The Hermean Helium Exosphere—Continuous and Sporadic Helium Release Processes

Since its detection by Mariner 10, helium has been a key focus in studies of Mercury's exosphere. Recently, Weichbold et al. (2025), https://doi.org/10.1029/2024je008679 provided the first in situ helium measurements, inferring density from Ion Cyclotron Wave (ICW) events observed by the MESSENGER spacecraft. This approach enables, for the first time, a helium density profile across a broad altitude range without relying on prior models. We present an ab-initio model for a steady state, solar wind-driven helium exosphere, which informed the interpretation of these ICW measurements. We discuss helium release processes and evaluate whether meteoroid impacts could account for specific instances of elevated helium measurements. We developed a global, semi-analytical model based on a helium-saturated regolith and an average helium source flux of He/s from solar wind ion implantation. We calculate the helium flux distribution using an analytical lateral transport model and then generate local radial density profiles from a numerical (Monte Carlo) radial transport model. Additionally, we applied the radial transport model to estimate the scale and duration of large, sporadic helium release events and assess the likelihood of detecting these events in situ. The strong agreement between our model and the novel measurements confirms that the measurable helium exosphere is dominated by thermally recycled particles. We show that elevated helium measurements can result from the vaporization and release of helium from large (1 m) meteoroid impacts, but it is statistically unlikely that more than one impact event is captured in the given set of measurements.