Planetary scientists generally accept that a giant impact into the newly formed Earth ejected a huge cloud of material that became the moon. In Crida and Charnoz’s conception, that ejecta first flattened into a ring around the planet, which then spread out and coagulated into the moon. But unlike Saturn’s ring, which would have leaked out numerous moonlets to form several moons, Earth’s relatively massive ring would have poured all its material into one large satellite before dissipating. “It spreads very fast,” Crida says of Earth’s hypothesized ring. “And if it spreads fast, only one moon has time to form.”
But the new hypothesis is not without its problems. First, it does not seem to apply to the satellites of Jupiter, which do not obey the same mass-to-distance correlation of the other moon systems. Crida notes that Jupiter was the first planet to form and may have coalesced under different conditions. “I was a bit disappointed to see that Jupiter did not fit the same distribution, but not too surprised,” he says. And then there is the obvious question: if extensive, Saturn-like ring systems once adorned Neptune and Uranus, where are they now? “It’s not that easy to remove them over time,” Nesvorny says. “There is a link that’s missing that needs to be understood.”
The French researchers concede that the fate of the rings is an open question. “I don’t know why Uranus and Neptune don’t have the rings anymore,” Crida says. “We have a few ideas, but nothing too convincing. Some people can see that as a weakness of our mechanism, because indeed where are the rings now? But I think we can find good reasons for the disappearance of the rings, and the satellites remain as the smoking gun."