a moderate energy of the order of ∼0.1-1 keV.

The fluences of these different populations at the surface are summarized in Tab. 1.

Our results on the proton energy distribution at the surface are key to reliably modeling the exosphere of Mercury.

Solar wind protons contribute to the exosphere of Mercury via ion sputtering.

Ion sputtering is usually included in exosphere models assuming a monochromatic energy of 1 keV amu−1.

This is a good approximation for protons in the solar wind, as shown by the orange curves in Fig. 7(a)-(b).

However, this is not a good approximation at the hermean surface where the proton distribution (in blue in Fig. 7(a)-(b)) shows a large population at lower energies, coming from the interaction with the bow shock.

Protons below ∼500 eV weakly contribute to ion sputtering (Eckstein 2007).

Our modeling suggests that only ∼60% of the total precipitating protons will have significant sputtering yields.

From the fluences in Tab. 1, we see that this value changes with latitude.

Around the poles, ∼50% of the protons are efficient for sputtering (fluence of ∼1.7 × 1014 protons cm−2); whereas around the equator, this value goes to 100% with a fluence of ∼0.7 × 1014 protons cm−2.

This mixture of spatial and energy dependence of the precipitating proton flux is a key ingredient that should be account for in the modeling of the Hermean magnetosphere and exosphere in the future.

Another major release mechanism for volatiles into Mercury’s exosphere is photon stimulated desorption (PSD) (Mc-Grath et al. 1986; Killen et al. 1990; Madey et al. 1998; Leblanc et al. 2022); a process that is enhanced by ion bombardment (McGrath et al. 1986; Mura et al. 2009; Leblanc et al. 2022).

The coupling of these two processes is thought to produce a sodium exosphere in regions typical of ion sputtering, but with the high efficiency and lower energy distribution of PSD.

Estimates and patterns of volatile release via PSD will be modulated by the actual flux and energies to the surface.

Solar wind electrons contribute to the exosphere of Mercury via electron stimulated desorption (ESD).

This process is usually neglected in state-of-the-art exosphere models due to the lack of quantitative information on the flux and energy of precipitating electrons.

Past works (McLain et al. 2011; Schriver et al. 2011) estimated that ESD can generate as many neutrals as ion sputtering at Mercury; however, such estimates come with large uncertainties.

In our work, we provided quantitative estimates of the electron fluence and energy distribution at the surface of Mercury that will help to better evaluate the relevance of ESD as a source process for the exosphere of Mercury.

The high-resolution maps in Fig. 5 or the coarse grid values in Tab. 1 as well as the energy distribution in Fig. 7(c)-(d) can be used to advance the exosphere modeling at Mercury including ESD.

Solar wind electrons in the range 0.5-10 keV drive X-rays emissions from the surface via X-ray fluorescence (XRF).

While this process does not affect the chemistry of the surface, it allows the detection and mapping of regions of electron precipitation.

Such emissions have been observed by the XRS instrument onboard MESSENGER for lines of Si Kα (around 2 keV) and Ca Kα (around 4 keV) (Lindsay et al. 2016, 2022).

These emissions are driven by keV electrons accelerated in the magnetosphere by magnetic reconnection, as shown by Lavorenti et al. (2023) and discussed in Sec. 3.2.

However, XRS observations had a limited energy resolution and the surface coverage was constrained by MESSENGER’s orbit.

The MIXS instrument onboard BepiColombo (Bunce et al. 2020) will extend XRS observations 11 by providing (i)