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same oxygen isotope ratios to within five parts per million. 10 The isotopic abund-
ances of the highly refractory elements titanium and tungsten are also identical
between the Earth and the Moon, as are the isotopes of chromium and silicon. Ex-
plaining how five markedly different chemical elements could have the same iso-
topic composition in the Earth as in the Moon places a heavy burden on the giant
impact theory, or any theory.
Then in 2007, Kaveh Pahlevan and David Stevenson of the California Institute
of Technology came up with a way to explain the oxygen isotope identity. 11 They
envision that in the collision, both Theia and the outer part of the Earth melted
completely. In a complex process, the isotopes of the gaseous oxygen would have
equilibrated between the two, while the nongaseous material that would form the
Moon was able to retain some of its original chemical distinction. Their mixing
model allows most of the Moon's material to come from Theia while allowing the
Earth and the Moon to have the same oxygen isotope abundances.
In the fall of 2012, Canup reported that Theia, by then thought to have been
only one-tenth to one-fifth as large as the proto-Earth, likely was nearly the same
size. 12 Her new model satisfies all the major constraints except one, the same old
one: it leaves the Earth-Moon system with more angular momentum than the pair
has today, with no obvious way of getting rid of it. But in a companion paper,
two authors came to the rescue by showing how, after impact, the rapidly spin-
ning Earth-Theia system could have been slowed by resonating tidal forces with
the Sun, lowering the angular momentum of the pair to the amount observed. 13
A conference put on by the Royal Society in September 2013, nearly forty years
after the 1975 meeting at which scientists first proposed the giant impact theory,
found the planetary scientists still puzzling, especially over the similarities in iso-
tope abundances between the Moon and the Earth. 14 Based on measurements they
had made on meteorites believed to have come from Mars, comets, and asteroids,
scientists concluded that isotopic compositions varied greatly across the early sol-
ar system. But what if that assumption were wrong? One way to tell would be to
analyze a sample of a rock from Venus, Earth's near neighbor in space. If Venus is
also isotopically similar to Earth, then such similarities might have been the rule
in the inner solar system. Theia might have had them as well, explaining how the
Moon and the Earth became isotopically identical and strengthening the giant im-
pact theory. A few decades ago, scientists had been surprised to discover that mu-
seum meteorite collections and the Antarctic ice held specimens blasted off the
Moon and Mars. Could a hardy Venus rock be hiding in some dusty cabinet draw-
er?
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