For a 4-billion-year-old planet, Saturn seems to have rather young rings. Planetary scientists have suspected that they came from a recent collision of a comet and a small, icy moon of Saturn--a cataclysmic event that would have spread debris around the planet to form a broad set of rings. Now, researchers have new evidence that such a collision actually happened.
Scientists recorded the new evidence in July 2004 when the Cassini spacecraft swept into orbit around Saturn by ducking under the rings (Science, 9 July 2004, p. 165). As Cassini flew by, its camera took the closest look ever at the rings in a search for, among other things, a distinctive pair of Morse-code-like dashes of light that dynamicists had predicted. If a huge collision did form the rings, there should be 100-meter-wide particles left from it--massive enough to perturb gravitationally nearby particles in their orbits but too small to be seen. Such particles would form dashes just ahead and behind them, giving away their existence.
At first, team members saw no such dashes. But one day last summer while Matthew Tiscareno of Cornell University was showing a visitor how there was nothing to be seen, he idly twiddled a dial on the monitor, and then "boom, we knew what we had," says Joseph Burns of Cornell. Tiscareno, Burns, and colleagues report in the 30 March issue of Nature that the two pairs of dashes they found represent two exceptionally large ring particles about 40 to 120 meters in diameter orbiting in Saturn's outermost A ring. Observation and theory fit, says planetary dynamicist Jack J. Lissauer of NASA's Ames Research Center in Mountain View, California. "That's why I believe" they've detected large particles, he says.
The two discoveries fill a gap between the centimeter-size to 10-meter-size ring particles inferred from Voyager spacecraft observations a quarter century ago and the two visible moons--Pan and Daphnis--embedded in the A ring. Such a continuum of sizes from small and abundant to large but rare is just what a collision would produce, Tiscareno and his colleagues note. Planetary dynamicist Heikki Salo of the University of Oulu, Finland, agrees that a violent, relatively recent birth of the rings fits the finds nicely, but there is an alternative, he notes. Rings that started with all small particles might have grown such large particles as many small ones glommed together to form a few big ones. Cassini will get another good look at the rings later this year.