Geoscience Reference
In-Depth Information
one or more previously existing craters. The larger craters had fea-
tures not displayed by volcanic craters: central peaks, terraced rims,
and rays of splashed debris. As this kind of evidence accumulated, it
gradually became clear that lunar craters were not volcanic but were
formed by impact. There was no scientific reason to believe that the
other inner planets would have had a different history.
Astronauts brought back from the lunar highlands samples of
impact breccia, rocks composed of broken, angular fragments em-
bedded in a fine-grained matrix. This is what would be expected
when impact breaks apart the rocks at ground zero, which are later
cemented back together. The first Apollo astronauts, however, stepped
out not on the highlands but onto a plain of basaltic lava, the type
extruded by the Hawaiian volcanoes, showing that although there
were no volcanic cones or craters any longer visible, at some earlier
time vast sheets of lava had flowed out onto the lunar surface. When
the lunar samples were dated, those from the highlands gave ages of
4.5 billion to 4.6 billion years, the same as the oldest meteorites and
the calculated age of the earth, thus supporting the view that all the
objects in the solar system have the same original age of formation.
The volcanic rocks, however, which appeared both to earthbound
geologists and astronauts to be the youngest lunar material, gave ages
ranging from 3.1 billion to 3.7 billion years. In other words, the
youngest moon rocks were almost as old as the oldest rocks on the
earth, which date to 3.8 billion years. Geologists quickly realized that
the moon had not had a continuous, steady geologic history like the
earth—everything had been crammed into the first 1.5 billion years or
so, after which the only significant process was meteorite impact.
The Apollo missions and the analysis of the returned samples had
the ironic effect of debunking all the existing theories of the origin of
the moon. As the full extent of cratering in the solar system came to
be appreciated, Donald Davis and the versatile William Hartmann—
scientist, artist, and author—proposed that early in the history of the
solar system, a protoplanet the size of Mars struck the earth and
blasted both itself and a large chunk of the earth into near-earth
orbit, where the debris gradually amalgamated into the moon.
6
The
mass that stuck became part of the earth's mantle. This has now
become the theory of choice among planetologists. (In yet another
example of a theory ahead of its time, Harvard's Reginald Daly pro-
posed this very idea in the 1940s, but no one paid it any mind.
7
)
The space age had hardly begun when it brought evidence of
cratering on other planets. The
Mariner 10
spacecraft missions to
Mercury in 1974 and 1975 found a surface as densely packed with
craters as that of the moon. On Venus,
Magellan
found huge active
