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Giant Impact Theory of Lunar Formation Gains More Credibility

A lingering problem in explaining the genesis of the moon appears to have been solved

By John Matson

The most commonly invoked explanation for lunar formation holds that a giant protoplanet, sometimes called Theia, struck the newly formed Earth 4.5 billion years ago and created a cloud of debris that quickly coalesced into the moon. But that hypothesis has suffered from a nagging flaw. Simulations of moon-forming collisions have shown Theia would have been the primary donor of lunar material. But analyses of Apollo moon rocks have shown that the moon seems in many ways a chemical clone of Earth, not Theia.

“The giant impact theory explains many traits of the system—that’s why it’s favored—but this discrepancy is a little tricky,” says planetary scientist Robin Canup of the Southwest Research Institute in Boulder, Colo., who played a key role in developing the Theia idea. “This has been a thorn in the side of the impact theory for some time.”

That thorn may be on its way out. A pair of papers published online October 17 in Science, one by Canup and one by planetary scientists at the SETI Institute in Mountain View, Calif., and Harvard University, demonstrate two different ways that a giant impact could produce a moon with the observed chemical similarities to Earth.

In Canup’s model, the impactor is substantially heftier than the canonical Theia—instead of a Mars-size object colliding with the much larger proto-Earth, her new study proposes a smashup of two comparably sized objects. “The set of impacts that I identify that can do this involve a much larger impactor than had been considered before,” Canup says. “The type of impact that I’m advocating here is the collision of two half-Earth-mass objects. They merge to form the Earth.” The moon would then form from the leftover debris, naturally explaining its similarities to Earth.

A different conception, from Matija Ćuk of the SETI Institute and Sarah Stewart of Harvard, invokes a small, high-velocity projectile smacking into a fast-spinning proto-Earth. Like an interplanetary mortar, that high-energy impact would fling out a cloud of debris composed primarily of material from Earth. “The crucial difference is that Earth is spinning faster,” Ćuk says. “If you hit it hard it’s easier for the pieces to fly into space.”

Both studies build on the recent finding by Ćuk and Stewart that gravitational interactions with the sun can quickly sap angular momentum from the newborn Earth-moon system. As a result, Earth may have been spinning much faster after lunar formation than had previously been thought plausible—a day on Earth may have lasted only two or three hours immediately after the impact. And the possibility of a fast-spinning Earth opens the door to types of collisions that had not been considered viable before.

Indeed, the true impact of the new studies is not in the specifics of the revised lunar-forming models but in the fact that such revisions now appear plausible, says Erik Asphaug, a planetary scientist at the University of California, Santa Cruz. “It’s not so much that they’ve come up with a model that works; it’s that they have taken away a constraint that we thought was sacrosanct for the last 20 years,” he says.

Ćuk, too, foresees the opening of a new chapter in unraveling the story of the moon’s birth. “This is going to be, I hope, the first of a new batch of papers, rather than the final word,” he says. “The thing that really surprised me and Sarah is, we didn’t try very hard—this kind of came out pretty much by itself. So that’s promising. We didn’t have to look far and wide for something that worked.”

The only catch is that Theia’s size and the magnitude of its impact, which once seemed to be fairly well understood, are now open to debate. And many more plausible scenarios that can explain the Earth-moon system may now come to the fore. “That is my worry—I wonder if moon formation may have become an unsolvable problem,” Asphaug says. “If you can have an Earth that is spinning with pretty much any spin rate, suddenly all bets are off.”