Surface Composition of Mercury from Reflectance Spectrophotometry

Reflectance spectra of Mercury have been obtained periodically from 1963 through 1984. Since 1969, these observations were made in an effort to learn about the surface mineralogical composition of Mercury. Using the phases of the planet around maximum elongations, Mercury's 6Y3851day rotational rate, and the theory of bidirectional rejectance spectroscopy, some spatial resolution across the planet has been obtained. A very shallow absorption feature, which has been attributed to Fe* + in orthopyroxenes, is evident in two recent, highquality spectra but is noticeably absent in a third. This dzfference cannot be explained by rejected light from dzfferent terrain. No noticeable spectral differences exist between the portion of Mercury photographed by Mariner 10 and the unimuged portion of the planet. All of the rejectance spectra display the same slope seen in lunar rejectance spectra, attributed to Feand Ti-bearing agglutinates in the lunar regolith. Gravitational focusing of meteoric material at Mercury's heliocentric distance and the extreme surface temperatures experienced by Mercury's sunward hemisphere both suggest that the regolith formation rate would be higher than the Moon's, and that agglutinates of unknown composition cause the spectral slope.