The Moon May Be Formed From Many Tiny Moons

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.

“We see that the moon and Earth are very similar in a lot of ways, in a lot of signatures that we measure— oxygen, tungsten, and so on,” Raluca Rufu, a researcher at the Weizmann Institute of Science in Israel, told me. “So you cannot explain the composition of the moon with a foreign material.”

Rufu, along with the planetary science professor Oded Aharonson and their colleagues, challenge the giant-impact model in a new study published Monday in the journal Nature Geoscience. The moon, they say, resulted not from a single impact with a Mars-size planet, but from a series of about 20 impacts with smaller objects over a period of millions of years. Each impact ejected debris, which formed into a disk in a matter of hours as it rotated away from Earth. The disks, known as moonlets, aggregated over several hundred years to assemble the moon.

Such collisions were common in the early solar system, as objects collided with each other to form planets, asteroids, and other objects. Multiple small, high-velocity collisions could chip off more material from Earth than a one-time, massive collision could, Rufu says. The objects likely each had their own unique chemical makeups, but their differences may have evened out over time as the debris coalesced. Variations in material composition would be more prominent in the aftermath of a one-time collision, rather than in the results of smaller collisions over millions of years.

Rufu conducted more than 800 simulations on a cluster of computers at the Weizmann Institute. Each run changed up the mass of the impacting objects, the angle of the collisions, and the speed at which they occurred.

The multiple-impact model raises some new questions, like how exactly the moonlets eventually merge to create a complete moon. Rufu says further study is needed to explore that process.

The new model, though it sounds like it, is not a complete departure from the leading theory. Rufu points out that her research relies on the knowledge gleaned from the impact simulations that were developed to test the giant-impact model. But she hopes the giant-impact hardliners don’t dismiss this theory outright.  

“I would like readers and scientists alike to not say that this is wrong because you were taught in school that it’s a giant impact” that formed the moon, Rufu said. “Try to be open-minded.”