United States Naval Research Laboratory

Understanding the Role of Solar Wind Irradiation in Lunar Space  Weathering through Analysis of Nanoscale Features in Permanently  Shadowed Soils using Scanning Transmission Electron Microscopy

A major outstanding question of space weathering on the Moon is how solar wind irradiation and micrometeoroid bombardment interact to create the physical and chemical changes characteristic of space weathered materials and whether both of these processes are necessary for formation of “typical” features, such as nanophase metallic iron inclusions. The new samples made available as part of the Apollo Next Generation Sample Analysis (ANGSA) Program, specifically the “frozen,” fully or partially shaded soil samples 72320 and 76240, will allow us to address fundamental open questions about lunar space weathering. The frozen samples provide us with the exciting opportunity to study in situ the relationship between volatiles and other space weathering features.

These newly available samples allow us to address two main objectives. Our first objective is to gain better understanding of how and when hydrogen or helium, the main components of the solar wind, are trapped in vesicles and how cold-storage has affected our ability to measure these volatiles. Cold storage of the samples of interest has likely slowed the loss of volatiles from vesicles, and thus these samples can help elucidate the mechanisms of vesicle formation based on whether hydrogen or helium is still present and can help link vesicle formation to other space weathering processes. Additionally, we aim to better understand how solar wind affects space weathering features by comparative analysis of permanently shadowed, partially shaded, and fully exposed soil samples, which could in turn help to better understand the mechanisms of vesicle formation or other features associated with space weathering. The proposed work could have implications for understanding the importance of long-term cold storage of planetary samples, as well as for interpreting albedo anomalies associated with reduction in solar wind, such as lunar swirls.

In order to achieve these objectives, we will use aberrations-corrected scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDS) to analyze portions of individual soil grains from soil samples 72320 and 76240 as well as soils collected close by but beyond the shadows of boulders #2 and #4, respectively. STEM-EELS will be used to analyze volatiles trapped in defects and vesicles in the frozen samples and the results compared to measurements of portions of each soil sample that was not frozen, as well as to the samples that were not shaded.

Contact Information: Kate Burgess,  kate.burgess@nrl.navy.mil