title: The surfaces of the Moon and Mercury: an experimental and numerical approach to ion sputtering
Creators: name: Jäggi, Noah
Publisher: name: Universität Bern
Year: 2023
Subjects: subject: 520 Astronomy
subject: 530 Physics
subject: 620 Engineering

description: Energetic solar wind ions erode the surfaces of the Moon and Mercury through sputtering. The process of sputtering ejects material with suprathermal velocities into the collisionless exospheres of their respective rocky body. The suprathermal populations in the exosphere thus directly sample the surface. Given the solar wind precipitation rates and areas, exospheric compositions can be interpreted if the sputtering yield of solar wind ions is known. A better understanding of ion sputtering will allow to quantify its importance relative to competing suprathermal processes such as photon stimulated desorption and micrometeoroid impact vaporization.
In the context of this thesis, mineral analogues and regolith samples were prepared, irradiated, and analyzed. A new preparation method using a custom pellet die was developed to obtain exceptionally resilient mineral powder pellets, without the use of a binder or adhesives. The analysis included infrared measurements in the 7–14 μm range, covered by BepiColombo/MERTIS (2.5–15 μm). We conclude that the interaction volume of infrared radiation exceeds the depth amorphized by average-velocity solar wind ions and does not lead to reliably detectable shifts in the spectrum. The efficient and rapid amorphization of the upper layer was determined computationally, and supported experimentally in collaboration with Biber et al. [1]. Therein, a crystal lattice effect on the sputter yield could not be detected between mineral pellets with microscopically rough surfaces and the glassy thin films produced from the same mineral powder. The differences in sputter yields could be attributed to roughness effects, suggesting extensive amorphization of the powder pellet surface.
The laboratory sputter yield results for flat surfaces were used to evaluate the established sputter code SDTrimSP and motivate the addition of two new models. The first model differentiates between oxide-bound elements and unbound elements in the sample and assigns density according to either the element or the oxide. It is capable of reliably reproducing mineral densities with simulated amorphization, causing only minor density changes at the surface that do not negatively affect the model. The second model expands on the commonly used surface binding energies by assigning a binding energy within the bulk sample. This bulk binding energy is based on the enthalpy of formation required to break up the oxides that make up the mineral. Both models rely solely on tabulated data and no parameter adjustment is necessary to fit laboratory data. The increased binding energies lead to a broadening of the energy distribution as observed in laboratory data of oxidized metal. At normal incidence, SRIM yields are up to a factor five above laboratory yields. SDTrimSP simulation results however are in unprecedented agreement with laboratory data when including the two newly implemented models.
In a sputter-unrelated part of this thesis, Mercury’s earliest magma ocean and atmosphere were modeled. A special focus was put on the loss or accumulation of sodium over the magma ocean lifetime, in an attempt to explain the exceptionally high surface concentrations of moderately volatile elements such as sodium, potassium, sulfur, and chloride on Mercury. Under average ‘young-Sun space weather conditions’, the combined atmospheric loss from plasma heating, photoevaporation, Jeans escape, and photoionization only accounts for a ≤0.02% decrease of the total sodium present in the mantle. This low degree of loss supports formation models which are based on the accretion of primitive chondrites to explain contemporary Mercury observations.
In conclusion, this thesis has advanced the understanding of sputtering on rock-forming minerals relevant for the Moon and Mercury. It includes data of lower than previously assumed yields, the ruling out of a crystal lattice effect on the sputter process, and the implementation of two new sputter simulation models, which in combination show results with unprecedented agreement with laboratory data. The laboratory data-verified computed sputter yields and their angular and energy distributions will help to differentiate solar wind ion sputtering from competing space weathering processes that provide suprathermal species to the exospheres of the Moon and Mercury.