The Bach quadrangle encompasses the south polar part of Mercury poleward of latitude 65° S. It is named after the prominent crater Bach within the quadrangle, which is in turn named after Baroque composer Johann Sebastian Bach.Gazetteer of Planetary Nomenclature, International Astronomical Union (IAU) Working Group for Planetary System Nomenclature (WGPSN),Bach The quadrangle is now called H-15.Map of the H-15 (Bach) Quadrangle of Mercury
Adjacent quadrangles to the north of Bach are Discovery quadrangle (0° to 90° W), Michelangelo quadrangle (90° to 180° W), Neruda quadrangle (180° to 270° W), and Debussy quadrangle (270° to 0° W). It is opposite the Borealis quadrangle at the north pole.
Mariner 10 photography
About half of the region was beyond the terminator during the three Mariner 10 encounters and hence not visible. The entire mapped area was covered by near-vertical photography from the second encounter, and the eastern part, from longitude 15° to about 110°, was covered by oblique photography from the first encounter. No third-encounter images were acquired. The entire visible area may be viewed stereoscopically by combining images from the first and second encounters taken at different viewing angles or by combining second-encounter images of the same area taken at different viewing angles. These combinations provided excellent qualitative control of topographic relief and a good quantitative photogrammetric base. However, sun-elevation angles of the images are limited to less than 25°, and image resolutions are no higher than about 0.5 km per picture element. Therefore, the south polar geologic map reflects mostly large-scale processes and topographic information, whereas other mercurian quadrangle maps benefit from greater albedo discrimination and, in some cases, higher resolution.
The imaged part of the Bach region covers about 1,570,000 km2. Its surface consists of craters of a wide variety of sizes and morphologies, as well as plains units, fault scarps, and ridges. It includes three double-ring basins that range from 140 to 200 km in diameter: Bach (after which the region is named), Cervantes, and Bernini. Another large crater, Pushkin, is 240 km in diameter and occurs at the map boundary at latitude 65° S., longitude 25° . Both Bach and Bernini display extensive fields of secondary craters. An unusual area between lat 69° and 80° S. and long 30° and 60° consists of young, relatively smooth plains marked by many flat-topped ridges unlike any seen in other areas of Mercury. Scarps similar to Discovery Rupes (in the Discovery quadrangle adjacent to the north) are relatively common throughout the Bach region. The most common terrain units in the region are the plains units, which display a wide range of small-crater densities.
MESSENGER photography
During MESSENGERs January 14, 2008 flyby, the probe photographed previously unseen portions of this region. More was imaged on its two subsequent flybys and the region was completely mapped during the orbital phase of the mission which ended in 2015.
While in orbit around Mercury, MESSENGER flew at a high altitude above the south pole. Using wide-angle camera images taken at 89 angles of illumination during a single Mercurian year, the percentage of time illuminating the regions around the pole were calculated as shown on the map to the left. Black regions are in permanent shadow.Illumination Map of Mercury's South Pole, released March 21, 2012. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.
Stratigraphy
Crater and basin materials Superposition relations among craters and basins, and their ejecta, provide the best means of establishing the relative time-stratigraphic order of crater and basin materials. Relative to the Moon, stratigraphic relations among mercurian craters are more clearly discerned because Mercury has a lower density of large craters, and its enhanced gravitational acceleration has restricted the distribution of ejecta. These attributes of the mercurian crater population allow stratigraphic sequences to be constructed over large regions.
The degree of crater degradation is determined by qualitative assessment of their landforms such as rim crests, interior wall terraces and slumps, central peaks, continuous ejecta deposits, and secondary crater fields (see Malin and Dzurisin, 1977; McCauley and others, 1981). To the extent that degradational changes are systematic with increasing age, they can be used to correlate local and regional stratigraphic sequences over the map region. On the basis of this morphologic evaluation, five crater ages are defined and used to make stratigraphic assignments. However, the low sun angle at which images in the region were acquired may make craters appear younger than in other parts of Mercury where images were taken at higher sun angles.
Of the region's three double-ring basins, Bach (200 km in diameter) and Bernini (140 km in diameter) are moderately fresh (of c3 age) and have well-defined secondary-crater fields, whereas Cervantes (200 km in diameter) is degraded (c1). The inner rings of the three basins are about half the diameter of the outer rings. Bach's inner ring, the most complete, is open only to the southeast; it consist of an almost continuous series of sharp-crested hills. The area within it and part of the area between it and the outer ring are filled with smooth plains material. The inner rings of Cervantes and Bernini consist of discontinuous, low, rounded hills, Bernini has a small central peak.
