Twenty years passed before the arrival of the next probe to Saturn, Cassini-Huygens, the result of a collaboration among seventeen nations and three space agencies. NASA’s Jet Propulsion Lab built the orbiter, the European Space Agency put together the Huygens probe, and the Italian Space Agency contributed Cassini’s main antenna.

Path of Cassini spacecraft during the engine burn (shown in red) that placed it in a closed orbit around Saturn; the burn was the closest approach of all orbits in the four-year mission, so Cassini took advantage of this opportunity to produce highly detailed images of the ring system. (image courtesy of NASA)

After a seven-year trip, including “slingshot” loops around Earth, Venus, and Jupiter, Cassini-Huygens reached Saturn on June 30, 2004. A 96-minute engine burn (see drawing) lowered its speed enough for it to settle into a closed orbit. During this burn, Cassini achieved its closest approach to the surface, 18,000 km (11,000 mi), or about one-sixth the planet’s diameter—much closer than Voyager’s 41,000 km (26,000 mi) —and Cassini made the most of its vantage point by capturing 61 detailed images of the ring system. Compare the detail of the Voyager and Cassini images showing wave-like ripples in the A-ring, which is the outermost wide ring.

In a different observation, a spectrometer on board Cassini analyzed visible and infrared light transmitted through the rings. One surprising result was the presence in the ring system of “dirt,” like that observed on Saturn’s moon Phoebe, which suggests that the rings are the remnants of a disintegrated moon. Supporting this theory is the fact that the total mass of the ring system is about the same as that of an average-sized Saturn moon, which raises the possibility that a former moon wandered in too close to the planet and disintegrated. The rings, made of boulder-sized chunks of ice, contain relatively little “dirt,” less than previously believed, with the “dirt” concentrated in the ring gaps. The image shows the region around the A ring in ultraviolet light, with ice in blue and dirt in red.

This pair of images, the one on the left from Voyager and the one on the right from Cassini, show the structure of Saturn’s A ring. Note the detail in the Cassini image. The moon Mimas, and its shadow, are visible in the Voyager image. (image on the left courtesy of Mark Showalter and the PDS Rings Node; image on the right courtesy of Cassini Imaging Team, SSI, JPL, ESA, and NASA)

The ultraviolet spectrometer on board Cassini produced perhaps the biggest surprise so far, a huge burst of oxygen in the E ring, with about four times the mass of the ring itself. Since presumably the oxygen comes from the decomposition on the ice in the ring—there’s not much other stuff around—the source of so much oxygen is quite a mystery. Scientists speculate that big hitherto-undiscovered ice chunks may have collided and released the oxygen, but E ring particles seen so far are very small.

More excitement is yet to come. In January, 2005, the Huygens probe will be parachuted onto Titan, whose atmosphere contains organic compounds. Huygens will take data all the way down and after landing, radioing its results to Cassini for transmission to Earth. Check your newspaper, and Physics Central too, for updates.

Images of the region around Saturn’s A ring. Blue indicates ice, and red indicates “dirt,” similar to the material on the moon Phoebe. The dirt is concentrated in the gaps—how this has happened is a mystery. (image courtesy of UVIS, University of Colorado, ESA, and NASA)