A centuries-old astronomical mystery may be finally solved. A scientist says she has figured out how Saturn's spectacular rings formed. The dramatic process could help explain other solar system mysteries as well.
Saturn's rings have mystified scientists since they were discovered in the mid-1600s. In particular, none of the hypotheses about their origin explain why individual ring particles, which range in size from hailstones to small boulders, average between 90% and 95% ice. If a moon disintegrated in Saturn's orbit, as some astronomers have suggested, the rings should be about half ice and half rock. That's the composition of most moons this far from the sun.
The new theory, set forth by planetary scientist Robin Canup of the Southwest Research Institute in Boulder, Colorado, and published online today in Nature, explains the ice-rich composition of the rings and accounts for the odd characteristics of some of Saturn's smaller moons.
Canup created detailed computer simulations, which suggest a violent origin for Saturn's rings. As the planet coalesced during the birth of the solar system more than 4.5 billion years ago, the swirling disk of gas surrounding it included several moons about the size of Titan, Saturn's largest remaining satellite, which is about 50% larger than Earth's moon. But gravitational interactions with the gas caused the moons' orbits to shrink, and one by one the satellites entered death spirals and plunged into the planet.
Before each moon collided, immense tidal forces produced by Saturn's gravity stretched and contracted it, stripping off much of its ice. Subsequent moons gravitationally captured this ice, but they were eventually stretched and contracted until they too shed their ice and plunged into Saturn. Today's ring system is the fossil remains of the last moon to fall prey to Saturn's immense gravity, Canup contends. This moon was basically a giant ice ball with a rocky center. After its ice-rich veneer was stripped away in large chunks, its rocky core disappeared beneath the saturnian clouds.
The fragments of that final doomed moon, each originally between 1 and 50 kilometers across, formed an icy ring system as much as 1000 times as massive as today's rings. In the subsequent 4.5 billion years, innumerable collisions between these large chunks produced the much-smaller ring particles now orbiting Saturn. What little rocky material occurs in today's ring system probably is the debris of collisions between icy ring particles and asteroids and comets swept up by the planet's huge gravitational field, says Canup.
The new hypothesis also explains how Saturn's moons that orbit just beyond the edge of today's ring system might have formed. Over time, the rings spread out, and the icy bits that drifted farthest from Saturn eventually reached distances where their gravitational attraction for each other could overcome the planet's tidal forces that tended to rip them apart—a process that is still happening today, according to observations by the Cassini spacecraft now touring the Saturn system. In particular, says Canup, the findings offer a good explanation for why the moon Tethys is apparently almost pure ice.
"This is a pretty impressive piece of work," says planetary scientist Joseph Burns of Cornell University. It's more comprehensive than previous theories and consistent with Cassini observations, he says, and "it tells a fun and convincing story."
The new study "provides a very compelling narrative," says planetary scientist Matthew Hedman, also of Cornell. "This is the first reasonably plausible scenario about how the rings could have formed."