Multispectral images of Mercury from the first MESSENGER flyby: Analysis of global and regional color trends

We have analyzed MESSENGER multispectral images of Mercury in order to gain information on the nature of materials present on the planet's surface. The quantitative analysis tools of Lucey, which are built on the Hapke conceptual framework for the reflectance of a silicate regolith, allow us to derive spectral parameter images that are sensitive to the abundance of ferrous iron in silicates, the abundance of opaque phases, and the state of maturity of the surface. Consistent with previous estimates, we conclude that the silicate portion of Mercury's surface is low in ferrous iron, with a small range of variation. The spectral changes that accompany regolith maturation on Mercury are consistent with lunar-style space weathering via the accumulation of nanophase iron. The spectral character of Mercury's major surface units (high-reflectance plains, intermediate terrain, and low-reflectance material) can be attributed to variations in the abundance of an opaque phase(s). A class of minor color unit, the red spots, occurs in at least three morphological varieties, two of which are associated with deposits interpreted to be of pyroclastic origin. The bright crater floor deposits, a second type of minor unit, have extremely shallow (“blue”) spectral slopes that are unrelated to the presence of immature material.