After a 7-year, 3.5-billion-mile journey, the Cassini spacecraft last week slipped through a gap between two of the icy rings circling Saturn and became the first spacecraft to orbit the distant planet. The probe, which will tour Saturn and many of its 31 known moons for at least 4 years, has already returned stunning images of the shimmering rings and recorded the sharpest images ever taken of smog-covered Titan, Saturn’s largest moon.
“The images are mind-boggling,” says Cassini imaging team leader Carolyn Porco of the Space Science Institute in Boulder, Colo.
In Cassini’s snapshots, striations within the rings resemble grooves in a vinyl phonograph record. Many of the images show wavelike features that form when the gravity of a passing moon perturbs icy particles in a ring.
Depending on the moon’s orientation relative to the ring’s plane, the interaction either forces icy ring particles to clump into bands or pulls them up and down into corrugations. Such features had already been spied in the early 1980s by the two Voyager spacecraft, which flew past Saturn, but the new Cassini images show the structures in far finer detail.
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Although most of the rings are composed of virtually pure water ice, a spectrometer on Cassini found that particles in the F ring, the sixth ring discovered, as well as those in the rings’ gaps, have a small dirt component with a composition resembling that of Saturn’s outlying moon, Phoebe, notes Roger Clark of the U.S. Geological Survey in Denver.
The similarity supports a theory in which the rings and Phoebe have a common origin. Scientists have suggested that the rings may be the shattered remains of one or more bodies that once resided in an icy reservoir at the outskirts of the solar system but were captured and torn apart by Saturn’s gravity about 100 million years ago. Phoebe, which Cassini imaged last month (SN: 6/19/04, p. 387: Available to subscribers at Portrait of Phoebe: Cassini images a large Saturn moon), may be an intact member of this same icy reservoir.
Flying within 339,000 kilometers of Titan on July 2, Cassini gave scientists their closest, albeit fuzzy, look at the surface of this enigmatic moon. Viewing Titan at infrared wavelengths, which can penetrate the moon’s thick haze, Cassini resolved some features as small as 10 km across.
The new observations show a field of methane clouds near Titan’s south pole that were first seen in ground-based images from 1999. The Cassini images reveal that the clouds change shape in a matter of hours. When a cloud grows smaller, it may be raining methane down on Titan’s surface, says Alfred McEwen of the University of Arizona in Tucson. That would dovetail with a theory that Titan contains lakes or ponds of hydrocarbons.
During its mission, Cassini will pass by Titan another 45 times, coming as close as 950 km. The nearest sweep is expected to resolve features one-hundredth as small as those seen in the July 2 flyby. In January, Cassini’s instrument-laden Huygens probe will plunge through Titan’s atmosphere.