Washington: Images captured by NASA’s Cassini spacecraft have finally revealed the answer to the perplexing behaviour of one of the most dynamic regions in Saturn’s rings.
The answer is: the rings are behaving like a spiral galaxy. Images of the B ring’s outer edge have also revealed at least two perturbed regions above the ring plane, which 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.
“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, at the Space Science Institute, Boulder, Colo.
The team has found 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.
“The ring, too, has its own natural oscillation frequencies, and that’s what we’re observing,” he added.
The new observations confirm the first large-scale wave oscillations of this type in a broad disk of material anywhere in nature. The results also confirm a Voyager-era predication that this same process can explain all the puzzling chaotic waveforms found in Saturn’s densest rings, from tens of meters up to hundreds of kilometers wide.
“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,” said Peter Goldreich, a planetary ring theorist at the California Institute of Technology in Pasadena.
The two perturbed B ring regions found orbiting within Mimas’ resonance zone stretch along arcs up to 20,000 kilometers (12,000 miles) long. Spitale and Porco propose that these regions very likely contain small moons, hundreds of meters to possibly a kilometer or more in size, that dramatically compress and force upward the ring material passing around them in this very agitated environment at the B ring’s edge.