CICLOPS: Cassini Imaging Central Laboratory for OPerationS

Saturn Equinox Provides Humanity Rare Close-up Glimpse Of Planet's Rings

Joe Mason (720)974-5859
CICLOPS/Space Science Institute, Boulder, Colo.

For immediate release: Sept. 21, 2009


In a rare and breathtaking display of nature, the setting of the sun on Saturn's rings, called equinox, has been captured in a series of striking Cassini images, uncovering a heretofore hidden dimension to this fabled disk of icy debris and highlighting new ring phenomena in the process. The new images, released today, have given excited Cassini scientists a truly unique and revealing glimpse into the physical processes underlying the rings' architecture and time-variability.

At the exact moment of equinox, which occurs twice every Saturn year (or once every 14.8 Earth years), the sun is directly overhead at the equator and illuminating the rings directly edge-on. But a few months before and after this moment, and especially during the four days surrounding exact equinox, the sun's rays are only skimming the ring plane, significantly darkening the rings, and disclosing out-of-plane structures by making these features anomalously bright and causing them to cast long shadows.

It is these novel lighting conditions surrounding Saturn's August 11, 2009 equinox that have made possible the bounty of recent discoveries in a slate of images released today by the Cassini imaging team.

The new images can be found at, and

Summing up the past several months of Cassini's exploration of Saturn during this unusual celestial event, imaging team leader Carolyn Porco in Boulder, Colo., said, "This has been a moving spectacle to behold, and one that has left us with far greater insight into the workings of Saturn's rings than any of us could have imagined. We always knew it would be good. Instead, it's been extraordinary."

In one unexpected equinox discovery, imaging scientists have uncovered evidence for present-day impacts onto the rings. Bright, and hence elevated, clouds of tiny particles, sheared out by orbital motion into streaks, up to 3,000 miles (5,000 kilometers) long, have been sighted in the A and C rings. These clouds -- very likely thrown up by impacts -- rising above the dark ring plane are more directly catching the sun's rays during equinox, and are hence well lit and easily visible by contrast.

By the brightness and dimensions of the streaks, scientists estimate the impactor sizes at roughly one meter, and the time since impact at one to two days. These equinox data now lend more confidence to the impact interpretation of earlier Cassini images, taken in 2005, showing similar streaks in the C ring. In the 2005 images, the impactors are likely much smaller than one meter, and yet have left a visible ejecta cloud. All together, these observations are heralded as the first visual confirmation of a long-held belief that bits of interplanetary debris continually rain down on Saturn's rings and contribute to their erosion and evolution.

Many of the images at equinox have yielded brand new insights into discoveries made earlier in the Cassini mission. Imaging scientists were especially startled to find that a gentle, spiraling undulation with a 19-mile (30-kilometer) wavelength, discovered in 2006 extending across 500 miles (800 kilometers) of the D ring, is now seen dramatically highlighted by the low-sun-angle illumination to extend fully across the C ring, right up to the inner B ring. In total, it covers a radial distance of about 11,000 miles (17,000 kilometers). This remarkable result calls into question the initial conjecture that an early-1980s impact of an object a few meters across into the D ring initiated the wave disturbance. The enormous extent of the corrugation now makes its existence more mysterious than ever, and imaging scientists are struggling to understand its origin. Dr. Matthew Hedman, an imaging team associate at Cornell University in Ithaca, N.Y., said, "It looks like something happened in the early 1980's to get this pattern going, but we are still trying to figure out what could have disturbed such a large part of the rings."

The equinox season has brought with it a fuller appreciation of the behavior of ring particles in regions perturbed by Saturnian moons. Early Cassini images had clearly shown that particle aggregates, or clumps, several miles or kilometers in extent, were formed in regions undergoing extreme, periodic compression of ring material due to the resonant gravitational forcing by Saturn's satellites external to the rings. Examples of these regions are the many strong density waves throughout the rings where very strong satellite gravitational resonances are known to be controlling influences. Now, narrow ring regions coincident with the excitation sites of these waves have been observed to be unusually bright as well, and hence elevated above the ring plane. Here the forced compression that is the hallmark of wave generation and propagation has resulted in vertical displacement.

Clumps have been observed in other perturbed ring locales, too, in the recent collection of images. The narrow ring in the Colombo Gap in the C ring, previously believed to be longitudinally smooth, now clearly sports clumps. And clumps within the kinky, discontinuous ringlets of the Encke Gap have in fact been spotted casting shadows long enough to reach across the 200-mile (325-kilometer) -wide gap and onto the A ring. Both regions are known to be affected by the gravitational forcing of moons: 29-kilometer-wide Pan orbits alongside the ringlets in the Encke gap, and Saturn's largest moon Titan controls the behavior of the Colombo Gap ringlet. Imaging scientists are seeing in all these observations the opportunity to understand, better than ever before, the three-dimensional response of planetary rings to external influences.

Yet another surprise has been the extreme heights to which the waves created in the edges of the Keeler Gap by the 8-kilometer-wide ring moon, Daphnis, can rise. Earlier Cassini images revealed the vertical extent of these waves to be about nine-tenths of mile, or 1.5 kilometers, high; one recent equinox image has caught them reaching 2.5 times higher. The most plausible explanation for this is the possibility that Daphnis can get closer to the inner gap edge, and can make an even greater disturbance there than previously believed.

The unusual equinox geometry has also thrown into relief small moonlets within the rings and the structures they create around them. Propeller-like features, a few kilometers long, centered on and created by the action of small embedded moonlets only about 330 feet (100 meters) across, were discovered early in the mission. These findings constituted the first recognition that bodies smaller than the 8-kilometer-wide ring moon, Daphnis, in the outer A ring and bigger than the largest ring particles (about 30 feet, or 10 meters, across) were present in Saturn's rings.

But the equinox images have revealed unusually large, 80-mile-long (130-kilometer-long) propeller features in the outer A ring. From the 220-mile (350-kilometer) length of the shadow cast by one of them, the heights of these features above the ring plane can reach approximately 660 feet (200 meters) indicating the moonlet responsible for this propeller is likely 1,300 feet (400 meters) across. A previously released early-equinox image had also revealed a moonlet in the outer B ring about 1,300 feet (400 meters) across. It has since become a growing realization resulting from Cassini's exploration of Saturn, that the objects forming Saturn's rings very likely span the full spectrum of sizes, from the smallest dust-sized ring particles to the ring-moons like Daphnis and Pan -- a significant advance in divining the origin of Saturn's rings.

Finally, a full-system mosaic of Saturn and its rings -- in natural color, taken a little over a day past exact equinox, showing one half of the rings only dimly lit by sunlight reflected off the planet, the other in near total darkness -- caps this series of unique images. Said Porco, "To know that in this one image we are witnessing, for the first time, up close, one of the solar system's greatest wonders is to feel truly blessed."

Cassini's Equinox Mission will continue through the end of September 2010.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team consists of scientists from the U.S., England, France, and Germany. The imaging operations center and team leader (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.


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