. Mercury is a unique case in the solar system: absence of an atmosphere and the weakness of the intrinsic magnetic field. The Hermean exosphere is continuously eroded and refilled by interactions between plasma and surface, so the environment is considered as a single, unified system surface- exosphere-magnetosphere. 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. A global description of planet’s exosphere is still not available: missions visited Mercury and added a consistent amount of data, but still the actual knowledge about the morphology of this tenuous atmosphere is anyway poor. This work is focused on study and modelling of the Mercury’s exosphere formation throught the process of Micro-Meteoroids Impact Vaporization (MMIV) from the planetary surface. 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 from different sources, as Jupiter Family Comets (JFCs), Main Belt Asteroids (MBA), Halley Type and Oort Cloud Comets (HTCs and OCCs), and to analyze particles ejected. We study how the impact vapor varies with heliocentric distance and the high impact velocity of these particles makes them critical for 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 assumed physical parameters of these Mercury-impacting grains are examined for consistency with the observations data. Furthermore, considering that Mercury is the target of the 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.