Craters on the Moon – Ancient Volcanoes or Impacts (63)

Introduction

The following excerpts from “Modern Mysteries of the Moon” by Vincent S. Foster explains the process of how the large craters on the Moon were discovered to be due to impact rather than the result of ancient dried volcanoes. This discovery actually shone new light onto the topic of how the Moon has developed through time.

Moon Craters Originally Thought to be Volcanoes

Until the late 1800s, most astronomers thought that volcanism formed the craters of the Moon. In the 1870s, the English astronomer Richard A. Proctor proposed correctly that the craters resulted from the collision of solid objects with the Moon. But at first, few scientists accepted Proctor’s proposal. Most astronomers thought that the Moon’s craters must be volcanic in origin because no one had yet described a crater on Earth as an impact crater, but scientists had found dozens of obviously volcanic craters (Fig. 1.4 ).

Idea of Impact Craters Proposed by Gilbert

“In 1892, the American geologist Grove Karl Gilbert argued that most lunar craters were impact craters. He based his arguments on the large size of some of the craters. Those included the basins, which he was the first to recognize as huge craters. Gilbert also noted that lunar craters have only the most general resemblance to calderas (large volcanic craters) on Earth. Both lunar craters and calderas are large circular pits, but their structural details do not resemble each other in any way. In addition, Gilbert created small craters experimentally. He studied what happened when he dropped clay balls and shot bullets into clay and sand targets.

Gilbert was the first to recognize that the circular Mare Imbrium was the site of a gigantic impact. By examining photographs, Gilbert also determined which nearby craters formed before and after that event. For example, a crater that is partially covered by ejecta from the Imbrium impact formed before the impact. A crater within the mare formed after the impact.”

Relevance to Lunar Evolution

“Gilbert suggested that scientists could determine the relative age of surface features by studying the ejecta of the Imbrium impact. That suggestion was the key to unraveling the history of the Moon. Gilbert recognized that the Moon is a complex body that was built up by innumerable impacts over a long period.

In his book The Face of the Moon (1949), the American astronomer and physicist Ralph B. Baldwin further described lunar evolution. He noted the similarity in form between craters on the Moon and bomb craters created during World War II (1939–1945) and concluded that lunar craters form by impact.

Baldwin did not say that every lunar feature originated with an impact. He stated correctly that the maria are solidified flows of basalt lava, similar to flood lava plateaus on Earth. Finally, independently of Gilbert, he concluded that all circular maria are actually huge impact craters that later filled with lava.”

Conclusion

The Moon has been orbiting in space for a long time and has actually received many impacts. However by studying the lunar craters scientists have been able to gleam information about how the various layers of the Moon’s surface has formed.

End (63).