Saturn's rings might be the remains of a giant "lost" moon that was stripped of its icy shell before it crashed into the planet, new research suggests.
What's more, the new theory could help explain why relatively similar Jupiter ended up with several huge moons and only thin, faint rings.
It's long been a mystery why Saturn's rings are made of 90 to 95 percent water ice. That's because most other materials in the outer solar system—where the gas giant planet dwells—are roughly half ice and half rock.
(Related: "Ice 'Tsunamis' Detected in Saturn Ring.")
The low densities of Saturn's small inner moons, which orbit within and just beyond the edge of the planet's main, bright rings, suggest these bodies are also strangely rich in ice.
Scientists have bandied about a number of theories for the rings' origins: Perhaps, for instance, the rings came from small moons, about 124 miles (200 kilometers) wide, that were broken up by meteors or comets.
But none of the previous theories could fully explain why the rings are so loaded with ice.
According to planetary scientist Robin Canup of the Southwest Research Institute in Boulder, Colorado, the answer may be that the rings resulted from the death of a moon thousands of kilometers wide, roughly as large as Saturn's biggest moon, Titan.
Saturn's Cycles of Doom and Destruction
About 4.5 billion years ago, soon after the birth of the solar system, giant planets such as Saturn and Jupiter would have been surrounded by disks of leftover gas and dust. As particles in these disks clumped together, moons made of ice and rock would have formed.
The moons that orbited close to their host planets would have experienced gravitational distortions, which generate heat, the new theory says. This heat would have caused the denser rock to quickly sink to the moons' cores, "somewhat like rocks dropping to the bottom of a swimming pool," Canup said.
But the newborn icy moons would have then experienced drag as they plowed through the remaining disks around the gas giants. This friction would have disrupted the moons' orbits and regularly sent the satellites spiraling down toward their massive host planets.
As the moons plummeted to their doom, their icy shells would have been stripped away, forming rings of virtually pure ice around the planets.
The initial rings would have been destroyed by interactions with other falling moons, the theory goes. But all this activity would have stopped once the disks around the planets were depleted.
In the end, the theory says, Saturn emerged with giant rings and one huge moon, Titan. Meanwhile Jupiter ended up without much in the way of rings but with four giant moons: Ganymede, Europa, Io, and Callisto.
Rocky Pollution Also a Factor
According to Canup's computer simulations, the demise of a particularly large moon late in this cycle would have led to rings initially a hundred to a thousand times as massive as today's rings.
Some of that ice could have spread inward and collided with Saturn, while some material that scattered outward could have clustered over time to form Saturn's oddly icy inner moons.
"Contamination" from rocky meteoroids hitting the rings over billions of years would then explain why the rings are not still pure ice.
Canup's new theory is the first "that's really comprehensive and consistent with the facts of Saturn's rings and its satellites," said planetary scientist Joseph Burns at Cornell University, who did not take part in this study.
NASA's Saturn-orbiting Cassini spacecraft may help Canup test this model: Mission managers plan to measure the rings' current mass and pollution rate.
The new Saturn-rings theory was published online ahead of print on December 12 by the journal Nature.