Tolstoj quadrangle
This basin, about 1550 km in diameter,[2] 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 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.
The plains were originally described as intercrater by Trask and Guest,[5] who noted their level to gently rolling appearance and their general lack of well-defined craters larger than about 50 km in diameter.
Malin[6] showed the plains to contain highly eroded remnants of large craters and basins that are only very shallow circular depressions.
On the other hand, the intercrater plains material partly postdates some of the major cratering events on Mercury, according to apparent superposition relations.
[6][7] In particular, the unit appears to overlap the entire northwest side of the Tolstoj Basin, a feature indicating that the intercrater plains in this region probably do not represent the remains of the primordial surface of the planet.
[8] Patches of less cratered, smoother, less rolling plains occur throughout the quadrangle, but their recognition is highly dependent on the resolution and lighting of individual Mariner 10 frames.
Despite Tolstoj's great age and its embayment by the ancient inter-crater plains, it retains an extensive and remarkably well preserved, radially lineated ejecta blanket around two-thirds of its circumference.
Analysis of stereo-photography of Tolstoj ejecta northeast of the crater suggests that this deposit has been upwarped to a higher elevation relative to the surrounding plains.
This stratigraphy is patterned after that used in and around the Orientale Basin on the Moon[13] and should aid in the future recognition of pre- and post-Caloris events over a broad expanse of the surface of Mercury.
The ubiquitous distribution of smooth plains in topographically low regions supports the hypothesis that these materials were deposited in a fluid or semifluid state as basin ejecta or volcanic flows.
The presence of large patches of smooth plains in the floor of the Tolstoj Basin and in irregular depressions in the extreme southeastern part of the map indicates that at least some of these materials may be volcanic.
The roughly rectilinear outlines of massifs within the Caloris Montes suggest structural control by a prebasin fracture pattern.
Its poor development and spacing much closer to the edge of the basin may be due to the greater mercurian gravity, as described by Gault and others.
[17] The Van Eyck Formation is characterized by an extensive radial ridge-and-valley system with minor concentric scarps and lineaments.
These features are considered for the most part as gouges and depositional plumes from secondary cratering within the Van Eyck; the remarkably straight ridges and steep walls, however, suggest formation by fracturing.
The position of Mozart at the west terminator of the Mariner 10 image data precludes visibility of its floor and thus hides any evidence of a possible central uplift or inner structural ring.
Some ridges, however, may represent flow fronts, but their estimated heights of several hundred meters would require formation by extraordinarily viscous lavas.
Numerous faint lineaments are visible within the quadrangle, especially in the area between the Tolstoj Basin and the large crater Zeami to the northeast.
Many of these lineaments may be faint secondary-crater chains or gouges; others may represent traces of an ancestral structural pattern that partly controlled the excavation of the craters and basin.
The largest lineament, which marks the northwest limit of recognizable Tolstoj ejecta, is a subdued scarp some 450 km long.
Thus, except for the lobate compressional scarps, it is difficult to separate internally produced structures from those of the complex impact history of Mercury.
