Cosmic Pinball - Four Revolutions in the Earth Sciences - page 192

vidual events.” 5 Hartmann gave as an example of a “class-predictable” event the
“probable Cretaceous-ending asteroid impact,” thus marrying the impact theory of
dinosaur extinction and the impact theory for the origin of the Moon. “As scient-
ists,” he said, “we should have pursued the geologic and climatic consequences
of these class-predictable events instead of waiting for iridium-rich layers to take
us by surprise.” 6 The writer Richard Kerr summed up: the giant impact idea had
“breathed new life into a long-stagnant field.” 7
Theia
Onewhohasfollowed theups,downs,andultimate demise ofthethreeclassic the-
ories of the Moon's origin could hardly be blamed for predicting that in time the
giant impact theory would likewise accumulate flaws and collapse. How ironic if
scientists could not explain the origin of the solar system's most looked-at object.
To gauge the fate of the giant impact theory, let us pick up the story again at a
conference held at Monterey, California, in 1998. Several from the 1984 confer-
ence presented, along with the astrophysicist Robin Canup of the Southwest Re-
search Institute in Boulder, Colorado, who would become a key figure in develop-
ing the giant impact theory. She and A. G. W. Cameron used a computer technique
called “smooth particle hydrodynamics” to simulate and explore just how the giant
impact might have unfolded.
The two reported that the Moon-generating impact would have occurred via a
bizarre double whammy. 8 After the first terrible strike, the “severely distorted” im-
pactor loses some of itself to the proto-Earth. The remaining material “pulls itself
together” and recedes. But escape from a body with the gravitational pull of even
a half-Earth proves impossible, and the impactor falls to Earth again in a second
collision. The metallic iron in the molten impactor sinks to the core of the proto-
Earth, whose outer section is also now completely molten. The cloud of debris left
in space coagulates into the Moon and gradually recedes.
In 2000, Alan Halliday proposed to name the impactor Theia, after the Titan
from Greek mythology who gave birth to the moon goddess, Selene. 9 Scientists
continued to refine the giant impact theory, coming up with new or more stringent
“constraints” that it would have to satisfy. They thought that Theia and Earth
would have arisen in different regions of the solar system and thus would have had
different oxygen isotope compositions, as noted earlier. If, as the computer simu-
lations suggested, most of the Moon's material came from the impactor, after col-
lision the Moon and the Earth would have maintained their distinctive oxygen iso-
toperatios.However,in2001scientistsfoundthattheEarthandtheMoonhavethe