When the sun set on Saturn's rings in August 2009, scientists on NASA's Cassini mission were watching closely. It was the equinox -- one of two times in the Saturnian year when the sun illuminates the planet's enormous ring system edge-on. The event provided an extraordinary opportunity for the orbiting Cassini spacecraft to observe short-lived changes in the rings that reveal details about their nature.
Like Earth, Saturn is tilted on its axis. Over the course of its 29-year-long orbit, the sun's rays move from north to south over the planet and its rings, and back again. The changing sunlight causes the temperature of the rings -- which are made of trillions of icy particles -- to vary from season to season. During equinox, which lasted only a few days, unusual shadows and wavy structures appeared and, as they sat in twilight for this brief period, the rings began to cool.
In a recent study published in the journal Icarus, a team of Cassini scientists reported that one section of the rings appears to have been running a slight fever during equinox. The higher-than-expected temperature provided a unique window into the interior structure of ring particles not usually available to scientists.
The researchers examined data collected by Cassini's Composite Infrared Spectrometer during the year around equinox. The instrument essentially took the rings' temperature as they cooled. The scientists then compared the temperature data with computer models that attempt to describe the properties of ring particles on an individual scale.
What they found was puzzling. For most of the giant expanse of Saturn's rings, the models correctly predicted how the rings cooled as they fell into darkness. But one large section -- the outermost of the large, main rings, called the A ring -- was much warmer than the models predicted. The temperature spike was especially prominent in the middle of the A ring.
The team's analysis suggested the best explanation for the A ring's equinox temperatures was for the ring to be composed largely of particles roughly 3 feet (1 meter) wide made of mostly solid ice, with only a thin coating of regolith.
"This particular result is fascinating because it suggests that the middle of Saturn's A ring may be much younger than the rest of the rings," said Linda Spilker, Cassini project scientist at JPL and a co-author of the study. "Other parts of the rings may be as old as Saturn itself."