On December 13, 1972, Apollo 17 astronaut Harrison Schmitt walked up to a boulder in the moon’s Sea of Serenity. “This boulder’s got its own little track, right up the hill,” he called to his commander, Eugene Cernan, pointing out the mark the boulder left when it rolled down a mountainside. Cernan bounded over to collect some samples.
“I’d rather not think about it,” Schmitt said.
The astronauts chiseled bits of the moon from the boulder. Then, using a rake, Schmitt scraped the powdery surface, lifting a rock later named troctolite 76536 off the regolith and into history.
That rock, and its boulder brethren, would go on to tell a story of how the entire moon came to be. In this creation tale, inscribed in countless textbooks and science-museum exhibits over the past four decades, the moon was forged in a calamitous collision between an embryonic Earth and a rocky world the size of Mars. This other world was named Theia, for the Greek goddess who gave birth to Selene, the moon. Theia clobbered Earth so hard and so fast that the worlds both melted. Eventually, leftover debris from Theia cooled and solidified into the silvery companion we have today.
But modern measurements of troctolite 76536, and other rocks from the moon and Mars, have cast doubt on this story. In the past five years, a bombardment of studies has exposed a problem: The canonical giant-impact hypothesis rests on assumptions that do not match the evidence. If Theia hit Earth and later formed the moon, the moon should be made of Theia-type material. But the moon does not look like Theia—or like Mars, for that matter. Down to its atoms, it looks almost exactly like Earth.
Confronted with this discrepancy, lunar researchers have sought new ideas for understanding how the moon came to be. The most obvious solution may also be the simplest, though it creates other challenges with understanding the early solar system: Perhaps Theia did form the moon, but Theia was made of material that was almost identical to Earth. The second possibility is that the impact process thoroughly mixed everything, homogenizing disparate clumps and liquids the way pancake batter comes together. This could have taken place in an extraordinarily high-energy impact, or a series of impacts that produced a series of moons that later combined. The third explanation challenges what we know about planets. It’s possible that the Earth and moon we have today underwent strange metamorphoses and wild orbital dances that dramatically changed their rotations and their futures.