Craters on the Moon from Galileo to Wegener: A Short History of the Impact Hypothesis, and Implications for the Study of Terrestrial Impact Craters

Abstract

The origin of lunar craters has been discussed for centuries, since they were discovered by Galilei in 1609. The majority of researchers were of the opinion that they are volcanic structures, but a variety of "exotic" explanations that included tidal forces, circular glaciers, and coral atolls was also considered. The meteorite impact hypothesis had been discussed a few times, starting with Hooke in 1665, and formulated in more detail by Proctor in 1873 and Gilbert in 1893. However, this theory only gained momentum early in the 20th century, after the identification of Meteor Crater in Arizona as an impact structure, and after specific and plausible physical models for impact craters formation were devised by Öpik in 1916, Ives in 1919, and Gifford in 1924. Nevertheless, despite growing evidence for the interpretation that most craters formed by impact, proponents of the volcanic theory impact were still vociferous as late as 1965, just four years before the first samples were brought back from the moon. Important lessons could have been learned for the study of impact craters on the Earth, especially in view of evidence that large impact events had some influence on the geologic and biologic evolution of the Earth.