Global Mineral Mapping of Diviner Data from 8-50 Microns

To date, the Diviner compositional investigation has been conducted using Diviner's three narrow band '8 μm channels' to model the position of the silicate Christiansen feature (CF), the position of which is sensitive bulk silica content. In this work, we extend Diviner spectral analyses to include additional broadband mid-infrared channels centered at 18, 31, and 66 μm. Incorporation of these channels into compositional analyses enables the use of spectral indexing and spectral mixture analysis techniques that were not previously available using 3-point spectra. We performed global compositional mapping using two approaches that complement previous CF mapping efforts. The first is mineral index mapping, which makes use of spectral shapes specific to the presence or absence of particular minerals. The second is spectral mixture analysis (SMA), which may provide a rough estimate of individual mineral distributions on the lunar surface. Analyses of global Diviner data show that emissivity varies considerably as a function of time of day, emission, and incidence angles. This can have strong effects on spectral indices and maps derived from SMA. Therefore, we have confined our initial analyses to a narrow range of emission (0-20 degrees) and incidence (20-40 degrees) angles that are experienced by a wide swath of the lunar surface, and over which emissivity values experience little variation. We are currently developing photometric corrections for the long wavelength Diviner channels analogous to those that have already been developed for the 8 micron channels. Our initial index mapping efforts have focused on olivine detection. Simulated lunar environment (SLE) spectra show that for the CF position of an anorthite-forsterite mixture to be elevated above average mare (~8.3 μm), there must be greater than about 90 weight percent forsterite present. It is clear from this work that non-linear spectral mixing makes it difficult to determine the presence of olivine from the spectral shape of channels 3-5 alone. However, these spectra show that with increasing olivine content, there is a concavity inversion between Diviner channels 4-6. We developed a concavity index using channels 4, 5, and 6 that is analogous to our previous index used to map highly silicic materials on the Moon. At local scales, the Diviner olivine index agrees well with M3 mineral detections at Aristarchus and other localities. At global scales, the olivine index is highest in the mare, although the index map differs substantially from previously reported CF maps. This is expected, given that the index is by definition more sensitive to olivine abundance than the CF. The addition of long wavelength channels to Diviner compositional analysis also enables rudimentary SMA. While the fine-grained nature of the lunar regolith likely precludes linear unmixing, spectral mixture analysis may prove to be a useful method to determine the presence and distribution of major silicate phases on the lunar surface. Global SMA results generally correlate well with the spectral index mapping, with higher olivine and pyroxene abundances in the mare and higher plagioclase abundances in the highlands. .Future work will focus on correlation and refinement of the deconvolution and index mapping methods