Geoscience Reference
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lision had even created Mare Imbrium, which at more than a thousand kilometers
in diameter dwarfs any earthly crater. In his presentation at the symposium, he said
that Imbrium's ejecta (material thrown out of a crater, whether by impact or vol-
canic eruption) created “a mantle of debris that buried more than half the visible
hemisphere of the Moon.” 13
Nicholas Short of Lawrence Livermore National Laboratory noted that impact
craters ought to bear a strong resemblance to the craters produced by underground
explosions in the American “Plowshare” program, which was exploring peaceful
uses of the atom. 14 He illustrated the point with two photographs. One was of the
crater left after the SEDAN nuclear test of July 1962, in which a hundred-kilo-
ton bomb was detonated at a depth of 635 feet. The other was a photograph of
Meteor Crater, Arizona, a cavity whose origin had long been claimed to be volcan-
ic but which by 1964, thanks to Shoemaker, geologists were beginning to consider
a meteorite impact crater. Without the roads and other manmade features that mark
Meteor Crater, it was hard to tell the two apart.
Robert Dietz had long believed that meteorite impact had produced many if not
most lunar craters. His short paper at the conference showed that Dietz had also
come to accept as probable meteorite impact craters not only Meteor Crater but a
number of other circular features on Earth. 15 Always ready to break new ground,
Dietz proposed that two of the largest and best-studied structures, the Vredefort
Ring in South Africa and the Sudbury Igneous Complex and ore body in Ontario,
are both “astroblemes” (star-wounds), as the inventive Dietz proposed to call met-
eorite impact craters. Heknewthat ithelped togiveanewidea amemorable name.
Walter Bucher titled his talk “The Largest So-Called Meteorite Scars in Three
Continents as Demonstrably Tied to Major Terrestrial Structures.” It was an abbre-
viated version of a paper he had published in the American Journal of Science the
year before. 16 In that paper, as obstinately opposed to meteorite impact on Earth
as he had been to continental drift, Bucher had dismissed Dietz's “astroblemes,”
renaming them “geoblemes.” 17 His main argument was that meteorites fall at ran-
dom, yet the so-called astroblemes are “decidedly not randomly aligned.” There-
fore “the meteorite impact hypothesis is rejected.” 18
The speakers at the two lunar conferences in 1964 knew that a Ranger mission
wouldsoonsucceedandendcenturiesoframpantandsafelyuntestablespeculation
about the Moon. No longer would our heavenly companion be an inaccessible and
unknowable object in space. Photographs with a resolution thousands of times bet-
ter than those from Earth-bound telescopes would soon be spread on scientists'
desks. The surface of the Moon would become better mapped than the surface of
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