Low-Reflectance Material in Mercury's Crust

Mercury's reflectance and spectral slope are broadly similar to that of the lunar nearside. However, a host of Earth-based measurements and spacecraft data indicate that the composition and physical makeup of their surfaces may exhibit significant differences. Apollo samples and orbital remote sensing show that the lunar nearside surface is generally high in FeO (6 to 20 wt %) while the farside surface abundance is somewhat lower (3 to 10 wt %). Earth-based remote sensing of Mercury indicates that its surface contains less than 6 wt % FeO and perhaps even lower than 3 wt %. The reflectance of the Moon, and most likely Mercury, is controlled to first order by variations in iron and titanium abundances. If Mercury's ferrous iron content is much lower than that of the lunar nearside, then why is its reflectance comparable (0.019 vs. 0.021 at phase angle 65°, respectively)? Two-color vidicon observations by Mariner 10 revealed patchy low-reflectance, relatively blue units within Mercury's crust. Hapke and coworkers first speculated that opaque minerals (most likely ilmenite) could explain the color and reflectance of this low-reflectance component. Multispectral image data obtained by MESSENGER during its January 2008 flyby of Mercury covered new terrain and provided higher resolution, better signal-to-noise ratio, and extended wavelength coverage above that obtained by Mariner 10. The new data confirmed the existence of the low-reflectance material (LRM) and its relatively blue color and provide much better geologic context to interpret the origin of this material. MESSENGER multispectral data show the LRM to be widespread across the surface and to occur at depth within the crust. Three key observations show the vertical distribution of LRM. First the rims and floors of 100-km-scale craters within the Caloris basin are composed of LRM, streamers of LRM occur in ejecta traceable back to outcrops in crater floors (e.g. Mozart), and large continuous sections of ejecta blankets of several craters and basins are composed of LRM (Tolstoj, Basho, Neruda). What is the composition and origin of the LRM? A significant abundance of ilmenite (low in reflectance and spectrally neutral) within the LRM is the most likely candidate material on the basis of MESSENGER multispectral data, laboratory spectral measurements of analog materials, lunar analogy, and cosmochemical arguments. Its widespread distribution deep within the crust is consistent with LRM forming as an original component of the crust or as a result of widespread later intrusion by magma. We note that no occurrences of LRM as surface flows or volcanic edifices have yet been identified. The widespread LRM material works to lower Mercury's overall reflectance along with space weathering processes. MESSENGER's second Mercury flyby (6 October 2008) will further elucidate the areal distribution of LRM and provide an opportunity for remote sensing over an extended wavelength range with the onboard spectrometer.