ORBITAL MULTISPECTRAL MAPPING OF MERCURY USING THE MESSENGER MERCURY DUAL IMAGING SYSTEM

One of the key observing campaigns during the MESSENGER’s primary orbital mission at Mercury was acquisition of a global multispectral map of surface reflectance in eight colors at visible to near-infrared wavelengths, at an average of 1 km/pixel or better, by the Mercury Dual Imaging System. These data have been calibrated, photometrically corrected to a standard geometry, map projected, and delivered to the Planetary Data System. Analysis of their global coverage reveals no spectral units not seen previously, and is consistent with the finding from previous analyses that most spectral variation (other than albedo) is related to changes in spectral slope and curvature between high-reflectance red plains (HRP) and low-reflectance material (LRM). Comparison between color properties of plains units and global mapping of smooth plains 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 frequency distributions of color properties distinct from other smooth plains and from intercrater plains. In contrast, other smaller deposits of smooth plains are indistinguishable from intercrater plains on the basis of the range of color properties exhibited, consistent with interpretations of intercrater plains as an older equivalent of smooth plains. A global search revealed no evidence for absorption features attributable to ferrous-iron-containing silicates or sulfide phases, consistent with results obtained from MESSENGER’s Mercury Atmospheric and Surface Composition Spectrometer. The only absorption is a broad band centered near 600 nm wavelength that is strongest in younger deposits of LRM; the feature is weak or absent in the oldest LRM. We modeled spectra of LRM by intimately mixing candidate low-albedo phases having this spectral feature with HRP. Of the phases examined, sulfides and coarse-grained metallic iron are too bright to produce LRM from HRP. Ilmenite is sufficiently low in albedo, but it would require a mass fraction of Ti inconsistent with upper limits obtained from MESSENGER X-Ray Spectrometer measurements. The remaining candidate for the darkening phase that cannot be rejected on the basis of our analysis or published elemental abundances is carbon possibly in the form of graphite.