Photos: Cassini Images

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  1. Rites of spring on Saturn

    Of the countless equinoxes Saturn has seen since the birth of the solar system, this one, captured by the Cassini orbiter, is the first witnessed up close by an emissary from Earth. Cassini took this picture on Aug. 12, a little more than a day after the planet's equinox. During the equinox, which happens only once every 15 years, Saturn's rings are edge-on with respect to the sun and virtually disappear. The exposure has been selectively brightened in this image so that the rings remain visible. (NASA) Back to slideshow navigation
  2. Gossamer rings

    This mosaic of Cassini images, captured just hours before the Saturnian equinox on Aug. 11, shows that spiral ripples in the planet's inner rings continue right up to the inner B ring. The wide extent of the ripples is an unexpected result that scientists are struggling to understand. "It looks like something happened in the early 1980s to get this pattern going, but we are still trying to figure out what could have disturbed such a large part of the rings," Cornell researcher Matthew Hedman said. (NASA) Back to slideshow navigation
  3. Shadows and more shadows

    Several sets of shadows are cast onto Saturn's A ring in this image, taken by the Cassini spacecraft's camera about a week after the planet's August 2009 equinox. (NASA) Back to slideshow navigation
  4. Fresh look at old finds

    These two Cassini images, taken four years before Saturn's August 2009 equinox, have taken on new significance. Take a look at the short white streaks that are parallel to other features in the ring. Observations made during the equinox have led scientists to believe that the streaks are likely evidence of impacts into the planet's rings. Other streaks that are not parallel to the rest of the ring features are actually star trails that were captured during the camera exposures. (NASA) Back to slideshow navigation
  5. Giant propeller

    An unusually large propeller feature is detected just beyond the Encke Gap in this Cassini image of Saturn's outer A ring, taken a couple days after the planet's August 2009 equinox. The "propeller" is caused by the action of a moonlet that is embedded in the ring but is too small to be seen itself. (NASA) Back to slideshow navigation
  6. Spiral corrugations

    Alternating light and dark bands, extending a great distance across Saturn's C and D rings, are seen for the first time in a mosaic of Cassini imagery that was captured as Saturn approached the equinox in August. (NASA) Back to slideshow navigation
  7. Ridges on Enceladus

    In addition to its equinox observations, the Cassini orbiter has been taking a close look at Saturn's moons. This perspective view shows the Baghdad Sulcus region on Enceladus, an ice-covered moon that may have liquid water beneath its frozen surface. Baghdad Sulcus is one of the linear structures known as "tiger stripes," where jets of water are thought to well up from the troughs situated between ridges of ice. (NASA) Back to slideshow navigation
  8. The face of Janus

    This picture, captured by the Cassini spacecraft on July 26, highlights a prominent crater on Janus, a tiny moon of Saturn. Janus, named after the two-faced Roman god of gates and transitions, is only 122 miles wide in its longest dimension. (NASA) Back to slideshow navigation
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  1. The Rite of Spring
    Above: Slideshow (8) A rare spectacle on Saturn - Cassini Images
  2. Image: USA Space Shuttle  Discovery Lifts Off  from Kennedy Space Center Florida
    Gary I Rothstein / EPA
    Slideshow (23) A rare spectacle on Saturn - Month in Space
updated 11/2/2009 12:12:21 PM ET 2009-11-02T17:12:21

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.

Slideshow: Month in Space: January 2014 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.

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