Ab-Initio Model for Mercury’s Helium Exosphere

In this study, we suggest a model for the origin and abundance of exospheric Helium at Mercury. It was derived ab initio, assuming He-saturated regolith at the surface and a “steady-state” Helium exosphere. A 1D Monte Carlo computer simulation [1] was used to calculate the exospheric He density profiles according to the model.

The Helium abundance in the Hermean exosphere was first constrained from UV Spectrometer measurements aboard Mariner 10 [2] and has been discussed extensively since, also in the light of probable analogies to the Lunar He environment. It is believed that there are two major sources for exospheric Helium: release of Solar Wind implanted He from the regolith of the Hermean surface and outgassing of radiogenic He from the interior [3]. However, there is no agreement on the quantitative contribution of the two possible origins. Through a larger volume and diversity of data, new insights concerning the origins and other aspects of the Hermean Helium system could be derived. Novel approaches are allowing the derivation of Helium density profiles from MESSENGER data [4], and soon the SERENA plasma/neutral particles package [5] on BepiColombo’s Mercury Planetary Orbiter (MPO) is expected to add the first ever in-situ density measurements to the picture.

The presented model shows that the Helium exosphere is dominated by exospheric recycling. This term describes the process in which particles that have been released into the exosphere at energies below E_esc return to the surface and bounce back into the exosphere immediately at the energy corresponding to the local surface temperature. The Helium accumulates in the exosphere, where its abundance is eventually limited by the exospheric loss processes of Jeans escape and ionization. This model can build the foundation for an evaluation of future data and can allow a quantification of the two exospheric Helium sources.

[1] Wurz, P. and Lammer, H. (2003). Icarus 164.1 (2003): 1-13.
[2] Broadfoot, A. L., et al. (1976). Geophys. Res. Lett., 3: 577-580.
[3] Hartle, R. E., et al. (1975),  J. Geophys. Res.,  80(25)
[4] Weichbold, F., et al. (2024), in preparation.
[5] Orsini, S., et al. (2021), Space Sci Rev 217, 11