Don't close down the lunar maternity ward just yet. New simulations suggest that seven of Saturn's moons were formed as recently as 10 million years ago—over 4 billion years later than the 55 other major bodies orbiting the planet. Researchers think even more new moons could be in prospect because the processes that produced the most recent examples are still active.
Even before the Cassini spacecraft arrived at Saturn 6 years ago this month, scientists wondered about the origins of the seven tiny moons orbiting either just beyond or inside the planet's magnificent rings. All seven, including Pandora and Epimetheus (pictured), are peanut-shaped like asteroids, suggesting that they formed at the beginning of the solar system and were grabbed by Saturn's gravity. But Cassini's instruments discovered that the density of the ring moons was closer to that of Swiss cheese than asteroid rock: less than 1 gram per cubic centimeter. That difference means that unlike the sun, planets, and other moons in the solar system, the ring moons didn't condense from a huge primordial disk of gas and dust. So how were they born?
The most obvious answer is that material from Saturn's rings clumped to produce the moons, but no one could develop a coherent computer model that mimicked the process. Now a team of researchers has done exactly that. By combining and adapting computer models designed to simulate solar-system formation and the orbital migration of planets, the researchers were able to show that the seven moons could condense directly from the rings and retain their wispy consistency.
The complicated process involves the behavior of the ring material, which is composed mostly of tiny grains of dust and ice crystals, just beyond what's called the Roche limit. That's the minimum distance—in Saturn's case, 147,000 kilometers from its center of gravity—an object such as a moon can approach a planetary body without being pulverized into a ring by that body's gravity. Just beyond the Roche limit, the ring material can begin to form clumps, or aggregates. Over time, those aggregates get pulled together by their own gravity, first to form tiny moonlets and then bigger—though still small—moons, the team reports in tomorrow's issue of Nature.
The gravitational interactions of ring material beyond the Roche limit made "the formation of aggregates unavoidable," says planetary scientist and co-author Sébastien Charnoz of Université Paris Diderot in Paris. As the simulation proceeded, he explains, it produced mergers among the aggregates, which yielded increasingly bigger objects. And soon, he says, tiny moons "just like Pandora and Epimetheus appeared."
Charnoz says the results should alter the way planetary scientists view the formation of the solar system. Previously, he and other researchers had thought that all of the solar system's bodies formed more than 4 billion years ago. But now "we see that it is possible, even today, to form new [moons]."
Astronomer and Cassini team member Joseph Burns of Cornell University says the researchers have made a case for how the small saturnian moons formed. They've also shown, he adds, that a similar process could have helped form Earth's moon, when a ring of material was created around our very young planet after it collided with a Mars-sized body.