Chips off the mantle: Saturn’s moons explained

Many years ago, astronomy students were taught that Saturn was, in essence, a mini-Jupiter. Sure, it had rings, but so, we now know, does Jupiter. Saturn has 31 known moons. Jupiter has 67, but most are probably captured asteroids, so…

Many years ago, astronomy students were taught that Saturn was, in essence, a mini-Jupiter. Sure, it had rings, but so, we now know, does Jupiter. Saturn has 31 known moons. Jupiter has 67, but most are probably captured asteroids, so the difference makes sense because Jupiter is closer than Saturn to the Asteroid Belt.
But that might be the only comparison between Jupiter and Saturn’s moons that has traditionally made sense. When you look closer, you find that Jupiter has four large moons (Ganymede, Callisto, Io, and Europa), while Saturn has only one, Titan.
Titan, however, is truly enormous. The largest moon in the Solar System, it’s about the mass of all four of Jupiter’s large moons, combined. Just as oddly, Saturn has a whole family of mid-sized moons that appear to be composed largely of ice, even though the outer Solar System should contain a roughly equal mix of ice and rock.
“It’s been a major puzzle,” says Erik Asphaug, a planetary scientist from the University of California, Santa Cruz. “Explaining the middle-sized moons of Saturn is quite a mystery.”
In a presentation this week at a meeting of the American Astronomical Society’s Division for Planetary Sciences, Asphaug proposed a solution for both processes. Titan, he suggested, initially had several large moons, just like Jupiter, each with a rocky core and an icy mantle – counterparts to Earth’s iron core and rocky mantle.
But Jupiter’s moons are locked into stable resonances in which they can never collide. “They are synchronised like gears on a wheel,” Asphaug said.
Saturn’s pre-Titan moons, he suggested, weren’t so lucky. One by one, they smashed into each other. In part, they merged to form Titan. But in the process, bits of their icy mantles were blown off into space.
These collisions would have been much slower-speed than the giant asteroid impact that formed our own Moon, which means that instead of vapourising the mantles of the colliding Saturnian moons, they would simply have basted solid bits off into space where they would rapidly form back into new moons – moons composed largely of ice.
Asphaug’s simulations, in fact, show how the giant Titan-building collisions might throw such small bodies off into space like a string of pearls or the arms of a spiral galaxy. “It’s a pulling apart of icy material,” he said. “It clumps up into beads, like beads on a string or water in a fountain. … The final result is Titan and the middle-sized moons.”
Better yet, the moons flung off by this process come from different sources: some from one of the colliding moons’ mantles, some from the other’s. That’s important because Saturn’s mid-sized, icy moons are otherwise inexplicably diverse. Some, like Enceladus, are “somewhat rock-rich,” Asphaug said, while others, such as Rhea, are 70% ice.
Why this happened at Saturn but not Jupiter, he admits, is unclear. Perhaps Saturn’s initial moons never managed to hit a stable configuration like what we now see among Jupiter’s. Or perhaps changes in the orbits of the major planets, late in the formation of the Solar System, caused Saturn’s early moons to be yanked out of stable orbits into ones that would eventually collide – while Jupiter’s were protected by its stronger gravity.
“Either [the system] didn’t stabilise to begin with, or something happened to destabilise Saturn’s satellites,” Asphaug said. “The Saturn system has only one-third the mass of Jupiter’s, so if [moons] were being yanked around, then Saturn’s would be several times less stable than Jupiter’s.”
It’s an interesting idea, says planetary scientist Matija Ćuk of the SETI Institute, Mountain View, California, who recently published a paper in Science examining the giant impact that most likely produced the Earth’s Moon. “I think it’s interesting because the pattern of composition of Saturn’s moons is that there is no pattern. This is one way to do it that I’ve never seen before,” she said.
But so far, he said, the simulation stops shortly after the collisions, without tracking what happens next to the icy objects being flung off from the colliding moons’ mantles. That’s a problem, he said, because there’s no guarantee that these objects go on to form moons like Rhea, Enceladus, or any of the others.
“I do orbital dynamics,” Ćuk said. “Erik does impacts. Impacts are complex but take a short amount of time – hours or days. After his simulations end, you have to find out what happens to all of these little moons in terms of orbital evolution.”
Ćuk’s moon-article collaborator, Sarah Stewart of Harvard University in the U.S., agrees. “You need to do the orbital dynamics because bodies that are blown off tend to come back in, or hit each other and accrete,” she says.
Meanwhile, the theory remains an intriguing solution to a knotty problem: a one-step method to make not only Titan but the intriguing family of other moons that have kept Saturn-system planetary scientists enthralled ever since NASA’s Cassini spacecraft arrive there and began cataloguing mysteries, the better part of a decade ago.
Richard A. Lovett is a science writer based in Portland, Oregon, USA.