Lunar science: An overview
Before spacecraft exploration, facts about the Moon were restricted to information about the lunar orbit, angular momentum and density. Speculations about composition and origin were unconstrained. Naked eye and telescope observations revealed two major terrains, the old heavily cratered highlands and the younger mostly circular, lightly cratered maria. The lunar highlands were thought to be composed of granite or covered with volcanic ash-flows. The maria were thought to be sediments, or were full of dust, and possibly only a few million years old. A few perceptive observers such as Ralph Baldwin (Baldwin 1949) concluded that the maria were filled with volcanic lavas, but the absence of terrestrial-type central volcanoes like Hawaii was a puzzle.
The large circular craters were particularly difficult to interpret. Some thought, even after the Apollo flights, that they were some analogue to terrestrial caldera (e.g., Green 1971), formed by explosive volcanic activity and that the central peaks were volcanoes. The fact that the craters were mostly circular was difficult to accommodate if they were due to meteorite impact, as meteorites would hit the Moon at all angles. The rilles were taken by many as definitive evidence that there was or had been, running water on the lunar surface. Others such as Carl Sagan thought that organic compounds were likely present (see Taylor 1975, p. 111, note 139).