Scientists believe they finally understand the perplexing behavior of one of the most dynamic regions in Saturn's rings thanks to images captured by NASA's Cassini spacecraft. And the answer is: The rings are behaving like a spiral galaxy.
This new insight, garnered from images of Saturn's most massive ring, the B ring, may solve another long-standing puzzle: What causes the bewildering variety of structures seen throughout the very densest regions of Saturn's rings?
Images of the B ring's outer edge have also revealed at least two perturbed regions, including a long arc of narrow shadow-casting peaks extending as high as 2 miles (3.5 kilometers) above the ring plane. The researchers suggest that these regions are likely populated by small moons that may have migrated across the outer part of the B ring sometime in the past to become trapped near the edge in a zone affected by the gravity of the moon Mimas. This process is commonly believed to have configured the present-day solar system.
"We have found what we hoped we'd find when we set out on this journey with Cassini nearly 13 years ago: visibility into the mechanisms that have sculpted not only Saturn's rings, but celestial disks of a far grander scale, from solar systems, like our own, all the way to the giant spiral galaxies," said Carolyn Porco from the Space Science Institute, Boulder, Colorado.
Since NASA's Voyager spacecraft flew by Saturn in 1980 and 1981, scientists have known that the outer edge of the planet's B ring was sculpted into a rotating, flattened-football shape by the strongest gravitational resonance in Saturn's rings due to the moon Mimas. Resonances in Saturn's rings occur where the relative orbital positions between ring particles and a moon continually repeat, altering the particles’ orbits. In the case of the Mimas resonance, the particle orbits are changed from circles to ellipses that form a two-lobed pattern rotating with Mimas. But it was clear, even in Voyager's findings, that the outer B ring's behavior was far more complex than anything Mimas alone might do.
Now, analysis of thousands of Cassini images of the B ring edge taken over the course of 4 years has revealed the source of most of the complexity: the presence of at least three additional, independently rotating wave patterns, or oscillations. These oscillations with one, two, and three lobes are not forced by any moons, but instead have spontaneously arisen in part because the ring is dense enough, and the edge of the B ring is sharp enough, for unforced "free" waves to grow on their own and then reflect at the edge.
"These oscillations exist for the same reason that guitar strings have natural modes of oscillation, which can be excited when plucked or otherwise disturbed," said Joseph Spitale, an imaging team associate. "The ring, too, has its own natural oscillation frequencies, and that's what we're observing."
Astronomers believe that such self-excited oscillations exist in other astrophysical disk systems, like spiral disk galaxies and protoplanetary disks found around nearby stars. However, motions within these remote systems cannot be directly observed, and astronomers have instead resorted to computer simulations to study these natural oscillations without confirmation in nature.
Now, that has changed. The new observations confirm the first large-scale wave oscillations of this type in a broad disk of material anywhere in nature.
Self-excited waves on small, 300-foot (100 meters) scales have been previously observed by Cassini instruments in a few dense ring regions and have been attributed to a process called "viscous overstability." In that process, the ring particles' small random motions feed energy into a wave and cause it to grow. The new results confirm a Voyager-era predication that this same process can explain all the puzzling chaotic waveforms found in Saturn's densest rings.
"Normally, viscosity, or resistance to flow, damps waves — the way sound waves traveling through the air would die out," said Peter Goldreich from the California Institute of Technology in Pasadena. "But the new findings show that in the densest parts of Saturn's rings, viscosity actually amplifies waves, explaining mysterious grooves first seen in images taken by the Voyager spacecraft."
The two perturbed B ring regions found orbiting within Mimas' resonance zone stretch along arcs up to 12,000 miles (20,000 km) long. The longest one was first seen last year during Saturn's northern vernal equinox season when the Sun's low angle on the ring plane betrayed the existence of a series of tall structures through their long spiky shadows. Spitale and Porco propose that these regions likely contain small moons that dramatically compress and force upward the ring material passing around them in this very agitated environment at the B ring's edge.
The researchers also theorize that the outer part of the B ring may at one time 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 — a suggestion supported by an earlier Cassini discovery of a lone moonlet about 1,000 feet (300 meters) wide casting a shadow in the outer portion of the B ring. The researchers suggest that such bodies may have migrated across the region in the past to become trapped in the strong Mimas resonance at the B ring's edge, where they remain to this day.