A side of Mercury not seen by Mariner 10

More than 60,000 images of Mercury were taken at ∼29° elevation during two sunrises, at 820 nm, and through a 1.35 m diameter off-axis aperture on the Southern Astrophysical Research telescope. The sharpest resolve ∼0.2″ (140 km) and cover 190°-300° longitude - a swath unseen by the Mariner 10 spacecraft-at complementary phase angles to previous ground-based optical imagery. Our view is comparable to that of the Moon through a pair of weak binoculars. Evident are the large crater Mozart shadowed on the terminator, fresh rayed craters, and other albedo features keyed to topography and radar reflectivity, including the putative huge "Basin S" on the limb. The classical bright feature Liguria resolves across the northwest boundary of the Caloris basin into a bright splotch centered on a sharp, 20 km diameter radar crater, and is the brightest feature within a prominent darker "cap" (Hermean feature Solitudo Phoenicis) that covers much of the northern hemisphere between longitudes 80° and 250°. The cap may result from space weathering that darkens via a magnetically enhanced flux of the solar wind, or that reddens low latitudes via high solar insolation