Abstract
Since its detection by Mariner 10, Helium has been a key focus in
studies of Mercury’s exosphere. Recently, Weichbold et al. (2024)
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
meteorite 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 2.5x10²³ 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 (1m) meteorite
impacts, but it is statistically unlikely that more than one impact
event is captured in the given set of measurements.