Planetary gamma ray spectrometer (GRS) has been used to map the elemental compositions of the surface materials of Mercury, the Moon and Mars. There were six times of GRS instruments have been on the lunar orbit for the past 40 years. Up to date, natural radioactive elements (U, Th, and K) and major elements such as Fe, Ca, Al, and Ti were globally mapped using data from Lunar Prospector, Kaguya, and Change-2 GRS, and are already released to the public.
This presentation demonstrates an investigation of the Si distribution based on the 4934 keV Si gamma ray peak. Our results are the first to detect Si concentrations at global scale similar to those discovered in lunar samples. We processed Kaguya GRS data using the gamma ray data analysis program using a grid of 10 degrees in longitude and latitude. Firstly, a Si peak analysis was accomplished by the Aquarius program and then, GRS data were corrected for altitude and thermal neutron density. Finally, a normalization process was performed using archived Si data of samples returned by the Apollo program.
For the Si linear regression model using spectral parameters, the continuum slopes, band depths, widths and minimum wavelengths of the absorption troughs near 1 µm and 2 µm were computed for a latitude range from 60°S to 60°N. These regression models were used to construct nearly global cpm (count per minute) maps of Si with a resolution of 20 pixels per degree. The regression-based cpm Si map was used for calibration to the elemental abundances inferred in Elphic et al. 2000 which is based on returned lunar samples for the nine Apollo and Luna sample return sites. The small range of Si abundances of only a few wt% across the landing sites did not allow for a robust estimate of both parameters of the linear calibration curve, so that we used a linear calibration curve with zero intercept. Latitudes beyond ±60° were excluded due to the low signal-to-noise ratio of the M³ data at high latitudes. The Si abundances estimated with our method are systematically higher than the LP GRS Si data but are consistent with typical Si abundances of lunar basalt samples and feldspathic mineral samples. The construction of a nearly global map of lunar Si abundances has been achieved by a combination of regression-based analysis of KGRS cpm data and M3 spectral reflectance data, and it includes a calibration with respect to returned sample-based wt% values. This study shows that the Si abundance on the Moon ranges from about 17 to 28 wt%. Highland terrains have a higher Si content (24–28 wt%) when compared to mare regions (17-21 wt%) consisting of mafic rocks and intermediate surface types (20–24 wt%). The Si map obtained by Kaguya GRS will provide an important aspect in both understanding mineralogical distribution and evolution of the lunar surface since its formation.