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A computer-simulated series of snapshots shows a collision between a protoearth and an impactor. COURTESY A. G. W. CAMERON

The early solar system was a very violent place," says Menzel professor of astrophysics Alastair Cameron. At the time of the creation of the moon, for example--now believed to be about 50 million years after the formation of the solar system, or 4.55 billion years ago--Earth was a fairly solid mass, probably nearly 70 percent formed, that exerted its gravitational pull on other masses. "There were lots of collisions," Cameron says. This basic premise led him, in 1972, to begin investigating the possibility that the moon was formed by a "giant impact" between the coalescing earth and a planetary body two to three times the size of present-day Mars.

Cameron first put forward his idea, which defied prevailing theories, in scientific papers in the late 1970s. By 1984, at a now-celebrated conference in Kona, Hawaii (called to consider findings from Apollo 13), the giant-impact theory had generated much interest. Since that time, astronomical science has come a long way. In preparation for a 1998 conference on the creation of the earth and moon, Cameron is pinning down the details of his theory, which has become the only remaining viable explanation of the moon's formation.

"The three classical explanations were entertained only as a matter of desperation," Cameron says. "Each was fundamentally flawed." One theory posited that the moon had formed independently of Earth, but nearby. In that case, they would have similar compositions--but they do not. The moon's density indicates that it has an insubstantial iron core, if any (at a quarter the earth's size, it has merely one percent of its mass). Alternatively, had the moon formed elsewhere in the solar system and then been captured by Earth's gravitation--a "near impossibility," Cameron states--it would show fewer similarities to Earth's composition than it does--for example, the relative abundance of oxygen isotopes on both bodies. The third theory, which proposed that part of the earth's mantle was flung off by the speed of its rotation, required the earth to have been spinning at four times the rate it did at the time of the moon's formation.

Cameron has honed his theory with the aid of computer simulations of the collision. There are two possible scenarios, both involving glancing blows. In one, the collision destroys the impactor, with most of its material falling into the earth, while some goes into orbit around it. The other version involves two collisions: the initial impact is much shallower in penetration, hence the impactor is less affected and can pull itself together with only slightly diminished velocity. It then returns to hit the protoearth a second time, but has now slowed down enough that it cannot escape the planet's gravitational pull. In either case, the collisions leave enough material in orbit to constitute the moon.

By adjusting variables like the ratio of the hypothetical masses of the impactor and Earth, the speed at impact, and the mass of the protoearth, Cameron has found that successful conditions for the formation of a moon require the earth to have been 60 to 70 percent formed after adding material from the impactor; the impactor's speed at impact to have been between 11 and 15 kilometers per second (about 24,550 to 33,000 miles per hour); and the impactor's mass to have been 20 to 30 percent that of the protoearth's.

The 'Giant Impact' was routine in one sense, Cameron says, because many such collisions occurred: one of them probably caused Earth's axis of rotation to tilt, so that the moon, unlike other planets' satellites, does not orbit our equator. One thing is clear, though. Says Cameron, "This would have to be the largest collision Earth ever withstood."

~ Daniel Delgado

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