NASA has photographed night falling over Saturn's entire ring system, an event which happens only once every 15 years.
As Saturn's rings orbit the planet, a section is typically in the planet's shadow, experiencing a brief night lasting from six to 14 hours. However, during Saturn's equinox, night falls over the entire visible ring system for about four days.
When the sun is directly over Saturn's equator, the rings, which also orbit directly over the planet's equator, appear edge-on to the sun. Light from the sun hits the ring particles at very low angles, accenting their topography and giving a three-dimensional view of the rings.
In orbit around Saturn, Cassini's wide angle camera was able to shoot 75 exposures in succession for this 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 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 Dr Michael Flasar of NASA's Goddard Space Flight Center.
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.
"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 center," said Flasar.
However, the CIRS team discovered that the A-ring – 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.