Mar 30, 2015: Return to Rhea - Cassini captured these views of Saturn's icy moon Rhea on February 9, 2015. After nearly two years orbiting at high inclination, the spacecraft officially returned to Saturn's equator plane on March 16. The near-equatorial orbits in February once again allowed close encounters with moons other than Titan. (Image Advisory can be found here.)
November 1, 2010
More than a year has passed since the last entry in this log, a time spent in serious contemplation of the many remarkable discoveries we have made in the images we have collected on this historic journey.
And today marks the announcement and publication of one of the most remarkable. After a journey of nearly 100 circuits of Saturn covering 830 million kilometers, we have finally solved a set of puzzles that goes back to the days of Voyager.
It became obvious soon after the Voyagers passed through the Saturn system 30 years ago that the behavior of the sharp, outer edge of Saturn's massive B ring was more complex than could be explained by the 2:1 orbital gravitational resonance with Mimas known to be responsible for its existence.
At the same time, Voyager uncovered a chaotic, baffling brew of ring structures throughout the B ring and even across the dense inner regions of the A ring. Unfortunately, the opportunities to observe the rings during the two brief Voyager flybys were insufficient to offer many clues to the origins of these phenomena.
Now, Cassini's 6-year stay in the Saturn system, and repeated occasions to observe the rings from four prolonged excursions into highly inclined orbits that carried us repeatedly above and below the ring plane, have revealed the cause of all the complexity: The outer several hundred kilometers of the B ring, bounded by its very sharp, resonance-maintained edge, is home to a set of traveling waves that are similar to those believed to give shape to spiral galaxies. Waves that spontaneously develop by deriving energy from the small random motions of the particles slosh back and forth across this region, propagating outward until they reflect off the outer edge and then propagating inward until they reflect off an inner boundary set by the wave shape and speed, and so on. This continuous back-and-forth reflection is necessary for these waves to amplify, grow and become visible as distortions in the outer edge of the B ring.
In order for this process to work, the ring particles must interact with these waves in a way that is counter to what we normally see in the propagation of waves through molecules of air or water. Instead of small random motions creating a resistance that damps the wave, in the rings they actually amplify the wave. This process has already been verified to produce wave features in Saturn's dense rings that are of small scale...about 150 meters or so. That it now also appears to produce waves of large, hundreds-of-kilometers scale in the outer B ring suggests that it can operate in dense rings on all spatial scales. How satisfying it is to find at last one explanation for most, if not all, of the chaotic looking structure we first saw in Saturn's dense ring regions long ago with Voyager and have since seen in exquisite detail with Cassini.
And as if that weren't enough, we've made another outstanding find in this part of Saturn's rings. At least two orbiting regions at the ring's edge, reported in the work published today, appear notably disturbed; one of these is inhabited by dramatic mountainous waves of ring debris reaching as tall as 3.5 km above the ring plane. These regions, we believe, are the sites of small moonlets, perhaps a kilometer or more in size, whose presence compresses and forces upward the ring material streaming by them in a process ring scientists refer to as `splashing'. If correct, this suggests that, at one time, the outer part of the B ring may have been populated by a collection of small bodies, in the same way that Saturn's outer A ring today is home to dozens of moonlets that create the famous 'propeller' features recently discovered by Cassini. This suggestion is supported by an earlier Cassini discovery of a lone moonlet, about 300 meters (1000 feet) wide, casting a shadow in the outer portion of the B ring. These bodies may have migrated across the region sometime in the past to become trapped in the strong Mimas resonance at the B ring's edge in a process mimicking the migration of the planets across the solar nebula in the early dawn of our solar system.
All in all, we have here a fascinating story of physical mechanisms at work in Saturn's rings that are at work today, and have been in the past, in other disks systems throughout the cosmos. In other words, we have uncovered one single physical mechanism that has the power to explain simultaneously a host of seemingly unrelated phenomena ... just the kind of discovery we scientists love to make.
Of course, the unveiling of this story isn't all that occurred in the last year.
In late 2009, we sighted Saturn's northern aurorae, the tallest yet seen in the solar system, shape-shifting and shimmering around the planet like a dancing curtain of light. The gradual recession of the dark northern polar night brought into clear view in visible light, at last, Saturn's famous, Voyager-discovered northern polar hexagon, a structure formed in a gaseous atmosphere and therefore surprising in its angularity. A close flyby last November of the perpetually fascinating moon, Enceladus, brought the closest look yet at Enceladus' jets, with the count now up to over 30 individual jets -- small and large -- erupting from the fractures that cross the south polar terrain.
Finally, the darkening of the rings that attended the arrival of northern vernal equinox last August also significantly reduced ring shine on the planet, permitting detection of a storm of faint flashing bolts of lightning in Saturn's atmosphere and their coincidence in time with the emission of powerful electrostatic discharges intercepted by the Cassini Radio and Plasma Wave experiment. Information derived from the study of these flashes -- their brightness, size, frequency -- helps us imaging scientists estimate the amount of energy entailed in their production and to locate the altitude of the water clouds in the atmosphere that are responsible for them.
Now, we on Cassini are fortunate to embark on yet another episode in this magnificent expedition around Saturn: an extension of our original mission that will see us exploring this distant planetary system and its marvels through 2017. And how sweet it is!
Carolyn Porco Cassini Imaging Team Leader Director, CICLOPS Boulder, CO