Global Distribution and Spectral Properties of Low-Reflectance Material on Mercury

Abstract:

Distinctive low-reflectance material (LRM) was first observed on Mercury in Mariner 10 flyby images. Visible to near infrared reflectance spectra of LRM exhibit a flatter slope than the average reflectance for Mercury, which is strongly red sloped (increasing in reflectance with longer wavelength). Moreover, LRM is lower in reflectance (<0.04–0.05 at 560 nm wavelength) than an already dark planet (global average 0.06–0.07). In some cases, LRM spectra also exhibit a curvature suggestive of a weak, broad absorption band near 600 nm. From Mariner 10 and early MESSENGER flyby observations, it was suggested that ilmenite, ulvöspinel, carbon, or iron metal could cause the characteristic dark, flat spectrum of LRM. However, once MESSENGER entered orbit, low iron and titanium abundances measured by the X-Ray Spectrometer and Gamma-Ray Spectrometer ruled out ilmenite and ulvöspinel as important constituents and implied a different darkening phase, such as carbon or small amounts of opaque minerals dispersed as micro- or nanophase particles in a silicate matrix. To test the various hypotheses for the darkening agent in LRM and on Mercury more generally, we present a global map of the distribution of LRM across Mercury. We also examine the spectral properties of major LRM deposits as measured by the Mercury Dual Imaging Spectrometer and Mercury Atmospheric and Surface Composition Spectrometer to further constrain the composition of the darkening agent. Our mapping and spectral analysis supports the inferences that LRM is excavated from depth by large impacts, and that the darkening agent is likely carbon that is native to Mercury rather than exogenic.