Quantifying sputter yields of lunar soils

One of the influences that the Moon experiences in the space environment is the bombardment of the surface by solar wind ions, mostly protons and alpha particles. A consequence of this irradiation is the liberation of material through the process of sputtering. The ejected particles subsequently take part in the formation of the lunar exosphere [1]. Understanding the sputtering of the Moon’s surface and experimentally constraining the process quantities like sputter yield and angular distribution of ejecta is thus necessary to properly model the exosphere creation [2].

For this purpose, previous studies used analogue materials to investigate their erosion. We now present studies of two types of samples prepared from actual lunar soil obtained during the Apollo 16 mission: First, regolith material was pressed into stainless steel holders to form pellets, analogue to the sample preparation described in [3]. Apart from the application of pressure necessary for the pellet formation, the specimens were not further altered. Moreover, pulsed laser deposition was carried out to grow thin films onto quartz resonators using one such pellet as donor. While these films were checked to have the same chemical composition as the source material, they are however flat and vitreous.

Using such a resonator with the deposited lunar material as a Quartz Crystal Microbalance (QCM), we studied the mass depletion of the sample layer due to He⁺ and H⁺ ion bombardment in situ and in real time. Because this direct means of measuring the sputter yield cannot be applied to the rough and more pristine regolith pellets, another QCM was used. This second microbalance maintains a fixed distance d to the centre of the irradiated target and allows for variation of the polar angle β with respect to the target surface normal. The setup enables us to probe the angular distribution of particle flux by collecting a fraction of the liberated material. It is sketched in figure 1. With the thin film irradiations used as calibration, these differential sputter yields give indirect insight into the total mass sputtered away as a function of ion incidence angle. This approach has already proven to work well with analogue materials for the surfaces of celestial bodies [4]. We will present our experimental findings for both thin film and pellet irradiations along with simulation approaches to model these results. This study represents an important extension of previous experiments to actual lunar surface samples and will thus provide essential insights into constraining sputtering of the surface of the Moon and other planetary bodies.

Figure 1: Illustration of the experimental setup. Using the catcher QCM, the sputtered ejecta flux can be probed along the emission angle β under various incidence angles α.

[1] Hapke, B. et al; J. Geophys. Res. 106 (2001): 10039
[2] Wurz, P. et al; Icarus 192 (2007): 486
[3] Jäggi, N. et al; Icarus 365 (2021): 114492
[4] Biber, H. et al; Planet. Sci. J. 12 (2022)