A Mariner 10 color study of Mercurian craters

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 ∼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.