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Airless body surfaces exposed to the solar wind are weathered by solar wind ions, which sputter atoms and molecules, participate to the creation of optically opaque particles, alter the regolith composition and create amorphous rims atop regolith grains. As protons and alpha particles dominate the density of solar wind ions, these two species may be the main contributors to ion weathering. However, the importance of solar wind ions heavier than alpha particles, sometimes referred as ‘minor’ ions, for the ion-induced alteration of airless body surfaces is an open debate. The fundamental question at stake is whether the variety of different minor ion species, their high masses, and their high charge states may overcome their low densities in the solar wind to enable them to significantly contribute to ion weathering processes. In this presentation, we investigate long-term effects which develop on geological time scales of hundreds to millions of years. The long-term averaged kinetic energy spectra of protons, alpha particles and heavy minor ions: We first estimate as best as possible the long-term averaged kinetic energy spectrum (thermal and suprathermal) of solar wind ions that impact airless body surfaces by compiling and contrasting ion measurements gathered over extended periods of time by the ACE, Wind, STEREO, ARTEMIS, and MAVEN missions. ACE and Wind are located at the Sun-Earth L1 Lagrange point, the STEREO A and B satellites are in orbit around the Sun at a distance of a 1AU, the two ARTEMIS probes are in orbit around the Moon and MAVEN orbits the red planet Mars. Among all these satellites, only ACE, Wind, STEREO A and STEREO B were equipped to observe solar wind minor ions, but ARTEMIS and MAVEN data help to estimate the flux of protons and alpha particles. Figure 1 shows the compilation of all available long-term spectra of solar wind iron ions. The green curve was published by Mewaldt et al. [2001] [1] and aggregates data obtained during 11 months in 1999 at “low” kinetic energies (Ek < 30 keV per nucleon) and measurements gathered from 1997 to 2000 at higher kinetic energies. For kinetic energies below 30 keV per nucleon, we find that the spectra published by Mewaldt et al. [2001] provide the most comprehensive picture to investigate the role of minor ions compared to protons and alpha particles. At higher kinetic energies, one can see on Figure 1 that all spectra computed over different periods of time with different ion measurement systems onboard different satellites are in excellent agreement. These ion spectra are the best available estimate of the number of ions impacting airless body surfaces over hundreds to millions of years.

On the Long-term Weathering of Airless Body Surfaces by the Heavy Minor Ions of the Solar Wind: Inputs from Ion Observations and SRIM Simulations