onto the surface of Mercury drives space-weathering processes such as ion sputtering, ion implantation, electron stimulated desorption (ESD), and X-ray fluorescence (XRF) (Domingue et al. 2014; Wurz et al. 2022). Ion irradiation affects the surface at an atomic level and the

[Figure: Figure 1. Sketch of plasma precipitation onto Mercury’s surface over orbital time. The 3:2 spin-orbit resonance is responsible for increasing the cusp precipitation at the hot poles (red dashed lines) compared to the warm poles (blue dashed lines).]

3 exosphere at a global level. Ion sputtering is thought to be one of the main source processes for high altitude sodium in the Hermean exosphere (Mangano et al. 2015; Killen et al. 2022). The maps presented in this work will allow researchers to better quantify ion sputtering at the surface of Mercury by relating the geographical distributions of ion fluxes with the surface distribution of sputtered species. ESD is another poorly understood source process of Mercury’s exosphere (Madey et al. 1998; McLain et al. 2011; Domingue et al. 2014). The electron maps computed in this work, coupled with maps of surface temperature and composition, will enable a precise description of ESD for exosphere models. XRF, driven by ∼keV electrons, converts precipitating electrons to X-rays photons at the surface of Mercury (Lindsay et al. 2016, 2022; Lavorenti et al. 2023). Future X-rays observations at Mercury by the joint ESA/JAXA BepiColombo mission (Benkhoff et al. 2021) will benefit from the electron-precipitation maps computed in this work. The BepiColombo/MIXS instrument (Bunce et al. 2020) will be able to constrain the surface composition and mineralogy, in part, by using solar wind electrons to “probe” the Hermean surface via XRF. Here, we present the first plasma precipitation maps at Mercury integrated over two full Mercury orbits (176 Earth days) to account for the spin-orbit resonance. We use the proton and electron precipitation maps published in Lavorenti et al. (2023) as a function of “time of day” as inputs to our computations over Mercury’s orbit. In this work, we neglect heavy solar wind ions (with atomic number Z ≥2), micro-meteoroids impacts, and thermal processes acting at the surface. The rest of this paper is organized as follows. Section 2 describes the methods used in this work. Section 3 presents the results with a focus on the spatial and energy distribution of particles at the surface of Mercury. In Section 4, we discuss the implications of our results for Mercury science and, more broadly, for space weathering of weakly magnetized bodies. Section 5 summarizes the conclusions of the paper.
2. METHODS We utilize the proton and electron precipitation maps published in Lavorenti et al. (2023), which were computed using a fully-kinetic, global, three-dimensional plasma model of Mercury’s magnetosphere. This numerical model solves the Vlasov-Maxwell system of equations using an implicit PIC (Particle-In-Cell) algorithm (Markidis et al. 2010). The model solves the plasma dynamics of the interaction between the solar wind and Mercury’s magnetic field, self-consistently including the kinetic physics of both protons and electrons. Kinetic models differ from other fluid plasma models (e.g., magnetohydrodynamic models) that do not take into account the velocity distribution functions of the particles but use only averaged quantities such as density, bulk velocity, pressure, etc. In our simulations, the normalized planetary radius, the proton-to-electron mass ratio, and the light-to-Alfv´en speed ratio are artificially reduced in order to be able to run on state-of-the-art HPC (High Performance Computing) facilities, while maintaining a good – although compressed – separation of scales between the planetary radius, the proton gyroradius, and the

[Figure: Figure 2. Magnetic field lines in the meridian plane (the X-Z plane in the Mercury-centered Solar Orbital (MSO) coordinate system) for our two simulations. The left (right) panel corresponds to a simulation with a northward (southward) IMF. Blue lines correspond to magnetic field lines not connected to the planet (i.e., solar wind field lines). Red lines correspond to magnetic field lines with one end connected to the planet (i.e., magnetospheric open field lines). Green lines correspond to magnetic field lines with both ends connected to the planet (i.e., magnetospheric closed field lines). The horizontal gray lines indicate the geographic equator of the planet.]

4 electron gyroradius.