The Moon’s Face - Four Revolutions in the Earth Sciences - page 155

The Moon's Face
The Next Generation Will Laugh
In reading the literature on lunar craters, one is surprised to find names familiar
from another of the great discoveries reviewed in this topic: continental drift. The
initial surprise comes because we expect scientists to have worked in narrow dis-
ciplinary niches, as they do today. We recognize that a few always defy pigeonhol-
ing, but we hardly expect them to be right and the experts wrong. Yet one lesson
of this topic is that these outsiders not only are often right; they are indispensable
to science.
During 1918 and 1919, while Alfred Wegener was a dozent at the University
of Marburg, he turned his attention to the Moon. In 1921 he published the results
of his investigations in a remarkably prescient pamphlet titled Die Enstehung der
Mondkrater , or The Origin of Lunar Craters , finally translated into English in
1975. 1 Wegener had carefully read Gilbert's The Moon's Face .
Wegener began by reviewing the various theories that had been offered to ex-
plain lunar craters. Like Gilbert, he pointed out that terrestrial volcanic craters and
lunar craters differ in size, shape, and features. On Earth, volcanoes typically dis-
play a “steep, cone-shaped mountain with a crater opening on the top.” 2 Yet “on
the Moon, the normal shape of a crater, especially the fresh ones, is that of a plate
whosefloorlieskilometerslowerthanthesurroundingterrain.”Manylunarcraters
havedistinctcentralpeaks,butthey“neverreachtheheightofthesurroundingring
walls and generally not even the level of the surrounding terrain.” The peaks that
are often centered in lunar craters never have an “opening at the top.”
Terrestrial craters, Wegener noted, “rarely reach a diameter of one or more kilo-
meters.”Oneswithdiametersoftenkilometersormoreare“extremelyrare”(218).
Lunar craters, on the other hand, form an unbroken series from the lowest limit
of resolution available at the time, about fifty meters, up to features so large we
need no telescope to see them, including (with dimensions as known today) Mare
Crisium at a diameter of 555 km, Mare Serenetatis at 674 km, and Mare Imbrium
at 1145 km. Younger craters scatter randomly atop the larger, older ones to such
an extent that “it seems doubtful to be able to find a single point which at least
once has not been part of the floor of a crater.” Wegener found the contradiction
between Earth and Moon “so flagrant that the next generation will only laugh at