MERCURY: EVIDENCE FOR ANORTHOSITE AND BASALT FROM MID-INFRARED (7.3-13.5 micron) SPECTROSCOPY


ANN L. SPRAGUE

Lunar and Planetary Laboratory
University of Arizona, Tucson, AZ 85721
sprague


R.W.H. KOZLOWSKI

Susquehanna University, Selinsgrove, Pennsylvania 17870


WITTEBORN, Fred C., NASA

Ames Research Center, Moffett Field, CA 94035




ABSTRACT

(ICARUS 109, 156-157, 1994)

Spectroscoic observations (7.3-13.5 micron) 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 degrees mercurian longitude on 8 December 1990, from the longitudinal region 22-44 degrees on 10 December 1990, and from the longitudinal region 110-130 degrees on 12 July 1992; all locations are primarily intercrater plains. Spectra indicate compositional differences amont these three locations. The emissivity maximum, or Christiansen emission peak, occurs at 8.1 microns in the 8 December 1990 spectr, but at shorter wavelengths in the data of 10 December 1990 and 12 July 1992. Emission peaks near 8 microns indicate rocks of intermediate or mafic composition. Spectra from 22 to 44 degrees longitude resemble spectra of terrestrial basalt and diorite with silicon dioxide content between 49 and 55%.The Christiansen feature in spectra from near 110-130 degrees 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 quarttzite in both reflectance and emittance illustrates the practical, spectral validity of Kirchoff's Law.