Saturn's Rings: Giant Structures

June 14, 2010

Saturn's Rings: Giant Structures 4 Kilometers High & 3-Dimensional Waves

Saturn's rings have been studied for the last six years by a school bus–sized spacecraft named Cassini that has been sending back astonishing pictures of the rings and the moons that help shape them. 

In August 2009, the rings were thrown into high relief, literally, during Saturn's vernal equinox. As the sun crossed into the northern hemisphere, its rays shone parallel to the rings for about four days—an alignment that happens once every 15 years or so. And as the rings themselves slipped into shadow, previously unseen features revealed themselves.

The equinox enabled Cassini to capture sharp relief images of the incredible rings around Saturn with the Sun shining sideway-on (when you're in space you're really at the mercy of natural lighting).  The results were amazing: rings thought to be ten meters thick, with variations of two stories at most, turned out to have vertical jumps the size of the Rocky mountains. The thing about space is you can hide things that big in it.  The results captured during the week of perfect plane illumination will be studied for years to come, and the great thing about the internet age is that they aren't just for the professionals - they stitched together a truly mind-boggling high resolution image just for us online-types to goggle at.

"It's like standing outside right before the sun sets. Your shadow gets very long. Anything that's a little bit bigger, or sticks up, casts a shadow,"  said JPL's Linda Spilker, Cassini's project scientist. The shadows revealed curtains of ice particles up to four kilometers tall, created by Daphnis, one of Saturn's moons. Daphnis orbits at a slight tilt with respect to the rings, and when it crosses the ring plane, it drags some of the ring material after itself.

Saturn's rings have weird wakes and waves of all sorts, sculpted by the gravity of passing moons. The equinox gave us a closer look at waves discovered by JPL's Voyager mission, including spiral density waves created by orbital resonances and compressional waves created from the repeated gravitational tugs analogous to the spiral arms of our own Milky Way. "They're much more tightly wound than the spiral structures that you see in galaxies, but they are in fact the same creatures. The physics behind them is the same, " according to the Space Science Institute's Carolyn Porco, head of the Cassini imaging team. If the resonant moon's orbit is slightly askew, three-dimensional bending waves—also spirals—form as well. The density waves and the bending waves propagate in opposite directions, says Spilker, adding a further level of complexity to the rings' structure.

The equinox images also revealed a new kind of three-dimensional wave, only about 100 meters tall. "From high above, the rippled surface of Saturn's D ring looks like a corrugated roof," says Spilker. "The ripple extends for more than 17,000 kilometers across the ring system. When the Voyagers flew by, it wasn't there." Scientists remain baffled about the ripple's origin, but one possible scenario has a meteoroid slamming into the rings.

Casey Kazan

Source: http://www.nasa.gov/mission_pages/cassini/whycassini/cassini-20090921.html