Cosmic Pinball - Four Revolutions in the Earth Sciences - page 191

Cosmic Pinball
Moon Over Mauna Loa
By the mid-1980s, though Penfield and Camargo had reported the existence of the
Chicxulub crater, no one seemed aware of it, and the controversy over the Alvarez
theory continued to bubble. But what was no longer in doubt was that many ter-
restrialmeteoriteimpactcratersexist.Alreadyby1970,somefiftyhadbeenidenti-
fied. Today the number exceeds 180. Hundreds, thousands more must have formed
but be inaccessible or have been removed by erosion.
But what of the largest collision of all, the giant impact that may have created
the Moon? How had that theory fared? By 1984, fifteen years after Apollo 11 , the
time seemed ripe to convene a new conference to review giant impact. Bill Hart-
mann was one of the organizers of the conference, held in Kona, Hawaii. “When
I went to the planning session for the conference to look over the abstracts for the
proposed papers,” he recalled, “I found, to my amazement and joy, that eight or
ten of the abstracts—independently of each other—were about the impact idea.” 1
Almost to a person, the presenters offered new support for the giant impact theory.
John Wood of Harvard set the stage in a paper titled “Moon Over Mauna Loa: A
Review of Hypotheses of Formation of Earth's Moon.” 2 He compared the various
models, including “collisional ejection,” his term for “giant impact,” giving each
hypothesis a letter grade forits ability to explain each ofseven “constraints.” Giant
impact was the only one not to get at least one grade of F, though it did get three
“incompletes.” Angular momentum did away with the sister and daughter hypo-
theses, and the Moon's paucity of iron scratched the intact capture hypothesis.
In the second of his two papers at the conference, Hartmann addressed the po-
tential criticism that the giant impact theory appeared ad hoc, ascribing the Moon's
origin to a one-off event dreamed up solely to explain it. He countered in a sec-
tion titled “Stochastic Does Not Equal Ad Hoc,” stochastic being a three-dollar
wordforrandom. 3 “Mypointwas,”hesaid,“thatalthoughwecannotpredictwhen
or where a catastrophic event will happen, we can be sure that catastrophic im-
pact events were happening all over the solar system, and therefore we shouldn't
rule one out on the basis that it is ad hoc .” 4 Hartmann distinguished between
events that “are class-predictable but not event-predictable: i.e., we believe the
class of events occurred, but we cannot determine times and magnitudes of indi-