Mercury: Evidence for Anorthosite and Basalt from Mid-infrared (7.3-13.5 μm) Spectroscopy

Spectroscopic observations (7.3-13.5 μm) of three locations on the surface of Mercury are reported. The observed spectral radiance emanated from equatorial and low latitude regions between 12 and 32° mercurian longitude on 8 December 1990, from the longitudinal region 22-44° on 10 December 1990, and from the longitudinal region 110-130° on 12 July 1992; all locations are primarily intercrater plains. Spectra indicate compositional differences among these three locations. The emissivity maximum, or Christiansen emission peak, occurs at 8.1 μm in the 8 December 1990 spectra, but at shorter wavelengths in the data of 10 December 1990 and 12 July 1992. Emission peaks near 8 μm indicate rocks of intermediate or mafic composition. Spectra from 22 to 44° longitude resemble spectra of terrestrial basalt and diorite with SiO2 content between 49 and 55%. The Christiansen feature in spectra from near 110-130° longitude strongly suggests the presence of plagioclase, in particular labradorite, while the overall spectrum resembles anorthosite. The spectra from all three locations on Mercury show distinct and recognizable features, the principal Christiansen emission peak being the most prominent, but they also contain features that we have not yet identified. The general indication from the spectra is that Mercury's surface consists of minerals more depleted in oxidized iron than those on the Moon. We also explore the theoretical and observational complexities of ground-based mid-infrared spectroscopy of airless bodies in general and Mercury in particular. A spectroscopic study of quartzite in both reflectance and emittance illustrates the practical, spectral validity of Kirchhoff's Law.