Hidden in Plain Sight : Spinel - Rich Deposits on the Nearside of the Moon as Revealed by Moon Mineralogy

Introduction: Investigation of spectral anomalies in global data acquired with the Moon Mineralogy Mapper (M) [1] has revealed a new, unique, and unexpected spinel-rich lithology on the central nearside. These spinel-rich deposits are found only among the Sinus Aestuum pyroclastic/dark mantle deposits (DMD) and are notably absent from the adjacent Rima Bode DMD. While these deposits are spatially extensive (10000’s km) [2], the most spinel-rich signatures occur at much smaller scales (<1 km). These newly identified regions are defined by their strong 2 μm absorptions and extremely weak or absent 1 μm absorptions as is characteristic of spinel-group minerals [3]. M’s combination of high signal-to-noise, extended spectral range (both 1 and 2 μm), and spatial resolution (140 m/pixel) has enabled this unique, previously hidden, spinel-rich deposit to be uncovered. Data and Methods: M is a 0.43-3.0 μm imaging spectrometer on-board India’s Chandrayaan-1 spacecraft. The M data used here include 85 bands with 2040 nm resolution and a spatial scale of 140 m/pixel. After radiometric and geometric calibration (including inflight darks and flatfields), the M data are calibrated to apparent reflectance by dividing by the solar spectrum, correcting for the cosine of the incidence angle, and normalizing to account for cross-track photometric effects (±12° FOV). Thermal emission is significant at wavelengths longer than ~2.2 μm. These effects have not yet been removed from the image data presented here. However, thermal contributions are modeled for individual representative spectra. Identification: The spinel-rich deposits were first recognized from analysis of global mosaics (spatially reduced by a factor of 10), where their strong 2 μm band and weak 1 μm band results in very high values in ratios of the integrated band depth (IBD) at 2 μm to that at 1 μm. They were also identified as spectral anomalies in statistical analyses of 85-dimensional space [4]. The only extensive spinel-rich deposits occur in DMDs on the central nearside. Additional small, isolated spinel deposits were found on the farside, but restricted to the inner ring of Moscoviense [5].