Geology Reference
In-Depth Information
tions to the rule, and those exceptions are set aside as curious anomalies. Venus rotates the
wrong way? Triton orbits the wrong way? No problem. Their aberrations are incidental to
the larger scheme.
Thesamekindofsituationcomplicatesmanypublicdebates,likethatoverglobalwarm-
ing.Manyscientistspredictthatalteredatmosphericconditionswillraisetheaverageglob-
altemperaturebyseveraldegrees.Butsuchchangescanalsocauseextremeweather,which
may mean worse snowstorms in the southern United States. Global warming may alter
oceancurrentsliketheGulfStreamandultimatelyturnnorthernEuropeintoamuchcolder
Siberian-type icebox. Anomalies like this fuel the global warming naysayers: scientists
say the world is getting hotter, but you've just suffered through the biggest snowstorm
in your region's history. How should you respond? A judicious response is that nature is
amazing—rich, varied, complex, and intricately interconnected, with a messy, long his-
tory. Anomalies, whether in planetary orbits or North American weather, are not just in-
convenient details to brush aside: they are the very essence of understanding what really
happened—how things really work. We develop grand and general models of how nature
works, and then we use the odd details to refine the original imperfect model (or if the ex-
ceptions overwhelm the rule, we regroup around a new model). That's why good scientists
revel in anomalies. If we understood everything, if we could predict everything, there'd be
no point in getting up in the morning and heading to the lab.
In the case of the origin of Earth's Moon, those exceptions to the systematic
trends—those niggling orbital anomalies—led to the concept of the “Big Splash” or “Big
Thwack” model, which arose in the mid-1970s. The original series of related but poorly
constrained hypotheses coalesced into conventional wisdom at a pivotal 1984 conference
in Hawaii, where planetary formation experts gathered to weigh all their options. In such a
heady environment, Ockham's razor—the demand that the simplest solution to a problem
consistent with the facts is likely to be correct—prevails. The Big Thwack fit the bill.
To understand this radical idea, think back more than 4.5 billion years, to the time when
the planets had just formed from all those smaller competing planetesimals. As Earth grew
closetoitspresentdiameterofeightthousandmiles,itswallowedupalmostalltheremain-
ing proximate bodies in a succession of huge impacts. Those penultimate collisions with
objects many hundreds of miles across would have been spectacular, but they had little ef-
fect on Earth, the much more massive proto-planet.
But not all impacts are equal. In Earth history, one single event—one day more memor-
able than any other—stands out. About 4.5 billion years ago, when the Solar System was
about 50 million years old, the black proto-Earth and a slightly smaller planet-size com-
petitor were jockeying for the same narrow band of Solar System real estate. The smaller
would-be planet (dubbed Theia, after the Titan goddess who gave birth to the Moon) was
worthy of planetary status—perhaps two to three times the size of Mars (or roughly a third
