Identification of Color Units From the Ultraviolet to the Near Infrared During the Second Messenger Flyby of Mercury

The first flyby of the planet Mercury by the MESSENGER spacecraft on January 14, 2008, provided a wealth of new, high-spatial-resolution remote sensing data. Analysis of spectra (wavelength range 220 - 1450 nm) from the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) produced several important new findings. Fresh crater deposits sampled by MASCS did not exhibit spectral features indicative of ferrous- iron-bearing silicates. The average visible to near-infrared spectral slope of the surface of Mercury was found to be significantly lower than that of the farside Moon. This slope difference is attributed to a greater abundance of opaque minerals in Mercury's regolith, a greater fraction of larger-sized particles of metallic iron, or a combination of these two effects. Finally, an absorption feature in Mercury's middle-ultraviolet spectrum, not seen in a lunar spectrum acquired with the same instrument, is interpreted as evidence for a low abundance of ferrous-iron-bearing silicates as well as a relatively lower modification of spectral properties by space weathering. The second flyby of Mercury will provide additional spatial coverage that will allow us to test and refine these initial conclusions. The brief opportunity to view the surface of Mercury during the first flyby allowed only a narrow strip of terrain to be sampled by MASCS. The two end-member color units (low-reflectance materials, and high-reflectance smooth plains) identified through an analysis of multispectral imaging from the Mercury Dual Imaging System (MDIS) acquired during the first flyby were not unambiguously sampled by MASCS. The hemisphere available during the second flyby was imaged previously by the Mariner 10 spacecraft, and these distinct units will now be observed by MASCS. In particular, MASCS will sample the plains areas west of Rudaki crater and the interior of the crater Homer. The Rudaki plains have been interpreted as a deposit of volcanically emplaced material embaying a local region of greater opaque content but similar iron abundance. The wide wavelength coverage and high spatial resolution of MASCS will provide a new dataset from which we will attempt to test these earlier inferences and elicit additional compositional details.