Meteoroids as Source for Mercury’s exosphere

Abstract

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. The ESA BepiColombo mission will study Mercury in details, by orbiting around the planet from 2025. For this reason, it is important to study the planet exospheric density and to develop a modelling tool ready for testing different hypothesis on the release mechanisms and for interpreting future observational data.In this work we focus the attention on one of the processes responsible of the Mercury’s Ca exosphere formation: micro-meteoroids impact vaporization (MMIV) from the planetary surface. 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 Mercury exosphere, demonstrating a persistent enhancement of dust/meteoroid at dawn, which should be responsible of the dawn–dusk asymmetry in Mercury’s Ca exosphere. The Sun Planet Interactions Digital Environment on Request (SPIDER) Virtual Activity of the Europlanet H2024 Research Infrastucture is funded by the European Union's Horizon 2020 research and innovation programme under grant agreement No 871149.