Worlds in Collision - Four Revolutions in the Earth Sciences - page 171

samples have the same oxygen isotope ratios as terrestrial rocks. The co-accre-
tion or double-planet (sister) theory would explain the oxygen isotope similarities
between the Earth and the Moon but not the other differences in chemistry. Nor
could it account for the angular momentum of the Earth-Moon system.
At first the fission (daughter) theory appeared to be the most promising. But
computer simulations showed that instead of flinging off a blob of material, a mol-
ten, rapidly spinning proto-Earth would have ejected a cluster of smaller frag-
ments. Fission might be salvaged if these fragments had settled into orbit as a ring
of planetesimals, which then coagulated into the Moon, something like what Gil-
bert had proposed with his astral pudding. Since in this model, the Earth and the
Moon would have been made from the same material, they would have similar
oxygen isotope compositions. One interesting result of the computer simulations
was that if fission were to have a chance of being correct, the ring of debris must
have had a mass at least one-tenth that of the Earth.
So far, so good for fission. But as always, the familiar nemesis of angular mo-
mentum loomed. To fling off the ring of debris necessary to make the Moon, the
Earth would have had to be spinning much more rapidly than the Earth-Moon sys-
tem is spinning now, with no obvious way of getting rid of the excess spin after the
two separated. And the more scientists learned about the chemistry of the moon
rocks, the more fission appeared unable to explain why the Moon has more iron
and more refractory elements than the Earth.
As the author Stephen Brush summed up Apollo's impact: “All theories are re-
futed by the data.” 3 By the early 1970s, selenology found itself where geology had
been during the first few decades of the twentieth century: a science without a the-
ory. But there was a big difference. The geologists had no definitive data that fals-
ified or corroborated continental drift. The lunar geologists of the 1960s and bey-
ond had a wealth of data yet at first could make no sense of it.
In a 1974 topic titled The Subjective Side of Science , the social scientist Ian
Mitroff reported the results of interviews during 1969-1972 with forty-two lunar
scientists. 4 He asked their opinions of five hypotheses for the origin of the Moon:
fission, capture, and three versions of the sister hypothesis: double planet, con-
densation from a primitive atmosphere, and accretion from a cloud of planetesimal
debris. Mitroff used a scale ranging from 1 (“agree strongly”) to 7 (“disagree
strongly.”) None of the five hypotheses rated better than 4. 5 Half the scientists “ex-
pressed little oronly moderate interest regarding the origin ofthe Moon” (58).One
told Mitroff, “You've got to realize that we've lived with some of these theories
for so long that they don't mean much to us anymore. We've heard the same old
people spin out the same old cobwebs and speculations for years without adding