Geoscience Reference
In-Depth Information
Cameron's method suggested that the Moon-creating impactor might have been
as large as Mars. His model showed how material blasted off the Earth could have
attained orbit rather than being caught by the Earth's gravity. The key is that the
impact would have generated temperatures as high as 8,000°C, enough to vaporize
the outer regions of both the Earth and the impactor. The rapidly expanding gases,
jetting above the proto-Earth, could have lofted more than enough mass into orbit
to make the Moon.
The giant impact theory combined the best features of its predecessors. As in
the modern version of the capture theory, the Moon accreted from fragments. The
fragments came not from some alien object but from the collision between a Mars-
sized impactor and the proto-Earth. As in the double-planet theory, prior to the col-
lision the two bodies were moving in the same region of space, having aggregated
from the same part of the original dust cloud, thus explaining their similar chem-
istry. As in the fission theory, the Moon was born from the Earth.
The Moon's iron deficiency may have arisen because by the time of the colli-
sion, most of the iron in the impactor and the Earth had already settled into their
cores. That left their outer regions, from whence came the material to make the
Moon, depleted in iron. The impact generated so much heat that the Moon melted
to its center. The outer region of the Earth also melted completely, covering our
planet with an ocean of magma. As the New Zealand scientist and author S. Ross
Taylor put it, the giant impact theory “cut the Gordian knot” that had fettered the
three classic theories for the origin of the Moon (80).
Like Vine, Wegener, and countless other young scientists, Hartmann thought
that his theory, which explained so much and, like Alexander, neatly cleaved Gor-
dian knots, would quickly earn a consensus. But later he realized that he had been
naïve: “No one had paid attention to our 1975 paper or Cameron's in 1976,” he
said. “I had thought that all you had to do was write a paper, and that was that—it
would sink or swim on its own merits. But that's not so. You have to push a new
idea” (12).
Six years after the 1974 conference, a team of scientists discovered evidence
of another collision between an extraterrestrial body and the Earth. This one, they
said,hadhappenednotatthebirthofthesolarsystem,but65millionyearsago—at
the time the dinosaurs went extinct. And in that, they detected cause and effect. By
this time, most geologists were willing to accept that there were meteorite craters
on Earth. But they were still unwilling to concede that a random, nonuniformitari-
an deus ex machina could solve one of the greatest mysteries of science.
