Saturn's rings may double up as a moon factory

Many of the moons in the solar system could have been spawned from giant rings around planets. According to a new model, Saturn, Uranus, Neptune and even the Earth may have once had ring systems that gave rise to satellites.

We used to think that moons form around planets in the same way as planets form around stars: coalescing from a gaseous disc that surrounded the planet as it formed. That model still applies to some moons, like those of Jupiter.

But Saturn's moons follow a peculiar pattern. Their orbits bunch near the edge of the rings, and the moons get more spread out and more massive as they get further away.

The rings mark Saturn's Roche limit: the distance from the planet beyond which its gravitational tidal forces are weak enough to let moons there survive.

Inside the Roche limit, however, Saturn's gravity would pull moons apart and add them to its rings. Some astronomers think this is how the planet gained its rings in the first place.

But theory says that such rings do not remain static. The constituent fragments that lie near the inner, planetary side of the ring should constantly exchange angular momentum with fragments further out. This means the inner fragments lose energy and fall towards Saturn while the outer ones gain energy and retreat from the planet.

Aurélien Crida at the Observatory of the Cote d'Azur in Nice, France, and Sébastien Charnoz at the Denis Diderot University in Paris have now run simulations of this effect. They showed that material leaving the outer edge of the ring would pool into a small moon, which then gradually migrates away from the planet.

When enough material is left in the rings, a second moon would grow where the first moon formed. This moon, too, would gradually move away, allowing a third moon to grow, and so on.

The earlier moons would probably be larger, because they had a bigger ring to draw material from than the later moons.

The early moons also have more time to collide with each other, fusing into larger satellites. "We see that for Saturn's moons, it fits quite well," Crida says.

Smoking gun Neither Uranus nor Neptune has a massive ring system today, but the distribution of moons around both planets is similar enough to the Saturn pattern to suggest that they once did – and that the rings gave rise to both of the ice giants' satellites.

"We think this is a smoking gun of this process," Crida says. Uranus and Neptune clearly lack big rings today, though, so we would need a better understanding of the ring-forming process to establish whether they might have done in the past.

Surprisingly, even the Earth could have had a ring once. Earth's moon probably formed when a large body collided with the young planet and sent hot mantle material flying into space.

"The process through which this material would eventually form the moon was not investigated in detail so far," Crida says. If the material settled into a massive ring, it could have spread and congealed into a single moon in as little time as a month, he says.

Journal: Science, doi.org/jvw