Erosion of the Lunar Surface by Solar Wind Ions

Abstract

indicated in fig. 1. To model the exosphere creation, physical quantities such as the sputtering yield and the angular distribution of sputtered particles must be understood. To achieve this, we have studied the erosion of two types of samples prepared from lunar soil obtained during the Apollo 16 mission in 1972. The lunar soil material was either pressed into pellets (analogous to the description in [2]), or used to grow thin films of typically 100 nm thickness onto quartz resonators through Pulsed Laser Deposition (PLD). Typical ions found in the solar wind (hydrogen, helium) were used as projectiles with an impact energy of l keY/amu, equivalent to the medium solar wind velocity of 440 km/s. To measure mass depletion caused by ion sputtering, thin PLD films were used in a Quartz Crystal Microbalance (QCM) setup that directly measured changes in the resonance frequency of the QCM. This methodology was previously tested and refined using Lunar and Mercury analog materials [3, 4]. In the case of pellet samples made from lunar soil material, an additional QCM was used to collect sputtered particles. By collecting sputtered material at different polar angles, the setup detern1ined the angular distribution of the emitted particle flux. These measurements indirectly allowed to access the total sputtering yield of pellet samples in relation to thin film samples through differential sputter yield measurements [5]. Figure 1: lnteraction processes leading to surface erosion ofairless planetary bodies such as the Moon and release of refractory and volatile species into the exospbere. Differences between thin film and pressed pellet samples could be attributed to different surface roughness. For this purpose, simulations with the codes SPRAY [6] and SDTrimSP-3D [7, 8] were conducted, which yielded very good agreement. We will present our experimental findings for both types of samples along with simulation approaches. References [1] P. Wurz et al., Space Sei. Rev. 218, 10 (2022) [2] N. Jäggi et al., 1 Icarus 365, (2021) 114492 [3] P. S. Szabo et al., Astrophys. J. 891, (2020) 100 [4] P. S. Szabo et al., J. Geophys. Res.: Planets 125, (2020) e2020JE006583 [5] H. Biber et al., Planet. Sei. J. 3, (2022) 271 [6] C. Cupak et al., Appl. Surf. Sei. 570, (2021) 151204 [7] U. von Toussaint, Phys. Scr. Tl 70, (2017) 014056 [8] P. S. Szabo et al., Nucl. Jnstr. Meth. B 522, (2022) 47