can provide key information to infer the role of solar wind ion irradiation on the spectral processing of the surface.

However, given the strong dependence of these maps on the time-variable IMF direction, we introduced the proton effective shielding parameter αp ≈20 in Eq. 2.

The use of this parameter will enable researchers to compare the partially shielded surface of Mercury with those unshielded of the Moon and of unmagnetized asteroids such as asteroids Ryugu (Sawada et al. 2017) and Bennu (Lauretta et al. 2017), paving the way to comparative space weathering studies.

Space weathering studies of the hermean regolith rely on comparisons with the Moon and with asteroids (Domingue et al. 2014).

Understanding how the magnetic field of Mercury shields the planetary surface compared to that of other unmagnetized bodies is a key point to study comparatively the effects of space weathering on solar system bodies.

At Mercury, in this work, we showed that the surface is exposed on average to ∼2 × 1014 protons cm−2 integrating over two full Mercury orbits.

At the Moon, using Eq. 1 and considering a solar wind flux of 3 × 108 protons cm−2s−1, we find a fluence of ∼1015 protons cm−2 for the same time period (this value would be reduced to ∼7×1014 protons cm−2 if considering zero proton flux when the Moon crosses Earth’s magnetosphere; Poppe et al. (2018)).

For main belt asteroids, this fluence is further reduced by a factor ∼5 −10 due to the increased distance from the Sun.

Therefore, Mercury is bombarded by roughly 3−5 times fewer protons as compared to the Moon and by roughly the same amount of protons as compared to a main belt asteroid.

At Mercury, the magnetic field screening compensates for the increase in solar wind flux, as compared to the Moon.

In this work, we neglected the interplay between ion irradiation and micro-meteoroids processes such as impact gardening and comminution.

Given that the micro-meteoroid impactor flux at Mercury is about a factor 5.5 higher than at the Moon (Cintala 1992), this interplay might be important and should be addressed in future works.

10 Space weathering of Mercury’s surface has consequences for the remotely sensed properties of the regolith, namely the determination of composition via color and spectral properties.

Composition, both the identification of specific minerals and their abundances, are based on the detection and strength of absorption features.

Space weathering, by both solar wind irradiation and micrometeoroid impacts, reduces the strength of absorption features.

For example, the 1 µm band diagnostic of the mineral olivine can be completely masked by the formation of a small amount of nanophase iron (∼1% in weight of np-Fe0), via solar wind irradiation and micro-meteoroid-impact-generated vapor (Kohout et al. 2014).

At Mercury, the MESSENGER infrared spectrometer MASCS/VNIR (McClintock & Lankton 2007) showed no distinct spectral features throughout the visible to near-infrared, except for the possible indication of sulfide mineralogy within the hollows (Vilas et al. 2016).

Given the reduced proton fluence onto Mercury’s surface compared to the Moon (by roughly a factor 3 −5), we suggest that ion irradiation is not the main process at play in reducing spectral band signatures at Mercury.
4.2.

Magnetosphere as an accelerator and decelerator: the effect of a magnetic field on the energy distribution of particles at the surface Our numerical simulations show that the energy distribution of both protons and electrons are affected differently by the hermean magnetosphere.

On the one hand, the protons impacting the surface are composed of one low-energy population (around ∼200 eV) and of one moderate-to-high-energy population (around ∼1 keV).

On the other hand, the electrons impacting the surface have 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).