Geoscience Reference
In-Depth Information
As the techniques of astronomy improved, the full extent of
lunar cratering emerged and needed to be explained. Many eminent
scientists and philosophers, including Robert Hooke, Immanuel Kant,
and William Herschel (polymaths all), took a crack at the question.
Almost to a person they concluded that the craters were the rem-
nants of lunar volcanoes. The idea that impact created the craters was
proposed from time to time but never taken seriously.
In 1892, the great American geologist G. K. Gilbert, whose in-
terest in craters was fostered by his research on one in northern Ari-
zona, began to study lunar craters by telescope. He observed that
their shape, and their central peaks and collapsed terraces, showed
them to be markedly different from terrestrial volcanic craters. For
that reason, he concluded that they could not be volcanic but in-
stead had to have been formed by impact. He conducted scale-
model experiments and found that impact could indeed form cra-
ters, but that when the experimental projectile struck at an angle,
the resulting crater was elliptical. Since meteorites arriving randomly
on the surface of the moon surely must strike at an angle most of the
time, at least some lunar craters should be elliptical, but as far as
Gilbert could determine, all were circular. To reconcile experiment
with observation, Gilbert proposed that a ring of solid objects in
orbit around the moon, like the rings of Saturn, gradually released
chunks that fell vertically onto the moon's surface. But since no evi-
dence supported this theory, it too sparked little interest.
Gilbert conducted his tests in a hotel room, which meant that
the experimental impactors fell at low velocities. He had no way of
knowing that a projectile arriving at interstellar speeds is destroyed
and its energy converted to an explosion that leaves a circular crater
almost regardless of the angle of incidence. That knowledge had to
await the early twentieth century and additional observations, many
of them on the bomb craters that soon became all too available in
the pockmarked fields of Europe.
Gilbert's negative conclusion essentially shut down research on
the origin of terrestrial craters until, in 1961, a new era began when
President John F. Kennedy, calling space the new ocean to be
explored, declared that within that decade the United States would
send a man to the moon and return him safely. Wisely, before astro-
nauts were sent, unmanned vessels such as
Ranger, Surveyor,
and
Orbiter
mapped the moon. They found it densely cratered on every
scale from thousands of miles to fractions of an inch. Missions from
the Soviet Union and the United States showed that the lunar far-
side was also heavily cratered. Craters of every size saturated many
lunar terrains, leaving no room for a new one without obliterating
