Cassini finds rhythm in Saturn's rings

Two of Saturn's rings have been found by NASA's Cassini spacecraft to contain orderly lines of densely grouped, boulder-size icy particles that extend outward across the rings like ripples from a rock dropped in a calm pond.
"Imagine going to a town that stretches from San Francisco to Los Angeles and seeing buildings spaced the same distance apart on every block," said Cassini radio science team member Essam Marouf of San Jose State University, San Jose, Calif. "All of these groups of particles within the rings are very close together, and the space between them is extremely small, only 100 to 250 meters (320 feet to 820 feet) wide, depending on where they are in the ring."

Normally, the distances between particles change with their velocity. In the case of Saturn's rings, the distances between these ring particles stay relatively equal even though their velocities may change. This type of pattern is completely new, according to Marouf.

"This particular feature is the smallest and most detailed of anything seen in Saturn's rings so far," Marouf said. "In the chaotic environment of the rings, to find such regularity in the most cramped areas is nothing short of amazing." The regular structure can only be found in locations where particles are densely packed together, such as the B ring and the innermost part of the A ring.

The unexpected pattern within Saturn's rings may give scientists some new ideas of what to expect from other similar planets and solar systems.

The pattern was detected when the radio on board the Cassini spacecraft sent out three signals toward Earth. The signals crossed the Saturn's rings, and their frequencies were separated by scattering from the ring particles. Once the signals were captured by Earth-based antennas of NASA's Deep Space Network, Cassini scientists saw a regular pattern in the received signal frequencies.

Credit: NASA/JPL/Space Science Institute

Radio signals sent by NASA's Cassini spacecraft to Earth through Saturn's rings revealed the presence of highly unusual regular formations of densely grouped ring particles. The harmonic ring structure caused the radio signal frequency to separate into three distinct components. The observed frequencies determine the regular spacing to be as small as 100 meters (320 feet), the finest-scale ring structure observed so far.

The regularly spaced yellow grid depicts the harmonic structure in Saturn's inner Ring A, and the image on the bottom right shows an actual observed frequency pattern (spectrogram). Color represents the observed signal strength. The structure acts like an enormously extended natural diffraction grating that separates the signal frequency into the three distinct components shown. The frequencies determine the regular spacing of the diffraction grating, 160 meters (500 feet) in this case. The image of Saturn was taken with Cassini's cameras and is shown here to illustrate the occultation.




Credit: NASA/JPL

For an Earth observer on May 3, 2005, the Cassini spacecraft appeared to pass behind the rings, then Saturn, then the rings again (the red line). The discovered harmonic structure was found on both the way in and out, but only in locations where particles are densely packed together, such as the B ring and the innermost part of the A ring.

The May 3 radio experiment and several others to follow in 2005 showed that the regular spacing of the harmonic structure vary from 100 to 250 meters (320 to 820 feet), depending on the location in the rings.

"The signals showed that the particle groups were arranged in an unexpectedly regular formation that had 'rhythm within the rings of Saturn'", said Marouf. "Each particle is in its own orbit, and sometimes they collide and move apart as their velocities change. As a result, you have particles bunched together into dense groups that extend across the ring in harmony with each other."

The pattern of particles is described as an enormously extended natural diffraction grating. A diffraction grating has parallel lines like a picket fence; when light hits this fence, it separates according to wavelength, from ultraviolet to infrared light.

The same thing happened when Cassini's radio signals hit the fencelike pattern of ring particles. The signals, sent out in 2005, were meant to capture a complete view of the rings.

The same thing happened when Cassini's radio signals hit the fencelike pattern of ring particles. The signals, sent out in 2005, were meant to capture a complete view of the rings.

This research appears as a cover story in the Dec. 28 issue of Geophysical Research Letters.

Source: European Space Agency
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