Once every 15 years, Saturn's ring system is plunged into a four-day-long night, throwing the orbiting dust particles into startling relief.
NASA's Cassini spacecraft observed this long night during Saturn's equinox earlier this year, measuring the coldest temperatures ever observed for the ring system.
The equinox observations can help astronomers better understand the structure and evolution of Saturn's rings, as well as the origin of the solar system.
A section of Saturn's rings are typically in shadow at any given time, experiencing a brief night that lasts from six to 14 hours. But during equinox, that night lasts for four whole days and affects the entire ring system.
Like Earth, Saturn experiences two equinoxes per orbit. During an equinox, each planet's equator lines up edge-on to its orbital plane, causing the sun to appear directly over the equator.
Of course, because Saturn is more distant from the sun, it takes much longer to complete an orbit — 29.5 Earth years, to be exact — so there is much more time between equinoxes than on Earth.
As equinox approaches, sunlight fades as the top edge of the solar disk appears to touch the ring (from the perspective of a viewer embedded in the rings). As the solar disk slowly crosses the rings, there is full darkness. Then the bottom edge of the sun rises above the ring planes, about four days after the sunlight originally began to fade.
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During equinox, light from the sun hits the ring particles at very low angles, which accentuates their topography, giving a 3-D view. (While Saturn's rings are wide, they are only about 30 feet (9 meters) thick.)
Saturn's most recent equinox occurred on Aug. 11, and NASA's orbiting Cassini spacecraft was there to capture the event.
Ring relief
Because of those low sun angles, the particles in the ring become very cold. Cassini's Composite Infrared Spectrometer (CRS) instrument took a ring temperature of minus 382 degrees Fahrenheit (minus 230 Celsius), the coldest temperature yet observed for the orbiting particles.
"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 Michael Flasar of NASA's Goddard Space Flight Center in Greenbelt, Md., and the principal investigator for the CRS instrument.
The temperature of various rings changes with their location.
"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 of with distance from Saturn's center," Flasar said.
Interestingly, the A-ring — the outermost of the wide, bright rings — did not cool off as much as expected. This anomalous temperature could be a clue to the ring's structure.
"One possibility is that the gravitational influence of moons outside the A-ring is stirring up wave in it," said Cassini team member Linda Spilker of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "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."
The effort to understand the rings could also help scientists paint a clearer picture of the origin of the solar system.
"Our solar system formed from a dusty disk, so by understanding the dynamics in a disk like Saturn's rings, we can gain insight into how Earth and the other planets in our solar system were made," Spilker said.