Impact Vaporization and Mercury’s Superthermal Exosphere

Mercury’s tenuous atmosphere leaves its surface exposed to continuous meteoroid bombardment, which vaporizes surface material and enriches the exosphere with various species. Ground-based observations (Bida et al., 2000; Killen et al., 2005) first detected calcium in Mercury’s exosphere; subsequent measurements by the MASCS spectrometer onboard MESSENGER confirmed that these Ca atoms can reach remarkably high temperatures (12,000–20,000 K, and occasionally up to ~70,000 K) despite Mercury’s surface being only a few hundred K (Killen et al., 2005). The Ca corona also displays distinct temporal and spatial patterns, suggesting that meteoroid impact vaporization—especially from the 2P/Encke meteor stream—is a significant source of these superthermal Ca atoms. It has been proposed that Ca-bearing molecules, such as CaO, are vaporized by impacts and subsequently dissociated into Ca atoms. In this work, we employ a time-dependent Monte Carlo model to simulate the expansion of gases released by impact vaporization, incorporating multiple species and photodissociation processes to determine the spatial distribution of fragments. These results will aid in interpreting future observations by the BepiColombo mission.