Shakespeare quadrangle

[1] Most images used in mapping the geology of the Shakespeare quadrangle were taken during the near-equatorial first pass, with close encounter or the dark side of the planet.

High-resolution images of small areas within the quadrangle were also obtained during the third pass, when the spacecraft was on a near-encounter north-polar trajectory.

Consequently, lighting conditions were favorable for determining fine-scale relief in the west, but progressively less so toward the east.

Conversely, albedo features such as bright crater rays, which are conspicuous in the eastern part, become increasingly difficult to recognize westward toward the terminator.

This range of lighting conditions across the quadrangle results in inconsistent geologic mapping, because topography, albedo, and surface texture are critical for characterizing individual materials units.

Caloris ejecta are embayed and partly covered by a plains unit that lies mostly in large, roughly circular depressions, some of which may be ancient degraded basins.

The eastern part of the Shakespeare quadrangle consists mainly of cratered terrain and intercrater plains.

The large extent of this surface compared to its counterpart on the Moon was thought to reflect the restricted distribution of ejecta around each individual crater caused by the relatively high gravity on Mercury.

The presence of this unit suggests that the plains-forming process spanned much of the early geologic history of Mercury and continued long after the peak of cratering.

Lineated plains material was recognized by Trask and Guest[2] as forming terrain consisting of lines of hills and valleys, some of which are as much as 300 km long.

Its features are similar to those of the lunar Imbrium sculpture (Gilbert 1893) and to the hills and valleys radial to the Nectaris Basin on the Moon (Stuart-Alexander, 1971).

Hilly plains material consists of low, rounded, closely spaced hills with relatively few superposed craters.

The hills range in size from 1 to 2 km across and were estimated to have heights of 100 to 200 m by Trask and Guest,[2] who first recognized this unit and named it hilly terrain.

The main tracts of hilly material occur in a roughly concentric band outside the Caloris ejecta.

It has been demonstrated that the history of the Moon was punctuated by a series of major impacts that have emplaced ejecta over widespread areas; the rock units associated with these impact basins were used to divide the lunar stratigraphic column into a series of well-defined time units (Shoemaker and Hackman, 1962; McCauley, 1967; Wilhelms, 1972).

On the basis of photometric evidence, Hapke and others (1975) suggested that the central part of the basin floor may be 7±3 km lower than the outer edge.

The smooth plains material forms essentially level tracts, flooring depressions in the mercurian surface.

Smooth plains are mapped in the Shakespeare quadrangle only where there is no clear evidence of small hills characteristic of the Odin Formation.

Interpretation of the origin of the smooth plains is difficult but significant, because it bears directly on the internal constitution and thermal history of Mercury.

On the basis of distribution and volume, Strom and others[5] argued that in most areas the smooth plains consist of extensive sheets of basic lava similar to the lunar maria.

As on the Moon, the principal eroding process is likely impact; thus, a fresh crater will be degraded systematically over time.

[7] O'Donnell and Thomas (personal communication, 1979) have suggested, on the basis of orientation of features outside Caloris, that these ridges and scarps largely follow preexisting radial and concentric fracture patterns in the mercurian lithosphere initiated by the Caloris impact, similar in character to those around Imbrium on the Moon (Mason and others, 1976).

A few sinuous scarps also occur in this quadrangle, including the Heemskerck Rupes which cuts the older intercrater plains.

Scarps of this type are considered by Strom and others[5] to be compressive thrust faults resulting from overall shortening of the mercurian crust early in its history.