Orbital multispectral mapping of Mercury with the MESSENGER Mercury Dual Imaging System: Evidence for the origins of plains units and low-reflectance material

A principal data product from MESSENGER's primary orbital mission at Mercury is a global multispectral map in eight visible to near-infrared colors, at an average pixel scale of 1 km, acquired by the Mercury Dual Imaging System. The constituent images have been calibrated, photometrically corrected to a standard geometry, and map projected. Global analysis reveals no spectral units not seen during MESSENGER's Mercury flybys and supports previous conclusions that most spectral variation is related to changes in spectral slope and reflectance between spectral end-member high-reflectance red plains (HRP) and low-reflectance material (LRM). Comparison of color properties of plains units mapped on the basis of morphology shows that the two largest unambiguously volcanic smooth plains deposits (the interior plains of Caloris and the northern plains) are close to HRP end members and have average color properties distinct from those of most other smooth plains and intercrater plains. In contrast, smaller deposits of smooth plains are nearly indistinguishable from intercrater plains on the basis of their range of color properties, consistent with the interpretation that intercrater plains are older equivalents of smooth plains. LRM having nearly the same reflectance is exposed in crater and basin ejecta of all ages, suggesting impact excavation from depth of material that is intrinsically dark or darkens very rapidly, rather than gradual darkening of exposed material purely by space weathering. A global search reveals no definitive absorptions attributable to Fe2+-containing silicates or to sulfides over regions 20 km or more in horizontal extent, consistent with results from MESSENGER's Mercury Atmospheric and Surface Composition Spectrometer. The only absorption-like feature identified is broad upward curvature of the spectrum centered near 600 nm wavelength. The feature is strongest in freshly exposed LRM and weak or absent in older exposures of LRM. We modeled spectra of LRM as intimate mixtures of HRP with candidate low-reflectance phases having a similar 600-nm spectral feature, under the assumption that the grain size is 1 mu m or larger. Sulfides measured to date in the laboratory and coarse-grained iron are both too bright to produce LRM from HRP. Ilmenite is sufficiently dark but would require Ti abundances too high to be consistent with MESSENGER X-Ray Spectrometer measurements. Three phases or mixtures of phases that could be responsible for the low reflectance of LRM are consistent with our analyses. Graphite, in amounts consistent with upper limits from the Gamma-Ray Spectrometer, may be consistent with geochemical models of Mercury's differentiation calling for a graphite-enriched primary flotation crust from an early magma ocean and impact mixing of that early crust before or during the late heavy bombardment (LHB) into material underlying the volcanic plains. The grain size of preexisting iron or iron sulfide could have been altered to a mix of nanophase and microphase grains by shock during those impacts, lowering reflectance. Alternatively, iron-bearing phases and carbon in a late-accreting carbonaceous veneer may have been stirred into the lower crust or upper mantle. Decoupling of variations in color from abundances of major elements probably results from the very low content and variation of Fe2+ in crustal silicates, such that reflectance is controlled instead by one or more minor opaque phases and the extent of space weathering. (C) 2015 Elsevier Inc. All rights reserved.