An analysis of the Mariner 10 color ratio map of mercury

The results of a geological analysis of the Mariner 10 orange/UV color ratio man of Mercury (B. Hapke, C. Christman, B. Rava, and J. Mosher, Proc. Lunar Planet Sci. Conf. 11th 1980, pp. 817–822) are given. Certain errors that occured in reproducing the published version of the 1980 map are pointed out. The relationships between color and terrain are distinctly nonlunar. There is no correlation between color boundaries and the smooth plains on Mercury, in contrast to the strong correlation between color and maria-highlands contacts on the Moon. There are no large exposures of low-albedo, blue material that could be considered to be Mercurian analogs of high-FeTi lunar maria basalts on any part of Mercury imaged by Mariner 10. Three lines of evidence imply that the crust is low in Fe²⁺ and Ti⁴⁺: rays and ejecta blankets are bluer than most areas on Mercury; the Fe²⁺ band in Mercury's reflectance spectrum is very weak or nonexistent and the albedo contrasts are smaller than those on the Moon. There is no evidence in the spectral or albedo data that a lunar type of second wave of melting ever occured on Mercury; rather, the observations are most consistent with the hypothesis that the smooth plains are extrusive landforms derived from local material, possibly mobilized by the Caloris event. In several places correlations between color and topography can be explained if older, redder, higher-Fe materials underlie younger, bluer, lower-Fe surfaces. There is some evidence of late Fe-rich pyroclastic-like activity.