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.