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 (Formula presented.) 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.

We present a study of an impacting descent probe that increases the science return of spacecraft orbiting or passing an atmosphere-less planetary bodies of the solar system, such as the Galilean moons of Jupiter. The descent probe is a carry-on small spacecraft (<100 kg), to be deployed by the mother spacecraft, that brings itself onto a collisional trajectory with the targeted planetary body in a simple manner. A possible science payload includes instruments for surface imaging, characterisation of the neutral exosphere, and magnetic field and plasma measurement near the target body down to very low-altitudes (~1 km), during the probe’s fast (~km/s) descent to the surface until impact. The science goals and the concept of operation are discussed with particular reference to Europa, including options for flying through water plumes and after-impact retrieval of very-low altitude science data. All in all, it is demonstrated how the descent probe has the potential to provide a high science return to a mission at a low extra level of complexity, engineering effort, and risk. This study builds upon earlier studies for a Callisto Descent Probe for the former Europa-Jupiter System Mission of ESA and NASA, and extends them with a detailed assessment of a descent probe designed to be an additional science payload for the NASA Europa Mission.