A Mariner 10 color study of Mercurian craters

[ 1] We have used recalibrated Mariner 10 color image data (UV and orange) to examine spectral trends associated with crater features on the "incoming'' hemisphere of Mercury. Spacecraft images of the Moon ( Galileo and Clementine) and lunar sample laboratory spectra provide a framework for interpreting the Mercurian data. Two spectral parameter images were constructed. One reveals opaque mineral abundance, and the other provides a map of variations related to optical maturity and FeO content. The crater Kuiper and its rays are bright mainly because they are fresh ( immature) but also because Kuiper has excavated material with a lower opaque content than the surroundings. Under the influence of space weathering, Kuiper will eventually lose its rays and evolve into a bright-floored crater like Lermontov. Lermontov and nearby smaller craters are examples of craters that are likely mature, but remain bright because of the low-opaque material exposed on their floors. In this portion of the planet, it appears that a surface layer with moderate-opaque abundance overlies low-opaque material found at depth. On the basis of estimates of crater excavation depths, we suggest that the moderate-opaque surface layer is similar to 3 - 4 km thick. This two-layer stratigraphy is reminiscent of the lunar highland crust and may have originated in similar magma ocean processes. We also describe a feature that exposes dark material with a relatively high opaque content. This feature could be a Mercurian analog to a lunar dark-halo impact crater or perhaps a pyroclastic deposit. This implies that opaque-rich, possibly mafic, magmas were generated in the interior and reached the surface through effusive or explosive activity.