Geoscience Reference
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volcanoes, vast lava flows, and a surface pocked with medium- to
large-size craters. (Smaller craters were not found, probably because
Venus's dense atmosphere causes small meteorites to burn up before
they hit.) The
Voyager
mission showed that Jupiter's moons—Cal-
listo, nearly as big as Mercury, and Ganymede, even bigger—are
cratered on a lunar scale. Mars is not only heavily cratered but con-
tains titanic volcanoes. And thousands of pockmarked asteroids float
in space, some of them in orbits that cross that of Earth. Indeed, of
all the bodies observed since the space age began, only three—Earth,
and Jupiter's moons Io and Europa, all of them with active sur-
faces—lack obvious and plentiful craters.
Thus over the course of the twentieth century, impact cratering
has gone from being viewed as extremely rare to being regarded as a
paramount process in the history of the solar system. To have existed
as a solid body in the solar system is to have been massively bom-
barded since the beginning. But where, then, are the craters that
must have formed on Earth?
WHERE
HAVE
ALL THE
CRATERS
GONE?
After all, Earth not only has a much bigger cross-section than the
moon to present to incoming meteorites, it also has a greater mass and
therefore exerts a greater gravitational pull. Calculations combining
area and mass show that at least 20 times as many meteorites should
have hit Earth as hit the moon. The moon has 35 impact basins larger
than 300 km in diameter, most of them nearly 4 billion years old. In
the same period in its early history, 700 giant basins should then have
formed on Earth. Such saturation bombing would have caused the
entire surfaces of the moon and Earth to melt, forming giant magma
lakes that persisted for millions of years (evidence for this is more vis-
ible on the moon than on Earth, where no trace remains of the early
bombardment). Subsequently, both the moon and Earth were struck
countless times, though not as often as during the first few hundred
million years. Why then could the Alvarezes not find ready support
for their theory in an abundance of terrestrial craters?
The first footprint at Tranquillity Base will outlast the pyramids
and the tallest skyscraper—the moon contains no wind or water to
erode the evidence of human visitation. Eternal, this giant fossil of the
early solar system awaits our return. Earth, on the other hand, has
been internally active since its creation, constantly renewing and
reworking its surface materials, altering them beyond recognition. The
erosive action of wind, ice, and water, and the large-scale effects of
