Turbulent clouds and a rock-steadied ring

Cassini's camera glimpses a familiar atmospheric process and snaps a shepherd moon tending its high-maintenance flock.
By | Published: October 26, 2004
Kelvin-Helmholtz clouds on Saturn
Familiar physics forms this wave-like pattern between two of Saturn’s cloud bands.
October 26, 2004
On October 9, the Cassini spacecraft captured a train of wavy clouds as it studied the ringed planet from a distance of 3.5 million miles (5.9 million kilometers). What makes the clouds curl is a process as familiar as wind-whipped waves on a quiet lake, or a flag rippling on a breezy day.
Kelvin-Helmholtz clouds on Earth
Kelvin-Helmholtz clouds resemble breaking ocean waves. On Earth, they often form in the lee of mountains or large hills. Benjamin Foster, courtesy of University Corporation for Atmospheric Research
Shear occurs wherever two fluids of different density flow past one another at different speeds, which is what’s happening where Saturn’s cloud bands meet. When the shear reaches some critical value, flow at the interface between the fluids becomes unstable, resulting in gentle waves in the denser fluid. These waves grow in size until they break and roll around a patch of the lighter fluid (picture ocean waves breaking near the shore). Turbulence spreads throughout the interface, mixing the two fluids and transferring momentum from one layer to the other. This, in turn, weakens the shear: turbulence dissipates, and the fluids flow smoothly again.

Scientists refer to this process as Kelvin-Helmholtz instability. Skywatchers on Earth may see the same rare, short-lived, but nonetheless distinctive, cloud pattern. The clouds permit us to trace the waves, but the swirling air may occur in air too dry for clouds to form. In fact, scientists suspect this process as one source of “clear-air turbulence” often encountered in air travel. A related instability creates wind-driven waves on water or fabric.

Prometheus herds the F ring
Prometheus ensures that particles from Saturn’s F ring don’t stray.
As a counterpoint to Saturn’s atmospheric disorder, the Cassini team offers an image of Prometheus, one of the plaent’s many small moons. Prometheus is one of the two shepherd moons that corral particles in Saturn’s thin F ring, visible here as the bright zone in the background. Prometheus, which is just 63 miles (102 km) across and serves as custodian of the F ring’s inner edge, looks distinctly potato-shaped. A smaller satellite, Pandora (52 miles, or 84 km), maintains the ring’s outer edge.

Cassini snapped Prometheus and the F ring as it entered into orbit around Saturn June 30. For this view, scientists combined nine images in a way that improves resolution and reduces noise, then magnified the result by a factor of five. Expect more detailed images of both shepherd moons later in Cassini’s mission.