Saturn’s magnificent ring system – a huge disc resembling an old gramophone record – turns out to share another property with the LP: it constantly emits a melodic series of musical notes.
The surprising discovery was made by radio and plasma wave detectors on board the Cassini spacecraft as it passed over Saturn’s rings during its arrival at the planet in July.
The tones are emitted as radio waves. Don Gurnett of the University of Iowa says his team reduced their frequencies by a factor of five to bring them into the range of human hearing. Gurnett says he was “completely astonished” when he heard the musical notes.
The tones are short, typically lasting between one and three seconds, and unlike the ethereal sliding tones associated with other cosmic processes, every one is quite distinct. The evidence suggests that each tone is produced by the impact of a meteoroid on the icy chunks that make up the rings.
Each hit, Gurnett says, creates a pulse of energy that is focused along the surface of a cone from the point of impact. By estimating the energy involved, he calculates that the impacting objects are about 1 centimetre across – although he cautions that his estimate could be out by as much as a factor of 10.
The findings were reported on Monday at the annual meeting of the American Astronomical Society’s Division of Planetary Sciences.
Planetary scientists have assumed that meteoroids constantly collide with Saturn’s rings, says Cassini imaging team leader Carolyn Porco, and that process has been suggested as a possible cause of the shifting, spoke-like formations seen in the rings by Voyager 2. But nobody thought it would be possible to detect the impacts so directly.
Cassini’s close-up observations have produced a wealth of new information about Saturn’s ring system, including complex details in the shapes and spacing of bands that have already revealed signs of three new moons – in addition to the three other moons Cassini had already discovered further out.
The craft’s discovery of one of the new moons, and a thin ring near the so-called F-ring, were reported by the International Astronomical Union on Monday.
An ultraviolet spectrometer is also yielding information about variations in the rings’ composition, reported Larry Esposito of the University of Colorado. The outer rings are mostly water ice, while the inner A and B rings are far richer in other elements, suggesting a kind of dirty ice.
Finer variations are seen over distances of just a few kilometres. These are so sharp and clear that they suggest the rings are even more dynamic and unstable than had been thought. If the rings had been in their present configuration for more than about 10 million years, their composition would have been thoroughly mixed and would now be uniform, according to dynamical models.
The persistence of the distinct zones suggests that they are variations produced by the break-up of one or more moonlets in the ring region within the past 10 to 100 million
years, Esposito says. This rate of change in the rings is about 10 times as rapid as that inferred from their physical features.
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