Galileo observations of post-imbrium lunar craters during the first Earth-Moon flyby

Copernican-age craters are among the most conspicuous features seen on the far side and western limb of the Moon in the Galileo multispectral images acquired December 1990. Among the new morphologic observations of far-side craters are bright rays, continuous ejecta deposits, and dark rings associated with probable impact-melt veneers. These observations suggest that the mapped age assignments of several large far-side craters (Ohm, Robertson, and possibly Lowell and Lenz) need revision. New crater size-frequency measurements on Lunar Orbiter images suggest the following age reassignments: Hausen (170 km diameter), Pythagoras (120 km), and Bullialdus (61 km) from Eratosthenian to Upper Imbrian, and Carpenter (60 km) and Harpalus (39 km) from Copernican to Eratosthenian. Colors and albedos of craters (away from impact-belt veneers)) are correlated with their geologic emplacement ages as determined from counts of superposed craters; these age-color relations are used to estimate the emplacement age (time since impact event) for other Copernican-age craters. These age-color relations indicate a probable Copernican age for 27 far-side or western limb craters larger than 10 km diameter that were not previously mapped as Copernican. The apparent deficiency of Copernican craters on the far side compared with the near side in published geologic maps is not present in our data. The soil-maturity trends represent longer geologic time periods in regions with thinner regoliths, such as the maria. -from Authors