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 low-albedo feature, Solitudo Maiae, appears to be associated with the Tolstoj Basin. Mercury’s rotation period of 58.64 days is in two-thirds resonance with its orbital period of 87.97 days Therefore, at its equator, longitudes 0° and 180° are subsolar points (“hot poles”) near alternate perihelion passage. The “hot pole” at 180° lies within the Tolstoj quadrangle; at perihelion, equatorial temperatures range from about 100 K at local midnight to 700 K at local noon. This daily range of 600 K is greater than that on any other body in the solar system. Mariner 10 photographic coverage was available for only the eastern two-thirds of the Tolstoj quadrangle. The MESSENGER spacecraft, which orbited Mercury from 2008 to 2015, mapped the planet in its entirety at much higher resolution and in more wavelengths of light than Mariner 10. It obtained topographic, reflectance, magnetic, gravitational, and other types of geophysical data from orbit in addition to the photography. Eminescu quadrangle is to the west of Tolstoj quadrangle, and Beethoven quadrangle is to the east. Raditladi quadrangle and Shakespeare quadrangle are to the northwest and northeast, and Neruda quadrangle and Michelangelo quadrangle are to the southwest and southeast. Stratigraphy Older plains materials The rolling to hummocky plains that lie between large craters in the southeastern part of the quadrangle make up the oldest recognizable map unit, the intercrater plains material. The plains were originally described as intercrater by Trask and Guest, who noted their level to gently rolling appearance and their general lack of well-defined craters larger than about 50 km in diameter. Malin showed the plains to contain highly eroded remnants of large craters and basins that are only very shallow circular depressions. These intercrater plains are, however, marked by a very high density of superposed craters that are small (5–10 km diameter),