More than four-fifths of the surface of the Moon consists of a profoundly cratered irregular surface designated terra or highlands by analogy with the terrestrial continents. These terra regions have much higher albedos than the physiographically lower and much smoother mare regions. The difference in albedo can now be ascribed to a fundamental difference in the chemical and mineralogical character of these two regions. Lunar samples from landing sites in the mare regions and high-resolution photographs taken from lunar orbit have shown that the lunar maria are underlain by extensive lava flows. Isotopic dating of samples from four mare regions (refs. 7-1 to 7-4) indicate that mare volcanism covered a time span of 600 million years, beginning approximately 3.7 billion years ago. The intensely cratered character of terra regions is due to both the greater antiquity of these parts of the Moon and the higher flux of incoming objects that hit the Moon during very early lunar history (ref. 7-5). In contrast with the mare region, the origin of the underlying material of the terra is not easily inferred from physiographic criteria. The surface manifestations of early plutonic or extrusive igneous activity — if indeed they ever existed — were erased from the terra regions by the intense early bombardment of the lunar surface. Some portions of the highlands may be exceptions to this generalization; in particular, large craters such as Ptolemaeus, Hipparchus, Albategnius, and Alphonsus. The regions bounded by these craters are much smoother than the typical densely cratered highlands. These regions are generally assumed to be physiographic lows that have, in some way, been filled by younger material. The nature of the material and the mechanism by which it was introduced into the basins are not well understood. On the basis of rather detailed studies of the physiographic and albedo characteristics of the basin material, it has been suggested (ref. 7-6) that the filling of the highland basins was a result of volcanic processes similar to those which filled the large mare basins. Some highland basins also contain hilly, hummocky regions that bear no relation to large crater rims or crater ejecta. These regions have been interpreted as extrusive igneous features formed by viscous, silicic, igneous liquids (ref. 7-6).
Additional Publication Details
Preliminary examination of lunar samples: Part A: a petrographic and chemical description of samples from the lunar highlands