Saturn's Moon Triggers Giant Snowballs in Planet's Ring
This mosaic of images from NASA's Cassini spacecraft depicts fan-like structures in Saturn's tenuous F ring. Bright features are also visible near the core of the ring. Such features suggest the existence of additional objects in the F ring.
Credit: NASA/JPL/Space Science Institute Full Story

A NASA spacecraft that orbits Saturn has captured new images that show icy particles in the planet's outermost discrete ring that are clumping into giant snowballs, created by the gravitational pull of a nearby moon.

The Cassini spacecraft, which has monitored collisions and disturbances in the gas giant's rings for the last six years, spotted the icy clumping process as the moon Prometheus makes multiple swings by Saturn's F ring ? a thin ring orbiting about 87,000 miles (140,000 km) out from the planet. [ Photo of the fan-like structures].

The moon's gravitational pull creates disturbances in the ring material, making wake channels that trigger the formation of objects as large as 12 miles (20 kilometers) in diameter, when smaller masses stick together through their mutual gravitational attraction.

The natural processes that occur within Saturn's rings can give scientists a glimpse into the mechanisms at work in our early solar system, as planets and moons coalesced out of disks of debris.

"Scientists have never seen objects actually form before," said Carl Murray, a Cassini imaging team member based at Queen Mary, University of London. "We now have direct evidence of that process and the rowdy dance between the moons and bits of space debris."

Murray presented the study July 20 at the Committee on Space Research meeting in Bremen, Germany. The findings were also published July 14 in the online edition of the Astrophysical Journal Letters.

Saturn's F ring

Saturn's relatively thin F ring was discovered by NASA's Pioneer 11 spacecraft in 1979. Prometheus and Pandora, the small moons found on either flank of the F ring, were discovered a year later by NASA's Voyager 1 robotic space probe.

In the years since its discovery, the F ring has undergone constant changes in appearance, and scientists have closely monitored the behavior of the two mischievous moons for answers.

Prometheus, which is larger and closer to Saturn, appears to be the main source of the disturbances in the F ring. At its longest point, the potato-shaped moon is 92 miles (148 km) across.

This moon swerves around Saturn at a speed slightly greater than the speed of the much smaller particles in the planet's F ring. As a result of this discrepancy, and the slightly offset orientation of Prometheus' orbit, the moon laps the F ring particles, stirring them up approximately every 68 days.

"Some of these objects will get ripped apart the next time Prometheus whips around," Murray said. "But some escape. Every time they survive an encounter, they can grow and become more and more stable."

Cassini scientists previously used the spacecraft's ultraviolet imaging spectrograph to detect thickened blobs of material near the F ring, after noticing starlight was partially blocked in that area. These objects may be related to the snowball-like clumps that were discovered by Murray and his colleagues.

Density and life span

The newly-found objects in the F ring also appear dense enough to possess what scientists call "self-gravity." This means that they can attract more particles and accumulate in size as ring particles bounce around in Prometheus' wake, Murray said.

As a result, the giant clumps could be almost as dense as Prometheus itself, but only about one-fourteenth as dense as Earth in comparison.

These snowballs in the F ring also have a higher chance of survival due to their serendipitous location in the Saturn system. The F ring is located at a point that is balanced between the tidal force of Saturn trying to break objects apart, and the self-gravity that pulls objects together.

Still, these giant snowballs have short life spans, likely forming and breaking apart within a few months.

The new study could also help scientists explain the origin of a mysterious object discovered by Cassini scientists in 2004. The object, which has been provisionally named S/2004 S 6 is about 3 to 6 miles (5 to 10 km) in diameter, and occasionally bumps into Saturn's F ring, producing jets of debris in the process.

"The new analysis fills in some blanks in our solar system's history, giving us clues about how it transformed from floating bits of dust to dense bodies," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "The F ring peels back some of the mystery and continues to surprise us."