MERCURY - EVIDENCE FOR ANORTHOSITE AND BASALT FROM MIDINFRARED (7.3-13.5-MU-M) SPECTROSCOPY

Spectroscopic observations (7.3-13.5 mum) of three locations on the su rface of Mercury are reported. The observed spectral radiance emanated from equatorial and low latitude regions between 12 and 32-degrees me rcurian longitude on 8 December 1990, from the longitudinal region 22- 44-degrees on 10 December 1990, and from the longitudinal region 110-1 30-degrees on 12 July 1992; all locations are primarily intercrater pl ains. Spectra indicate compositional differences among these three loc ations. The emissivity maximum, or Christiansen emission peak, occurs at 8.1 mum 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 mum indicate rocks of intermediate or mafic composition. Spectra fro m 22 to 44-degrees longitude resemble spectra of terrestrial basalt an d diorite with SiO2 content between 49 and 55%. The Christiansen featu re in spectra from near 110-130-degrees longitude strongly suggests th e presence of plagioclase, in particular labradorite, while the overal l spectrum resembles anorthosite. The spectra from all three locations on Mercury show distinct and recognizable features, the principal Chr istiansen emission peak being the most prominent, but they also contai n features that we have not yet identified. The general indication fro m the spectra is that Mercury's surface consists of minerals more depl eted in oxidized iron than those on the Moon. We also explore the theo retical and observational complexities of ground-based mid-infrared sp ectroscopy of airless bodies in general and Mercury in particular. A s pectroscopic study of quartzite in both reflectance and emittance illu strates the practical, spectral validity of Kirchhoff's Law. (C) 1994 Academic Press, Inc.