Long Night Falls Over Saturn's Rings
As Saturn's rings orbit the planet, a section is typically in the planet's shadow, experiencing a brief night lasting from 6 to 14 hours.
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. At this time, the rings, which also orbit directly over the planet's equator, appear edge-on to the sun. During equinox, light from the sun hits the ring particles at very low angles, accenting their topography and giving us a three-dimensional view of the rings.
"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, Md.
During Saturn's latest equinox August 11, the rings reached a temperature of 382 degrees below zero Fahrenheit, the coldest yet observed, as seen by the Composite Infrared Spectrometer (CIRS) instrument on board the Cassini spacecraft in orbit around Saturn. CIRS was developed at NASA Goddard, and Flasar is the Principal Investigator for the instrument.
"The whole point of the CIRS observations of Saturn's rings, other than producing some cool pictures, is to learn something about the physical properties of the ring particles: their spin rates, how sluggish they are in storing and radiating heat (a diagnostic of size and composition), and their vertical distribution in the ring 'plane'," said Flasar.
Although the rings are thousands of miles wide, they are only about 30 feet thick. They are made of particles that are mostly water-ice. Scientists continue to debate the rings' origin and age. Some think they could be remnants of a shattered moon or captured comets, while others think they could have formed along with Saturn from the primordial disk of gas and dust that gave birth to our solar system.
"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 Dr. Linda Spilker, of NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif.
"Seen from our planet, the view of Saturn's rings during equinox is extremely foreshortened and limited. But in orbit around Saturn, Cassini had no such problems. From 20 degrees above the ring plane, Cassini's wide angle camera shot 75 exposures in succession for this mosaic showing Saturn, its rings and a few of its moons Aug. 12, 2009, beginning about 1.25 days after exact Saturn equinox, when the sun's disk was exactly overhead at the planet's equator. (Credit: NASA/JPL/Space Science Institute)"
Source: NASA
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