Ring Shots

Add another glimmer of glamour to the icy jewels that adorn the outer solar system’s largest planet. On Sept. 15, the Cassini spacecraft spied two new rings around Saturn. The craft had an unusual viewpoint. It was in Saturn’s shadow on the opposite side of the planet from the sun. For 11 hours, as the sun lit the rings from behind, Cassini recorded the vast system of ice particles that stretches across a region of space greater than the distance between the Earth and its moon.

The brilliant back lighting provided a rare opportunity for Cassini to see microscopic ring particles that are usually too faint to be recorded.

“It’s like the sun coming directly through a really dusty windshield,” explains Cassini scientist Linda Spilker of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Just as the dust particles on a windshield are brightened by illumination, so, too, are ice particles in the rings. In August 2005, Cassini had viewed the rings under the same conditions, but for only about 2.5 hours, notes Cassini scientist Matt Hedman of Cornell University. The September session is the longest such observation that the craft will make during its mission.

“It’s very exciting to see the rings in a way that no one has ever seen them before,” says Spilker.

Moonmade

The billions of ice particles that make up Saturn’s rings circle the planet like cars on racetracks. The particles’ gyrations sculpt waves, wakes, and other structures (SN: 11/19/05, p. 328: Groovy Science). The rings that Cassini sighted in September are the first discovered in more than 25 years. The brighter of the two rings is 5,000 kilometers wide and lies between Saturn’s main rings and the faint, outer G ring. That new ring’s location coincides with the orbits of Janus and Epimetheus, two tiny moons of Saturn. The fainter new ring is about half as wide and overlies the orbit of another tiny moon, Pallene, that lies between the two outermost rings.

Saturn’s moons are bombarded by comets or micro-meteoroids. Those collisions knock off ice particles and send them into orbit about Saturn, forming rings.

Despite myriad observations of Saturn’s majestic rings since the time of Galileo, no one knows how long rings survive or how often they’re replenished by the planet’s small moons, notes planetary scientist Josh Colwell of the University of Colorado in Boulder.

Dynamic bands

Other images taken during the Sept. 15 event reveal the ephemeral nature of the rings. Thousands of narrow bands of icy particles, collectively called ringlets, appear within gaps in each of the rings. Cassini’s September images confirmed two ringlets first spied earlier in the craft’s mission.

These ringlets showed up in the Cassini Division, a large gap between Saturn’s third and fourth main rings. The ringlets must have formed recently—they weren’t seen when the Voyager craft passed by 25 years ago, notes Cassini scientist Joe Burns of Cornell University. Indeed, the color and brightness of one of the ringlets suggest that it’s composed of fresh ice, Burns adds.

Cassini also uncovered evidence that a comet or an asteroid recently collided with Saturn’s innermost, D ring. The craft found what appears to be a series of bright ringlets spaced 30 km apart in the outer part of the ring. In 1995, the Hubble Space Telescope recorded similar features spaced about twice as far apart. Furthermore, Cassini images show that the D ring has corrugations, as a tin roof does.

These features are best explained by a collision that knocked the ring about a kilometer out of the plane in which the main rings orbit Saturn, Burns notes.

“Saturn’s rings are dynamic and have changed before our eyes,” he says.