Interactions of galactic cosmic rays with the lunar surface

Abstract

This dissertation is focused on improving understanding of the composition and structure of the Moon's crust and the processes that have affected it over its history. This is accomplished through integrating remote sensing datasets with models of modifying processes such as basin-forming impacts and galactic cosmic ray radiation. I focus on two areas of active study for the Moon: the nature of the mafic component of the highlands crust, and the unique nature of the Moon's polar regions. The first part of this dissertation integrates three remote sensing data sets: 1) mineral maps derived from ultraviolet-visible spectroscopy from the Clementine mission, 2) neutron counts from the lunar surface from the Lunar Prospector neutron spectrometer, and 3) elemental abundances from the Lunar Prospector gamma ray spectrometer. Systematic discrepancies between the three datasets are identified and resolved, resulting in new abundance maps of the major lunar minerals (olivine, clinopyroxene, orthopyroxene, plagioclase, and ilmenite) and an associated map of magnesium number for the mafic minerals. The second part of this dissertation employs the new mineral maps as constraints on mixing models. The mixing models assess contributions to the current lunar highlands crust of primary magma ocean anorthosites, later igneous products, and ultramafic mantle material excavated by large basins. We conclude that the magma ocean likely did not predominately produce anorthosites containing 15 vol% mafic minerals, and that the combined contribution to the highlands crust of ultramafic mantle material excavated by large basins and post-magma ocean igneous activity is between 15 and 45 vol%. The mixing models also support the conclusion of Weiczorek and Phillips (1999) that the excavation cavities of the largest lunar basins had depth-to-diameter ratios shallower than 1/10 (ratios between 0.03 and 0.07 are consistent with our mixing models). The final part of this dissertation uses the radiation transport code MCNPX 2.6.0 to model the radiation dose from galactic cosmic rays absorbed at different depths in the lunar regolith. We conclude that if simple ices are present in polar cold traps, they could have accumulated radiation doses equivalent to those used in experiments to stimulate synthesis of complex organic species.