Violent Origin for Saturn's Rings
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.