It has taken centuries for scientists to settle on a creation story for our moon, the most popular of which is depicted on the July cover of National Geographic magazine. But as I learned at a recent lunch with Erik Asphaug, a planetary scientist at Arizona State University, the debate is still far from finished.
Before the giant impact model gained traction nearly four decades ago, three other models were in contention. One said the moon condensed from the same whirling cloud of dust that created Earth. But this "binary" model couldn't explain why the moon, far from being a smaller twin of Earth, is much less dense than our planet, with no iron core.
A second model held that the young molten Earth spun so rapidly that it split apart, flinging a giant blob of magma into space. But Earth's spin today and the moon's orbit don't fit the pattern predicted by the "fission" model.
In the third model, Earth's gravity lassoed the moon as it wandered through from some distant part of the solar system. This "capture" scenario was appealing until the Apollo astronauts brought their moon rocks back home. The minerals in them turned out to be similar to those in Earth's mantle—not exotic at all.
The giant impact model avoided all these problems. When it came along in the 1970s, the model fit an emerging view of how the solar system as a whole had formed. In that view, gaseous and rocky protoplanets grew within a disk around the young sun, competing for space, for tens of millions of years. Collisions were inevitable.
As Earth got bigger, it absorbed several Mercury-size or Mars-size objects. The final major blow was an impact so fierce that it left a permanent reminder in orbit around us. According to the impact model, the moon coalesced mostly from the shattered debris of the impactor, a rocky protoplanet similar to Earth. Because the impactor's own iron core sank into Earth's core, the moon is all rock.
Giant Impact, Big Questions
It's a tidy picture, and it has become conventional scientific wisdom. "Five years ago, this article would have said the moon's story was figured out," Asphaug said. "But it's not."
Today, he explained, scientists generally agree that something smashed into Earth to give birth to the moon—but new evidence has cast doubt on details of the giant impact model.
Ongoing analysis of moon rocks, for instance, has shown that the moon and Earth's mantle are not just similar—they're nearly identical. Elements such as oxygen, silicon, and titanium come in several varieties, or isotopes. The blends of these isotopes match up so closely that it seems the moon must have formed almost entirely from fragments of Earth, rather than from the impactor.
One way around this problem is to resurrect the old fissioning Earth idea, but this time assisted by impacts. Repeated small impacts could have spun the growing Earth like a child's merry-go-round, Asphaug said, until it was rotating once every two hours—a breakneck speed for a rocky body that big. "It would have looked oblong," Asphaug said, twirling an egg-shaped roll on the lunch table.
A furiously spinning egg-shaped planet would have been under such strain that a single smaller impact—perhaps by a body just one-tenth the size of Mars—could have made it "explode," Asphaug went on. (A colleague of his calls this the Pinto model, a reference to the 1970s cars that allegedly tended to burst into flame when rear-ended.) Most of the material splashed into orbit would have been from Earth, thus creating a moon with the right chemical blend.
Another possibility is a "hit-and-run" collision by a bigger but speedier impactor that kept going after it blasted huge chunks of Earth's mantle into space; the moon would have formed from those pieces. Or maybe the moon did form from the impactor, but it was then coated with a thick layer of Earth material, which had remained in orbit for at least a century after the impact as an ultrahot disk of rocky vapor.
A second challenge for the giant impact model is to explain why the far side of the moon is so much more mountainous and thick-crusted than the side we see. Asphaug has proposed that Earth briefly had a second moonlet, which plastered itself onto the far side of its larger companion.
"The barn door is wide open, and now we have lots of ideas," Asphaug said. "There probably will be another 'aha' moment in five years or so."
But for now, the moon holds on to its mystery.
Robert Irion directs the science writing program at the University of California, Santa Cruz.