The role of Meteoroid Impacts in the generation of Mercury’s Calcium exosphere: Modeling and Observations

Abstract: 

Mercury’s calcium (Ca) exosphere was first detected through ground-based telescopes and later confirmed by the Ultraviolet and Visible Spectrometer (UVVS) onboard the MESSENGER spacecraft. Observations of Ca column densities exhibit a scale height consistent with a temperature > 50,000 K, with a source located mainly on the dawn-side and seasonal variations, with enhanced emission shortly after Mercury passes through perihelion. Micro-Meteoroid Impact Vaporization (MMIV) process is considered as the main source of exospheric Ca, while meteor stream impacts possibly contribute to seasonal enhancements.

In this study, we use a 3D Monte Carlo model developed at the Institute for Space Astrophysics and Planetology (IAPS) to simulate the distribution of atomic Ca and Ca-bearing molecules produced by MIV, including both background meteoroid fluxes and dust flux associated with comet Encke. The model includes two distinct populations: a high-energy Ca component (> 20000 K), originating from shock-induced, non-equilibrium dissociative ionization and neutralization of Ca+ during the vapor cloud expansion, and a lower energy Ca component (< 10000 K), generated through the photo-dissociation of Ca-bearing molecules. Mercury’s non-uniform surface composition and its 3:2 spin-orbit resonance modulate the exposure of Ca-rich regions to meteoroid fluxes, producing distinct variations in the exospheric Ca profile over its 2-year orbit. Key model parameters, such as the photolysis lifetime of the molecules and the relative abundances of atomic and molecular Ca components, play a crucial role in shaping Ca exosphere, but they are still not well constrained by observations. To better understand the mechanisms governing the release and distribution of the Ca exosphere, we compare model simulations with data acquired by the PHEBUS spectrometer (Probing of Hermean Exosphere By Ultraviolet Spectroscopy) onboard ESA-JAXA’s BepiColombo spacecraft, currently in the cruise phase towards its destination. During the first three flybys, PHEBUS observed the Ca emissions near closest approach using its two visible channels. Preliminary results suggest that a high energy Ca component is consistent with the observed intensities, while the contribution from a low-energy component seems to be negligible. PHEBUS will collect extensive data along Mercury’s orbit, so we’ll be monitoring seasonal variations too and these data could be used to compare with the model.

This study allows evaluation of the consistency between the modeled and observed Ca distributions, and helps constrain the physical parameters governing exospheric source processes. Our results advance our understanding of the MMIV at Mercury, providing a valuable tool for interpreting BepiColombo observations and guiding future mission strategies.