Two-mile-high mountains discovered in Saturn's rings

At the edge of Saturn's B ring, the Cassini spacecraft has spotted a huge mountain range of debris stretching up over two miles above the ring plane. In this picture you can see the mountains' long shadows.

And below, you can see the motion of the B ring, in a series of photos taken every few minutes over a period of 9 hours. As the ring thickens, you're seeing the mountainous region from above, so you can't see the shadows but you can see how big this area really is.

The ring changes shape so much because waveforms caused by the gravity of local moons and the planet are oscillating back and forth through the rings, transforming their shapes over time. However, not all of these shapes can be explained by moons. Now scientists believe that the rings themselves are generating their own waves, in a process called self-exciting oscillation, which they believe is caused by "viscous overstability" in the material that makes up the rings.

Joseph Spitale, author of a paper published today about the B ring, says:

These oscillations exist for the same reason that guitar strings have natural modes of oscillation, which can be excited when plucked or otherwise disturbed. The ring, too, has its own natural oscillation frequencies, and that's what we're observing.

According to a release from the Cassini imaging team:

Self-excited waves on small, 100-meter (300-foot) 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, from tens of meters up to hundreds of kilometers wide.

"Normally viscosity, or resistance to flow, damps waves — the way sound waves traveling through the air would die out," said Peter Goldreich, a planetary ring theorist at 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."

Humans have never observed the way waves behave in such vast structures of material - we've got only the ocean to study, and it's responding to very different conditions (as well as being made of fluid inside an atmosphere, rather than chunks of rock and ice in vacuum). Scientists are using these observations to speculate about why spiral galaxies have the shapes that they do, though of course spiral galaxies are many magnitudes more vast and may be subject to forces we don't yet understand. However, their shapes do resemble those of Saturn's rings.

via Cassini Imaging Central Laboratory for Operations

download a PDF of Spitale, et. al.'s paper about the self-exciting oscillations in Saturn's B ring, via Arxiv