Geoscience Reference
In-Depth Information
much to them. For the first time in history we have the hard data, the evidence to
test a lot of these things. It's the data that is important now” (60).
A successful theory of lunar origin would have to incorporate the new evidence
and the best features of each of the classic theories while avoiding their venerable
flaws. Somewhere in the vast amount of new, Space Age data were the clues that
would allow scientists to decipher the Rosetta Stone, if only they were clever
enough. By now, it will come as no surprise that the idea that scientists would
eventually accept had been floated two decades before.
A Drifter Turns to the Moon
In 1946, Reginald Daly, by this time retired from Harvard, published a thoughtful
article titled “Origin of the Moon and Its Topography.” Daly was more impressed
by the “contrasts of topographic form in the two bodies” than by their similarities,
which were “remarkably few.” Scientists had found “30,000 visible pits” on the
Moon's surface, for example, and interpreted them all as volcanic vents. Though
he did not cite Wegener's paper, Daly no doubt would have agreed that the enorm-
ous number of lunar craters, so vastly larger than the number of volcanic craters
on Earth, is evidence in itself that not all lunar craters can possibly derive from
volcanism. Daly noted that Gilbert, “one of the keenest observers and most logic-
al thinkers who have worked in earth science,” had written a “classic paper [that]
contained a powerful argument for the very different impact or bombardment the-
ory of lunar pits.”
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Daly was primarily concerned with the hypothesis that “some or all of the
tailed analysis, he concluded that fission had failed, leading him to propose a new
version of the capture theory: that the “moon's substance represents a planetoid
which, after s
triking the earth with a glancing, damaging blow
, was captured”
(108, italics added). In Daly's model, the critical event was more collision than
capture, so we might think of it as a
fourth
theory of the Moon's origin. The colli-
sion would have been especially violent, Daly wrote, “if the 'planetoid' had a large
fraction of the moon's present mass” (109). The explosion then would “drive out
terrestrial material into a limited belt ringed about the planetoid,” from where it
wouldbe“gravitativelyaggregatedbythepullof[the]masterfragmentorcaptured
'planetoid'tomakethesubstanceofourmoon,andthesomewhatdiminishedearth
felt a prolonged rain of other earth-fragments, large and small” (109, 118). Daly
listed ten observations that his “earth-fragment” hypothesis explains, including the
