[1] Gamma ray spectroscopy data acquired by Lunar Prospector are used to determine global maps of the elemental composition of the lunar surface. Maps of the abundance of major oxides, MgO, Al 2 O 3 , SiO 2 , CaO, TiO 2 , and FeO, and trace incompatible elements, K and Th, are presented along with their geochemical interpretation. Linear spectral mixing is used to model the observed gamma ray spectrum for each map pixel. The spectral shape for each elemental constituent is determined by a Monte Carlo radiation transport calculation. Linearization of the mixing model is accomplished by scaling the spectral shapes with lunar surface parameters determined by neutron spectroscopy, including the number density of neutrons slowing down within the surface and the effective atomic mass of the surface materials. The association of the highlands with the feldspathic lunar meteorites is used to calibrate the mixing model and to determine backgrounds. A linear least squares approach is used to unmix measured spectra to determine the composition of each map pixel. The present analysis uses new gamma ray production cross sections for neutron interactions, resulting in improved accuracy compared to results previously submitted to the Planetary Data System. Systematic variations in lunar composition determined by the spectral unmixing analysis are compared with the lunar soil sample and meteorite collections. Significant results include improved accuracy for the abundance of Th and K in the highlands; identification of large regions, including western Procellarum, that are not well represented by the sample collection; and the association of relatively high concentrations of Mg with KREEP-rich regions on the lunar nearside, which may have implications for the concept of an early magma ocean.

[1] The MCNPX radiation transport code is used to simulate cosmic ray interactions within the Moon. Accurate source, geometric, and physics models are developed to successfully benchmark neutron density results with Apollo 17 measurements. The peak of the MCNPX lunar neutron density profile is shown to be within a few percent of the measured value, using a galactic cosmic rays modulation parameter that is consistent with the timeframe of the Apollo 17 mission. Sensitivity of the density profile to various input parameters and physics options is considered. Details of the simulation input are provided, along with neutron production and flux results, to facilitate additional benchmark efforts in the future.

[1] New models have been computed for the Lunar Prospector (LP) thermal and epithermal neutron counting rates using the particle transport code MCNPX. This work improves upon previous studies by using one code to model the neutron production, transport, and detection processes, and by examining the sensitivity of epithermal neutrons to elements other than hydrogen. Our modeling results for standard anhydrous lunar soils show that when hydrogen is not included in a soil, epithermal neutrons are most sensitive to variations in the abundances of Fe, Gd, and Sm, which is consistent with measured epithermal neutron data. We use our current modeling results, in conjunction with known mineral compositions of lunar soils and other lunar global data sets to conclude that the best explanation for a decrease in the counting rate of epithermal neutrons near both lunar poles is the presence of hydrogen. We have further concluded that the average hydrogen abundance near both lunar poles is 100-150 ppm and is likely buried by 10 ± 5 cm of dry lunar soil, a result that is consistent with previous studies. The localized hydrogen abundance for small (<20 km) areas of permanently shaded regions remains highly uncertain and could range from 200 ppm H up to 40 wt% H 2 O in some isolated regions.

[1] We have used improved knowledge of the spatial distribution of thorium (Th) on the lunar surface, in conjunction with a forward modeling analysis of Lunar Prospector gamma ray data, to estimate the thorium abundances of lunar red spots. The results from this study can be combined with preexisting compositional and morphologic evidence to suggest that Hansteen Alpha, the Gruithuisen domes, and the Lassell massif are silicic, nonmare, volcanic constructs, similar in nature to terrestrial rhyolite domes. We propose that either silicate liquid immiscibility or, more likely, basaltic underplating could have produced lunar rhyolite domes. Thus the Lunar Prospector data presented in this study provide new information about the full range of volcanic and crustal processes that could have occurred on the Moon.

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Abstract-The 50,000 year old, 1.8 km diameter Lonar crater is one of only two known terrestrial craters to be emplaced in basaltic target rock (the 65 million year old Deccan Traps). The composition of the Lonar basalts is similar to martian basaltic meteorites, which establishes Lonar as an excellent analogue for similarly sized craters on the surface of Mars. Samples from cores drilled into the Lonar crater floor show that there are basaltic impact breccias that have been altered by post-impact hydrothermal processes to produce an assemblage of secondary alteration minerals. Microprobe data and X-ray diffraction analyses show that the alteration mineral assemblage consists primarily of saponite, with minor celadonite, and carbonate. Thermodynamic modeling and terrestrial volcanic analogues were used to demonstrate that these clay minerals formed at temperatures between 130°C and 200°C. By comparing the Lonar alteration assemblage with alteration at other terrestrial craters, we conclude that the Lonar crater represents a lower size limit for impact-induced hydrothermal activity. Based on these results, we suggest that similarly sized craters on Mars have the potential to form hydrothermal systems, as long as liquid water was present on or near the martian surface. Furthermore, the Fe-rich alteration minerals produced by post-impact hydrothermal processes could contribute to the minor iron enrichment associated with the formation of the martian soil.

Granular zircon in impact environments has long been recognized but remains poorly understood due to lack of experimental data to identify mechanisms involved in its genesis. Meteor Crater in Arizona (USA) contains abundant evidence of shock metamorphism, including shocked quartz, the high-pressure polymorphs coesite and stishovite, diaplectic SiO 2 glass, and lechatelierite (fused SiO 2). Here we report the presence of granular zircon, a new shocked-mineral discovery at Meteor Crater, that preserve critical orientation evidence of specific transformations that occurred during formation at extreme impact conditions. The zircon grains occur as aggregates of sub-micrometer neoblasts in highly shocked Coconino Sandstone (CS) comprised of lechatelierite. Electron backscatter diffraction shows that each grain consists of multiple domains, some with boundaries disoriented by 65° around <110>, a known {112} shock-twin orientation. Other domains have {001} in alignment with {110} of neighboring domains, consistent with the former presence of the high-pressure ZrSiO 4 polymorph reidite. Additionally, nearly all zircon preserve ZrO 2 + SiO 2 , providing evidence of partial dissociation. The genesis of CS granular zircon started with detrital zircon that experienced shock twinning and reidite formation at pressures from 20 to 30 GPa, ultimately yielding a phase that retained crystallographic memory; this phase subsequently recrystallized to systematically oriented zircon neoblasts, and in some areas partially dissociated to ZrO 2. The lechatelierite matrix, experimentally constrained to form at >2000 °C, provided the ultrahigh-temperature environment for zircon dissociation (~1670 °C) and neoblast formation. The capacity of granular zircon to preserve a cumulative pressure-temperature record has not been recognized previously, and provides a new method for investigating histories of impact-related mineral transformations in the crust at conditions far beyond those at which most rocks melt.

[1] We have completed the first global spatial deconvolution analysis of planetary gamma-ray data for lunar Th abundances as measured by the Lunar Prospector Gamma-ray Spectrometer. We tested two different spatial deconvolution techniques -Jansson's method and the Pixon method -and determined that the Pixon method provides superior performance. The final deconvolved map results in a spatial resolution improvement of a factor of 1.5-2. The newly deconvolved data allow us to clearly delineate nearside Th enhancements and depressions, validate enhanced Th abundances associated with specific lunar red spots, and reveal new details of the Th distribution at the Aristarchus plateau. Citation: