Article Posted on : - Oct 24, 2009

Saturn's rings backlit by the Sun. Image Credit: NASA/JPL/Space Science Institute.

WASHINGTON (PTI): Astronomers have discovered evidence that a long night, lasting from 6 to 14 hours, falls over Saturn's rings.

However, once approximately every 15 years, night falls over the entire visible ring system for about four days.

This happens during Saturn's equinox, when the sun is directly over Saturn's equator.

"The equinox is a very special geometry, where the sun is turned off as far as the rings themselves are concerned, and all energy comes from Saturn," said Dr Michael Flasar of NASA's Goddard Space Flight Center in Greenbelt.

Although the rings are wide, they are only about 30 feet thick. They are made of particles that are mostly water-ice.

"At first glance, Saturn's rings look broad and bland, but then we got close-up images from the Voyager flybys, and our reaction was: oh, my gosh, there's structure everywhere ? what's going on?" said Linda Spilker of NASA's Jet Propulsion Laboratory (JPL), Pasadena, California.

Researchers have discovered that while most of the ring particles are as small as dust and pebbles, there are a few chunks as big as mountains, and even some small moons several miles across embedded in the rings.

Instead of orderly orbiting around Saturn, the particles clump together and drift apart, and the rings ripple and warp under the gravitational influence of Saturn's swarm of more than 60 moons.

"The closer we look at the rings, the more complex they get," said Spilker, Deputy Project Scientist for the CASSINI mission and a Co-Investigator on CIRS. She is leading the instrument team's investigation of the rings.

"Because Saturn's rings are so extended, going out to more than twice Saturn's radius (from the cloud tops), the furthest rings get less heat from Saturn than the innermost rings, so the ring temperatures at equinox tend to fall off with distance from Saturn's centre," said Flasar.

However, the CIRS team discovered that the A-ring which is  the outermost of the wide bright rings did not cool off as much as expected during the equinox. This might give clues about its structure and evolution.

"One possibility is that the gravitational influence of moons outside the A-ring is stirring up waves in it. These waves could be much higher than the typical thickness of the rings. Since the waves rise above the ring plane, material in the waves would still be exposed to sunlight during the equinox, which would warm up the A-ring more than expected," said Spilker.