Tolstoj quadrangle
The Tolstoj quadrangle in the equatorial region of Mercury runs from 144 to 216° longitude and -25 to 25° latitude. It was provisionally called "Tir", but renamed after Leo Tolstoy by the International Astronomical Union in 1976. Also called Phaethontias. It contains the southern part of Caloris Planitia, which is the largest and best preserved basin seen by Mariner 10. This basin, about 1550 km in diameter, is surrounded by a discontinuous annulus of ejecta deposits of the Caloris Group that are embayed and covered by broad expanses of smooth plains. The southeast half of the quadrangle is dominated by ancient crater deposits, by nondescript rolling to hummocky plains materials between individual craters, and by isolated patches of nondescript plains. The ancient and degraded Tolstoj multiring basin, about 350 km in diameter, is in the south-central part of the quadrangle. The large, well-preserved crater Mozart (285 km diameter) is a prominent feature in the western part of the area; its extensive ejecta blanket and secondary crater field are superposed on the smooth plains of Tir Planitia. Low-albedo features Solitudo Neptunii and Solitudo Helii, adopted from telescopic mapping, appear to be associated with the smooth plains material surrounding Caloris; a third