The icy particles that make up Saturn's rings may owe their existence to a big moon that smacked into the planet about 4.5 billion years ago.NASA/JPL/University of Colorado

Saturn’s majestic rings are the remnants of a long-vanished moon that was stripped of its icy outer layer before its rocky heart plunged into the planet, a new theory proposes. The icy fragments would have encircled the solar system’s second largest planet as rings and eventually spalled off small moons of their own that are still there today, says Robin Canup, a planetary scientist at the Southwest Research Institute in Boulder, Colo.

“Not only do you end up with the current ring, but you can also explain the inner ice-rich moons that haven’t been explained before,” she says. Canup’s paper appears online December 12 in Nature.

The origin of Saturn’s rings, a favorite of backyard astronomers, has baffled professional scientists. Earlier ideas about how the rings formed have fallen into two categories: either a small moon plunged intact into the planet and shattered, or a comet smacked into a moon, shredding the moon to bits. The problem is that both scenarios would produce an equal mix of rock and ice in Saturn’s rings — not the nearly 95 percent ice seen today.

Canup studied what happened in the period just after Saturn (and the solar system’s other planets) coalesced from a primordial disk of gas and dust 4.5 billion years ago. In previous work, she had shown that moon after moon would be born around the infant gas giants, each growing until the planet’s gravitational tug pulled it in to its destruction. Moons would have stopped forming when the disk of gas and dust was all used up.

In the new study, Canup calculated that a moon the size of Titan — Saturn’s largest at some 5,000 kilometers across — would begin to separate into layers as it migrated inward. Saturn’s tidal pull would cause much of the moon’s ice to melt and then refreeze as an outer mantle. As the moon spiraled into the planet, Canup’s calculations show, the icy layer would be stripped off to form the rings.

A moon so large would have produced rin

The icy particles that make up Saturn's rings may owe their existence to a big moon that smacked into the planet about 4.5 billion years ago.

gs several orders of magnitude more massive than today’s, Canup says. That, in turn, would have provided a source of ice for new, small moons spawned from the rings’ outer edge. Such a process, she says, could explain why Saturn’s inner moons are icy, out to and including the 1,000-kilometer-wide Tethys, while moons farther from the planet contain more rock.

“Once you hear it, it’s a pretty simple idea,” says Canup. “But no one was thinking of making a ring a lot more massive than the current ring, or losing a satellite like Titan. That was the conceptual break.”

“It’s a big deal,” agrees Luke Dones, also of the Southwest Research Institute, who has worked on the comet-makes-rings theory. “It never occurred to me that the rings could be so much more massive than they are now.”

Another recent study supports the notion that today’s rings are the remnants of massive ancient rings of pure ice. In a paper in press at Icarus, Larry Esposito, a planetary scientist at the University of Colorado at Boulder, calculates that more massive rings are less likely to be polluted by dust, and hence could still be as pristine as they appear today even after 4.5 billion years.

Some questions still linger about Canup’s model, says Dones, like why some of Saturn’s inner icy moons have more rock in them than others.

The theory will be put to the test in 2017, when NASA’s Cassini mission finishes its grand tour of Saturn by making the best measurements yet of the mass of the rings. Researchers can use those and other details to better tease out how the rings evolved over time.