As first noted by Gault and others, the continuous ejecta blankets and secondary crater fields surrounding mercurian craters are smaller than their lunar counterparts, and the boundary between the two features is much less distinct. As a consequence, continuous and discontinuous ejecta are mapped together in the Bach region as "radial facies." With this exception, the morphological elements of mercurian craters are virtually identical with those on the Moon. Therefore, all of the craters within the Bach region are probably the result of impact by meteorites, small planetesimals, and possibly comets.
Plains materials About 60 percent of the mapped area consists of tracts of planar surfaces having a variety of small-scale textures. These tracts range in size from a few square kilometers within craters to areas larger than 10,000 km2 that surround and separate large craters: the so-called "intercrater plains". The origin of the plains material is uncertain. Strom and others, Trask and Strom, Strom (1977), and Leake (1982) presented arguments in favor of volcanism, whereas Wilhelms and Oberbeck and others (1977) argued for an impact-related origin through processes similar to those responsible for the lunar Cayley Plains (fluidized ejecta sheets or ballistically deposited secondary-crater ejecta). Plains formation occurred throughout the period when visible craters were formed and most likely throughout the period of intense impact cratering (Strom, 1977). The time scale for production and retention of plains units is crudely similar to that for the production and retention of craters.
The oldest and most extensive plains material in the Bach region, the intercrater plains material, is characterized by a gently rolling surface and a high density of superposed craters less than 15 km in diameter. Most of these small craters occur in strings or clusters and are irregularly shaped; they appear to be secondaries from craters of c2 through c5 age. Therefore, the intercrater plains unit is thought to be older than most c2 craters. Its relation to c1 craters is not clear. The highly degraded nature of c1 craters makes it impossible to determine whether the craters predate, postdate, or are contemporaneous with the intercrater plains unit. However, the presence of shallow depressions, which may be ancient craters, within this plains material suggests that the unit flooded a preexisting population of craters and therefore was emplaced sometime during the period of late heavy crater bombardment. The two proposed origins for this plains unit, as volcanic or basin-ejecta material, cannot be unambiguously resolved by geologic relations in the Bach region. However, a volcanic origin is favored because of (1) the widespread distribution of the plains material throughout the imaged regions of Mercury, (2) the apparent lack of source basins large enough to supply such great amounts of impact melt, and (3) the restricted ballistic range of ejecta on Mercury.
The intermediate plains material is concentrated mostly in the northeastern part of the Bach region. It is similar in morphology to intercrater plains material but has a lower density of small craters. On the basis of the reasoning applied to the intercrater plains material, the intermediate plains unit is also tentatively ascribed a volcanic origin.
Materials of the smooth plains and very smooth plains are also concentrated mainly in the eastern part of the map area. The smooth plains unit has a lower density of small craters than does intermediate plains material and a somewhat hummocky surface with scattered small hills and knobs. The hummocks within fresh c5 craters may be mantled floor materials or incipient peak rings (see, for example, crater Callicrates at lat 66° S., long 32°; FDS 27402). The very smooth plains unit has virtually no visible small craters and displays smoother planar surfaces than those of the smooth plains unit. It occurs in the lowest areas within smooth plains material (including areas within buried crater depressions) and commonly within older craters. The areas of greatest concentration of smooth and very smooth plains materials also contain the most ridges, which suggests that ridges and the younger plains units are genetically related. Very smooth plains material for instance, commonly lies at the base of ridges or scarps. It occurs as small patches within the smooth plains unit that fills the crater Pushkin. Smooth plains material embays the ejecta blanket of a c3 crater on Pushkin's rim at lat 66° S, long 28° (FDS 27402) and fills the interior and part of the outer-ring area of Bach. The distribution of these two youngest plains units may indicate that the smooth plains material as mapped is nothing more than a thin, discontinuous layer of very smooth plains material that mantles the older units. In this respect, it is similar to the lunar Cayley Formation, which is probably basin ejecta. However, unlike plains material of the lunar uplands, no source basin is evident for the mercurian smooth and very smooth plains units within the imaged part of the Bach region. Although such a source basin may lie within the part not imaged, intervening areas do not contain smooth or very smooth plains materials. For these reasons we tentatively ascribe a volcanic origin to most of the smooth and very smooth plains material. The ridges appear to be of volcano-tectonic origin; the fracturing may have provided the means by which lavas reached the surface to form these younger plains units. Some very smooth and smooth plains materials that form the floors of c5 and c4 craters may be impact melt.
Structure The map region displays a wide variety of structural features, including lineaments associated with ridges, scarps, and polygonal crater walls. Joint-controlled mass movements are most likely responsible for the polygonal crater-wall segments; segments as long as 100 km suggest that these fractures extend deep into the lithosphere. The most conspicuous trends of these lineaments are east-west, N.50° W., and N. 40° E. More trends are north-south, N.20° E., and N.70° E.
Large ridges and scarps are the most prominent structural features in the low-sun-angle Mariner 10 pictures of the Bach region. They are most numerous between long 0° and 90°, where they have no preferred orientation.