Cassini Shows Saturn Rings In New Detail

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Temperature changes mapped with Cassini's composite and infrared spectrometer throughout Saturn's main rings show the ring temperatures decreasing with the increase of the Sun-spacecraft-ring angle (called phase angle) on both the lit and unlit sides of the rings. (See larger image)These temperature changes indicate that the ring particles spin slowly compared to their orbital periods of 6 to 14 hours. They may spin several times per orbit to less than one time per orbit.

Four scans are shown for the lit and unlit rings, at relatively low (less than 60 degrees) and high (more than 130 degrees) phase angles. Warmer temperatures about minus 262 degrees Fahrenheit (110 Kelvin) are shown in red and cooler temperatures about minus 343 degrees (65 K) are shown in blue. Other colors indicate temperatures between minus 343 degrees and minus 262 degrees (65 K and 110 K). The top two scans are for the lit rings and the bottom two scans are for the unlit rings. The change in ring temperature between each scan can be seen clearly.

The thermal characteristics of each main ring vary noticeably with phase angle. Radial scans of the A, B and C rings show a decrease in temperature with increasing phase angle for both the lit and unlit sides of the rings.

The C ring and Cassini Division exhibit the largest change in temperature. The temperature of the lit C ring decreases by about 22 degrees (12 Kelvin) between low and high phase angles. A similar contrast is present for the unlit side of the C ring. The C ring and Cassini Division are darker than the A and B rings so they can absorb more heat from the Sun. The lit B ring shows a temperature contrast of approximately 18 degrees (10 K) while the unlit B ring shows very little thermal contrast. Very little sunlight may make it through the thick B ring to its unlit side. The lit A ring is particularly interesting because the magnitude of the thermal contrast decreases with increasing radial distance from Saturn. The outer A ring shows only a small temperature change with phase angle, possibly because it contains smaller, or more rapidly rotating ring particles, which would have more uniform temperatures with phase angle.

Moffett Field CA (SPX) Sep 06, 2005
Cassini scientists studying Saturn's rings have made several new findings that further our knowledge of how this beautiful and dynamic system continues to evolve before our eyes.

"Understanding the dynamics of Saturn's rings provides on a miniature scale a better understanding of how our solar system formed from a disk of particles surrounding the Sun," said Dr. Jeff Cuzzi, Cassini interdisciplinary scientist, NASA Ames Research Center, Moffett Field, Calif.

Cassini's composite infrared spectrometer has found that particles within Saturn's main rings (the A, B, and C rings) are spinning slower than anticipated. This occurs even in areas where particles are so densely packed that they frequently bump into each other and where researchers expected them to spin quicker. From the planet outward, the order of Saturn's rings is D, C, B, A, F, G and E. The rings were assigned a letter in the order they were discovered

Scientists determined the spin rate by studying the temperature profiles of the particles. They hypothesized that collisions in the dense A and B rings would have resulted in faster-spinning ring particles that would have a more uniform temperature. Instead, results show that the A and B ring particles spin slowly, just like particles in the sparser C ring.

"It would be wonderful if we could scoop up a ring particle and bring it back to Earth to study it but we can't do that, so using an instrument like this one can help reveal what a ring particle might actually look like," said Dr. Linda Spilker, deputy project scientist for the Cassini-Huygens mission at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "A ring particle probably looks more like a fluffy snowball than like a hard ice cube," she said.

Saturn's outermost main ring, the A ring, is also causing a stir. Cassini's ultraviolet imaging spectrograph shows that particles in that ring are trapped in ever-changing clusters of debris. These clusters are torn apart and reassembled by gravitational forces from the planet, indicating that the A ring is primarily empty space. The particle clusters range from the size of sedans to moving vans.

"The spacing between the clumps is greater than the widths of the clumps themselves," said Dr. Joshua Colwell, team member of the ultraviolet imaging spectrograph, University of Colorado, Boulder. "If we could get close enough to the rings, these clumps would appear as short, flattened strands of spiral arms with very few particles between them."

Colwell likened the process to a handful of marbles placed in orbit around a beach ball. The marbles closest to the ball would orbit more quickly and drift from the pack before reorganizing themselves into new, orbiting clumps.

Imaging scientists have also made some surprising discoveries. Part of the D ring (the ring closest to Saturn) has gotten dimmer and moved inward, toward Saturn, by about 200 kilometers (125 miles), since it was observed by NASA's Voyager spacecraft, some 25 years ago.

Dr. Matt Hedman, an imaging team associate at Cornell University, Ithaca, N.Y. said, "Cassini's high resolution images of the D ring are providing new information about the dynamics and lifetimes of ring particles in a new regime, very close to the planet."

Among their biggest surprise is that a spiral ring encircles the planet like a spring. This unexpected "spiral arm" feature exists in the vicinity of the F ring and actually crosses the F ring's orbit.

"It is very possible that the spiral is a consequence of moons crossing the F ring and spreading particles around," said Dr. Sebastien Charnoz, imaging team associate at the University of Paris. "The F ring might be a very unstable or even a short-lived structure."

These ring results were acquired over the summer as Cassini was in a favorable ring-viewing period after the spacecraft's orbit was raised to look down on the rings. These and other results were presented in a press briefing at the 37th Annual Meeting of the Division for Planetary Sciences meeting held this week in Cambridge, England.

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Mysterious Enceladus

Moffett Field CA (SPX) Sep 05, 2005
Evidence is mounting that the atmosphere of Enceladus, first detected by the Cassini Magnetometer instrument, is the result of venting from ground fractures close to the moon's south pole.

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