Cosmic-ray exposure ages of pristine sample horizons
Science Goals: We propose to conduct noble-gas cosmic-ray exposure dating on a subset of samples from the vacuum-sealed and frozen lunar samples. As exposure dating depends on understanding the nature of the sample, we will also collect high-quality mineralogy and petrology of the samples. These data will be used to understand the surface exposure history, gardening rate, and potential for volatile formation and retention in the samples. Surface interaction with the solar wind and cosmic rays influences the measurement of multiple characteristics which are of importance to the community. Examples include organic load analysis, where organic materials degrade and change with exposure to UV, indigenous volatile analysis, where the solar-wind and cosmogenic contributions must be known and corrected for, and regolith layering and gardening, where successive generations of impact ejecta mix the surface components to varying depths. Cosmic-ray exposure ages will also be of great use when interpreting the history of the shadowed soils and determining their volatile content as a function of exposure time at the lunar surface.
Methodology: We have request samples from the drive tube 73001, and from permanently shadowed soils 72320 and 76240. The samples do not need to be processed frozen or under vacuum for our analyses. We would then use ~10 particles from each soil and horizon interval for our analyses. We prefer particles from the 2-4 mm size fraction for multiple reasons – the 2-4 mm fraction is well-studied in other lunar sites so forms a basis for comparison, the samples are large enough to determine petrology, mineralogy, and exposure age on the same sample, and it would free up larger samples for other analyses that require them (e.g., isotopic studies). However, we would work within the consortium framework to determine whether there are other studies that would benefit from exposure age determination on specific particles as well. We will mount the particles in CrystalBond epoxy and polish them to present a flat surface. We will use a scanning electron microscope and an electron microprobe to determine each particle’s petrology and mineralogy while they are mounted. We will then extract a subset of particles of interest, clean the epoxy off, and use noble-gas mass spectrometry to measure cosmogenic noble-gas isotopes. We will focus on isotopes of 38Ar and 36Ar produced by spallation of Ca, K, Ti, and Fe by cosmic rays in the lunar regolith, 21Ne and 22Ne produced by spallation of Mg, Al, Si, and Na, and Kr isotopes produced from Rb, Sr, Y, and Zr.
Science team members are from NASA Goddard Space Flight Center, Smithsonian, Carnegie Institute and NPP.