Cassini's camera captures night falling on Saturn's rings

Washington, October 24 (ANI): The wide angle camera aboard the Cassini spacecraft has captured images of Saturn's latest equinox that shows night falling over the planet's rings.

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.

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. rom 20 degrees above the ring plane, Cassini's wide angle camera shot 75 exposures in succession for a mosaic, showing Saturn, its rings and a few of its moons on August 12, 2009, beginning about 1.25 days after exact Saturn equinox, when the sun's disk was exactly overhead at the planet'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, Maryland.

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.

Ring material pulled to spectacular heights above the ring plane by the gravity of the moon Daphnis casts long shadows on Saturn's A ring in a Cassini image taken about a month before the planet's August 2009 equinox.

The structures casting the shadows reach heights of almost 4 kilometers (2.5 miles) above the ring plane.

Saturn experiences two equinoxes per orbit, just as Earth does, when the planet's equator lines up edge-on to its orbital plane, causing the sun to appear directly over the equator.

For a viewer on Saturn, the sun would seem to move from south to north around the time of the August 11 equinox.

Technically, the equinox is the instant when the sun appears directly over the equator, but Saturn's situation gives the rings an extended twilight.

Before the August 11 equinox, a viewer embedded in Saturn's rings would have seen sunlight fade as the top edge of the solar disk appeared to touch the rings first.

This would be followed by darkness around the equinox as the solar disk slowly crossed the ring plane. (ANI)