In comparison to other planets in the solar system, our single glowing lunar orb seems to pale. Saturn may have up to 62 moons. Jupiter could have 67. Even Mars has two moons, Phobos and Deimos. So it seems odd that the Earth only has one moon. But new research suggests that may not have always been the case. Earth may have had several moons that at some point crashed together to form our current celestial buddy.
For many years researchers believed in something called the “giant-impact” model of the moon, reports Sarah Kaplan for The Washington Post. The idea is that a large protoplanet called Theia bumped into Earth about 4.4 billion years ago, knocking off a moon-sized cloud of debris that eventually coalesced into our favorite satellite. But in the last 20 years, researchers have started doubting that idea—especially since moon rocks collected by the Apollo missions are chemically similar to Earth. If there was an impact with Theia, researchers would expect there to be chemical traces of the mystery planet as well.
The new study published in the journal Nature Geosciences proposes a different model. They ran over 1000 simulations modeling this ancient impact, which suggest that instead of one giant collision the Earth may have experienced many smaller ones. Each of these smaller impacts would have ejected earth debris that could have coalesced into a moonlet.
“Our model suggests that the ancient Earth once hosted a series of moons, each one formed from a different collision with the proto-Earth," says co-author Hagai Perets of Israel’s Technion. “It’s likely that such moonlets were later ejected, or collided with Earth or with each other to form bigger moons.”
According to Kaplan, about 20 moonlets likely combined to form the current moon. While the idea behind the many-moons theory has been around since the 1980s, the research teams' simulations demonstrate for the first time that the process would have been possible.
“We see that multiple impacts will have a high probability of building a moon with similar composition to the Earth,” co-author Raluca Rufu tells Ian Sample at The Guardian. “With 20 impactors, it would take about 100 million years to build the moon.”
But there are still problems with the hypothesis. For instance, Kaplan reports, the researchers do not yet have a coherent model for how the 20 mini-moons would have merged into one supermoon.
Researchers may get a few more clues to the moon’s formation later in 2017, however, when China attempts to bring back the first moon rocks since the Apollo missions ended in the 1970s, Sample reports. “If we had more lunar samples, that would be very helpful,” Rufu tells Sample. “One giant impact should produce a more homogenous rock, but under our scenario, I’d expect the composition to vary between different regions.”
But the giant-impact camp isn’t ready to roll over yet. In November, researchers proposed a new model of the giant impact in the journal Nature. In that scenario, they suggest that the impact between Earth and the protoplanet vaporized a good portion of each, mixing them up and making their chemistries similar. The impact also tilted the Earth, putting the seasonal wobble in its orbit and sent it spinning incredibly fast for a few million years.
No matter how the moon was formed, it must have been an unusual event. Among all the moons in the Solar System, the stabilizing effect our moon has on the planet is unique. Perhaps our celestial buddy isn't so boring after all.