Ring menagerie. Corrugation (upper right) and spiraling (lower left) caused by the gravitational tugs of small moons hint at the properties of Saturn's rings.

Perfection at Saturn's Rings

Cassini's arrival at Saturn this week proved to be a triumph. The $3.3 billion spacecraft and its 5.7 tons of technology performed flawlessly, promising beleaguered NASA and its European partners years of positive public interest. And researchers can look forward to years of returns from a planetary system that in many ways evokes the early days of our solar system (Science, 28 May, p. 1230).

Cassini's images of Saturn's rings were taken from so close and with such a capable camera that even the camera's principal investigator--Carolyn Porco of the Space Science Institute in Boulder, Colorado--was flabbergasted, despite her 15 years of planning for the moment. "Oh-my-god, look at that!" she exclaimed on seeing phonograph-like "grooves" in Saturn's A ring, the outermost of the three main rings. They and other so-called spiral density waves had been discovered by the two Voyager spacecraft that flew by Saturn in 1980 and 1981, but Cassini captured them with a more massive camera benefiting from 20 years of electronic technology development and mounted on a far steadier spacecraft. "That's gorgeous, just gorgeous," said Porco.

The Cassini camera caught the whole menagerie of known ring structures: spiral density waves (which look like ripples on a pond, but are actually a single band where ice particles are more tightly packed), bending waves (corrugations in the ring), scalloped gap edges, ringlets, moonlet wakes, moonlet-shepherded rings, and more. There were also a few new species to add to the list. Close inspection of the various waves should provide insights into how the rings operate.

Other instruments on Cassini delivered new perspectives on the rings. Roger Clark of the U.S. Geological Survey in Denver, Colorado, reported that the Visible and Infrared Mapping Spectrometer found that--for reasons yet unknown--the 99% pure ice of ring particles is dirtied more in thinly populated gaps and in the F ring. And Donald Shemansky of the University of Southern California in Los Angeles said that early this year the Ultraviolet-Visible Spectrograph had detected a massive outburst of atomic oxygen off to one side of the main rings. Because the outburst began at the orbital distance of the faint, diffuse E ring, Shemansky speculates that a surge of charged particles from the saturnian magnetosphere somehow eroded the yet undetected moonlets thought to supply the E ring's micrometer-size particles.

As all this is just the early days of a 4-year mission, Porco was no doubt right when she noted that "I and my ring colleagues will be busy."

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