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

Abstract

Apollo 17 core sample 73001/2 was recently made available to researchers for analysis using state-of-the-art techniques in the framework of a modern understanding of lunar surface processes. In this work, we employ transmission electron microscopic analysis to observe the mineralogy, microstructural, and chemical characteristics of space weathering and solar energetic particle (SEP) track distribution in soil grains in the <20 μm size fraction in core sample 73002. The modal mineralogy and stratigraphic space weathered grain abundance suggests that a geologically recent mixing event affected the top 3 cm of 73002. Surface exposure age distributions derived from SEP tracks demonstrate that individual regolith grains rarely reside on the surface for longer than ∼4 million years. The abundance of surface exposed monomineralic fragments with respect to depth correlates well with bulk measurements of space weathered soils using other techniques, such as ferromagnetic resonance. Exposure age distributions suggest the presence of two unique in situ reworking zones spanning the top 8 cm of the core and median exposure ages decrease with increasing depth for both reworking zones, albeit at different rates. These rates were compared to reworking models and suggest a relationship between median exposure age and reworking rate with respect to depth. Applications of modern transmission electron microscopy to core sample 73001/2 have proven useful in understanding lunar regolith evolution both within the context of the Apollo 17 field site and more broadly via in situ reworking.