Analyzing the Mineralogy and Space Weathering Characteristics of the Finest Fraction in Apollo Core Sample 73002

<p>In 2019, NASA released Apollo core sample 73002/1 for scientific study for the first time. The sample has previously remained under vacuum to retain it's scientific integrity. 73002 is the top ~20 cm of lunar regolith and exhibits evidence of long durations of exposure to the space environment. The collective mechanisms that alter lunar regolith is known as space weathering. These mechanisms include irradiation exposure from solar wind and flares, as well as impacts from micrometeorites. Irradiation from solar wind will produce a layer (up to ~100 nm thick) of damage on the outer rim of crystalline materials. Particles with higher energies, typically from solar flares, will penetrate millimeters in depth leaving behind trails of irradiation damage. Micrometeorites will produce melt, vapor, and reduced metallic iron particles. All of these features can be observed via transmission electron microscopy (TEM). Core sample 73002 was split into 0.5 cm intervals. The dataset presented here reflects the stratigraphic analysis of space weathering features of individual regolith grains found in core sample 73002. We used intervals representing the top 8 cm as this region has the highest degree of space weathered material, and the bottommost interval in order to compare the top 8 cm to the least space weathered region in the core.</p>

<p>TEM imaging and chemical analysis was used to characterize grain microstructure and mineralogy in order to assess characteristics linked to space weathering. Bright field (BF) and dark field (DF) scanning transmission electron microscope (STEM) images were primarily collected at NASA Johnson Space Center using a 200 keV JEOL 2500SE STEM, equipped with a 60 mm<sup>2</sup> ultra-thin window silicon drift energy dispersive X-ray (EDX) spectrometer. Additional images were collected at Purdue University using a 200 keV Talos 200X TEM with a dual Super-X SDD EDX. Analysis was focused on monomineralic crystalline grains due to their abundance in the <20 μm size fraction and to avoid complications in quantification that may be associated with utilizing polymineralic fragments, such as differences in SEP track retention and solar wind damaged rim development (e.g., the solar wind damaged rim of olivine widens at a faster rate than plagioclase). Modal mineral analysis and space weathering evaluations were made simultaneously for each grain. Mineral types were determined by acquiring quantitative compositional maps and point measurements of multiple locations in each grain via EDX analysis using the Cliff Lorimer method.</p>