On the long-term weathering of the lunar surface by the heavy minor ions of the solar wind: inputs from ion observations and SRIM simulations

The lunar surface and the regolith that covers it 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 on regolith grains by displacing atoms inside the material. 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 minor ions heavier than alpha particles 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, long-term effects which develop on geological time scales are investigated. To do so, the long-term averaged energy spectrum of thermal, suprathermal, and energetic solar wind ions is estimated by compiling and contrasting ion measurements gathered by the ACE, Wind, STEREO, ARTEMIS, and MAVEN missions. The long-term ion environment to which the lunar surface is exposed to in the solar wind is then convolved with Stopping and Range of Ions in Matter (SRIM) simulations. Combining these data and ion effect models, we find that solar wind minor ions significantly alter the lunar surface and airless body surfaces in general, as they contribute to 12% to 22% of the total sputtering and create 20% to 50% of atomic displacements at depths greater than 100 nanometers. This new approach therefore confirms that solar wind minor ions play an important role in ion weathering of airless surfaces throughout the Solar System.