The Influence of Exposure History and Mineral Phase Composition on the Expression of Space Weathering Features in Lunar Soils

Abstract:

The primary weathering processes observed on the lunar surface are solar wind irradiation and micrometeoroid bombardment. These space weathering processes produce characteristic features such as thin (<200 nm) amorphous rims, which can be enriched in vesicles, nanophase metallic iron inclusions (npFe⁰), or a combination of these. An unresolved question in lunar space weathering concerns the individual and tandem influences of solar wind irradiation and micrometeoroid bombardment in the formation of these ‘typical’ features. Within this broad question, exposure history and bulk composition are two variables that have received limited attention.

This investigation explores lunar soil samples kept at ambient temperatures since their return that will later be compared with frozen samples that are part of the “new” lunar collection. The samples represent two different modal compositions and maturities and also differ in the amount of exposure they had to space weathering processes. We will focus on the presence of systematic differences in amorphous rim thickness; the size, abundance, and distribution of npFe⁰ and vesicles; and any detectable relationships between these features and sample chemistry. Characterization of these aspects will be carried out using scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS), and energy dispersive spectroscopy (EDS) analyses on samples prepared by ultramicrotomy.

By constraining these relationships, we are developing a better understanding of which lunar materials are more susceptible to vesicle formation and volatile trapping, and whether or not micrometeoroid bombardment is a necessary precursor to their nucleation. Ideally, these data can shed light on phenomena such as lunar swirls, albedo anomalies hypothesized to be associated with variation in solar wind exposure. Moreover, the efforts here underpin a broader initiative within the Apollo Next Generation Sample Analysis (ANGSA) program to develop new perspectives on the mechanisms that shaped the evolution of the Moon through the examination of pristine samples. Ultimately, determination of these aspects will provide reference data for future lunar exploration efforts and investigations of space weathering on other airless bodies in the Solar system.