Beethoven Quadrangle - Geologic History

Geologic History

Geologic evidence for the reconstruction of the evolutionary history of Mercury is less complete than for the Moon and Mars, for which orbiting spacecraft and landers have provided total or near-total coverage and high-resolution images. However, available data allow certain parallels to be drawn with respect to the bombardment and accretionary histories of the three bodies. The geologic record shows a period of decreasing meteoroid flux on all three, wherein the basins and large craters formed early in their crustal evolution were superseded by impacts of progressively smaller size. The relative paucity of mappable c5 craters in the Beethoven quadrangle is indicative of the decreasing crater-production rate in the younger crater classes. The low density of small craters in the oldest class, c1, results from their destruction by impacts and obscuration by ejecta and volcanic material over a long period of mercurian history.

The intercrater plains and younger plains materials probably have mixed origins, and they consist of both volcanic and impact ejecta-related deposits. The plains materials accumulated mostly in low-lying areas and have buried or partly buried older craters and surfaces. Their relative ages and thicknesses are reflected by the number of craters visible on their surfaces: where crater densities are high, the plains material is relatively old or thin; low crater densities indicate relatively thick, young deposits. Where superposed craters can be distinguished from partly buried craters, relative ages of the plains units can be established. Crater counts indicate that the intercrater plans unit, whose crater density is twice that of the intermediate plains unit, is significantly older.

Whether the mercurian plains materials are analogous to volcanic flows of the lunar maria is unknown. In this quadrangle, the former lack many characteristics of mare materials, including low albedo and strong albedo contrasts with other units, lobate flow fronts, sinuous rilles, and numerous wrinkle ridges and domes with summit craters. Possibly the plains units on Mercury are similar to the Cayley Formation on the Moon and consist largely of finely divided ejecta materials. Whatever the origin and composition of the plains units in the Beethoven quadrangle, they represent late stages in the crustal evolution of this region.

Other differences between the Moon and that part of Mercury observed in this quadrangle are the absence in Beethoven of distinct highlands and lowlands, as well as the preservation in the quadrangle of secondary crater chains around older craters and basins (Scott, 1977).

The geologic history of Mercury has been summarized by Guest and O’Donnell (1977), Davies and others, and Strom.