Integrated topographic, photometric and spectral analysis of the lunar surface: Application to impact melt flows and ponds

Abstract

This study provides an analysis of the impact melt deposits (melt flows and ponds) associated with eight lunar craters with diameters between 9 and 96km in terms of their topographic, photometric and spectral properties. Our analysis is based on an integrated method for construction of digital elevation models (DEM) of high lateral resolution, pixel-wise photometric modelling, construction of topographically corrected normalised reflectance maps, extraction of spectral parameters and estimation of elemental abundances based on Moon Mineralogy Mapper (M3) hyperspectral data and data of the Lunar Prospector Gamma Ray Spectrometer instrument. Most of the examined geologically fresh impact melt flows are characterised by lower albedos than the surrounding surface and tend to originate from topographically low parts of the source crater rim. As an exception, we find that the impact melt deposits northeast of crater Aristillus lack any optical signature and are not visible in monochrome images, but can only be observed by their anomalously smooth surface inferred from the DEM. Some impact melt flows and ponds exhibit photometric anomalies in terms of the asymmetries in the single-particle scattering function and the slope of that parameter with respect to wavelength. These photometric anomalies may be due to impact melt specific anomalies of the surface structure, e.g. consolidated rock rather than regolith, or anomalies in size or shape of the geometric surface elements (e.g. individual grains or their aggregates) on submillimetre spatial scales. The strength of the hydroxyl absorption displays a negative anomaly with respect to the surrounding surface for the majority of the examined impact melt flows. As an impact melt specific spectral behaviour, we find a decrease in the absorption band depth around 2000nm and (in several cases) a simultaneous increase of the absorption band depth near 1000nm. These observations are consistent with the laboratory analyses by Tompkins et al. (Tompkins, S., Pieters, C.M. [2010]. Meteorit. Planet. Sci. 45 (7), 1152–1169) of chemically uniformly composed mixtures between crystalline and glass material. For several impact melt flows, their data allow to roughly estimate the fraction of glass material based on our spectral analysis.