When the Apollo astronauts flew to the moon in the 1960s, scientists eagerly awaited the return of lunar rocks they hoped would reveal the origins of the moon billions of years ago. One theory for the moon’s formation, proposed by Charles Darwin’s grandson, George, posited that the early Earth had spun so fast that part of it had flown off. Another theory suggested the moon was born from the same primordial dust that made the Earth and other objects in the solar system. A third said the Earth’s gravity had lassoed the moon into its orbit as it passed, like a cosmic flytrap.
The first examinations of the rocks favored none of these explanations. In the mid-1980s, scientists began to coalesce around another theory, proposed a decade earlier. The moon, many now believed, was formed from the debris of a powerful collision between the early Earth and a planet the size of Mars.
“The giant-impact scenario seems to have cut the Gordian knot of the three classic theories,” a geophysicist told The New York Times in 1986. “It requires no magic, no special pleading, no extra twiddling and no deus ex machina. It just works.”
It worked enough to become the leading theory of moon formation, gaining support through impact models simulated on, as one astrophysicist in the 1986 article put it, “huge computers.” But further research revealed a limitation. The model assumes that most of the material of the moon comes from the Mars-sized planet that struck the early Earth. Yet analysis shows the chemical composition of moon rocks is nearly identical to the composition of Earth.