October 7, 2010 | 15
PASADENA—This week I’m here at the annual Division for Planetary Sciences meeting. Much as I enjoy Pasadena, it’s rather a comedown from last year’s meeting place in Puerto Rico. Leave aside the natural attractions: even the freeways in Puerto Rico are in better repair than California’s. Then again, we don’t come here for the earthly sights, but for the celestial ones.
On Tuesday, we heard the latest chapter of a bizarre discovery about the rings of Saturn that I wrote about from Puerto Rico. Last year, Matt Hedman of Cornell University showed conferees pictures of a gentle ripple in Saturn’s rings that stretches for tens of thousands of kilometers. Such a vast and regular disruption means that a fairly dramatic occurrence must have befallen the rings. Something managed to torque the entire ring system or else the giant planet itself lurched; either way, the rings fell out of alignment with the equator of the planet. The giant planet’s gravity tugged on this imbalance and wound it up into a spiraling ripple.
Based on the current spacing of the pattern, Hedman and his colleagues estimated that the initial disruption occurred in the mid-1980s. At a loss to explain what could possibly have torqued an entire ring system, they suggested that Saturn itself shuddered for some unknown and almost unimaginable reason.
Now one of Hedman’s colleagues, Mark Showalter of the SETI Institute, has announced that it has happened again—this time to the rings of Jupiter. In fact, not just once, but twice. Using images taken by the Galileo space probe in October 1996 and June 2000, Showalter has found two overlapping sets of ripples in the main ring of Jupiter. One has a spacing of 600 kilometers and a height of 600 meters, the other a spacing of 2,000 kilometers and a height of 3 kilometers. The first appears to date to early 1990, the second to mid-1994. (The ripples are visible in the alternating pair of images above—they rise above and below the central plane.)
Those dates ring a very loud bell. The second is when Comet Shoemaker-Levy 9 crashed into Jupiter, and the first is when the comet made an earlier pass by the giant planet. The comet left giant scars in the planet’s atmosphere, but scarcely budged it. It did, though, dribble a trail of fine dust that the gentle pressure of sunlight could then have pushed into the rings. "Over a one-week period in 1994, all this crap hit the ring," Showalter says. The shower of dust would have perturbed the ring and sent the ripples into motion.
If no planetary lurch was needed in this case, then maybe Saturn’s ripple, too, came from a shower of incoming debris. In an accompanying talk, Joe Burns, also at Cornell, described how a comet a kilometer or so in size could break up and rain down on the rings. To create an imbalance, the debris had to have been unevenly distributed; that would have happened if the planet created a rain shadow, blocking debris from reaching one section of rings. Burns estimates that objects of the requisite size should strike both the Jovian and Saturnian rings once a decade on average.
"All of us came in somewhat skeptical," Burns says. But the Jupiter discovery and the dawning implausibility of explaining how an entire planet might abruptly lurch tipped the balance in favor of meteor showers. It’s amazing to think that the rings absorb these blows and yet remain the jewels of the solar system.