Rosetta Stone - Four Revolutions in the Earth Sciences - page 161

lay in the possibility that the Moon could tell us much about the early history of
the Earth.
Scientists had long known that because of the tug of the Earth's gravity, the
Moon bulges slightly toward the Earth. Since over geologic time the protrusion
has not receded, Urey speculated that it must have formed early in the Moon's his-
tory and been frozen in. Therefore the Moon must not have isostasy—its interior
must be cold and rigid. Such a moon was likely to be primordial, Urey thought,
and thus represent a fossil from the earliest history of the solar system. By study-
ing the Moon, we could learn about the starting conditions of all the planets and
moons, including our home planet. Urey published his ideas in a 1952 topic called
The Planets . It was widely read and so persuasive that it caused many physical sci-
entists to change the direction of their research. 5
Harold Urey, given his age, stature, and university tenure, was free to let his
mind roam wherever he wished, even to the faraway Moon. Shoemaker's remark-
able life has been well recorded by his friend and colleague David Levy. 6 In the
1950s, his career was just getting off the ground. 7 Shoemaker worked for the
USGS, which still lay in thrall to G. K. Gilbert. For half a century, the Survey had
discouraged its employees from studying terrestrial craters, much less lunar ones.
As a way to pursue his interest in the Moon while acceding to the boundaries es-
tablished by the Survey, Shoemaker hit on the idea of studying the Hopi Buttes,
whichhethoughtresembledvolcanicnecksandchainsofcratersontheMoon.The
inventive Shoemaker justified this work by convincing his superiors that, as the
volcanic material that was to become the buttes had risen to the surface, it might
have gathered up uranium deposits, a major focus of the USGS at the time. From
there Shoemaker moved on to study actual craters produced not by meteorites or
volcanoes but by underground nuclear explosions at the Nevada Test site. This nat-
urally led him to nearby Meteor Crater.
Havingnoevidencetothecontrary,ShoemakerstartedoutassumingthatGilbert
had been right to consider Meteor Crater volcanic. But by 1959, Shoemaker had
laid to rest the notion that Meteor Crater is of terrestrial origin. He showed that
meteorite impact generates two shock waves that interact in complex fashion to
destroy the incoming meteorite and excavate the resulting cavity. When the met-
eorite, traveling at cosmic velocities, first makes contact with the surface of a sol-
id body, one shock wave moves back into the meteorite to engulf and blast it to
pieces. Ahead of the incoming projectile, another shock wave races down into the
incipient crater, compressing the target rocks, then ejecting them at great speed.
Fragments of target rock thrown out on low trajectories land in the reverse order
in which they departed, stacking upside down relative to their original layering.