On Dec. 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.
“Think how it would have been if you were standing there before that boulder came by,” Cernan mused.
“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.