October 8, 2009 | 5
FAJARDO, Puerto Rico—I first heard about Matt Hedman’s talk while going out to dinner on Tuesday night. Best talk of the meeting, I was told. Everywhere I went yesterday, I kept hearing about this guy Matt Hedman. A former professor of mine chided me for missing his presentation. The problem with the Division for Planetary Sciences meeting is that you have to make hard choices about which of many parallel sessions to go to, and clearly I’d made the wrong choice on Tuesday afternoon. Fortunately, I caught up with Hedman today and learned what is causing all the fuss.
Hedman, a Cornell University researcher, and his team have discovered a very curious feature in the rings of Saturn. Although it has gotten much less attention than Saturn’s newly discovered ring, it may well be more significant. In mid-August, Saturn passed through its equinox and sunlight shone directly along the rings, so that even the most subtle vertical structure cast long shadows. Images from the Cassini spacecraft revealed that the planet’s C ring has a gentle corrugation that extends across tens of thousands of kilometers. (Saturn has seven main rings, named A to G in order of discovery.) The ripples are barely 100 meters high and recur with a wavelength varying from 30 kilometers toward the planet to 80 kilometers farther out. The pattern appears to be an extension of a corrugation that scientists first noticed in Saturn’s D ring back in 2005. What’s more, the pattern seems to be evolving with time, its wavelength steadily diminishing. Extrapolating back in time, it must have been set in motion 25 years ago.
It looks like the ring—the whole ring—was abruptly yanked out of Saturn’s equatorial plane. Pulled out of the plane, ring particles began to bob up and down, creating the corrugation; the giant planet’s oblong shape tugged on the pattern, causing it to evolve. But what could possibly do that to a ring? A passing moon could scarcely cause such a vast disruption, Hedman says. Instead, he suggests that it wasn’t the ring that shifted, but Saturn’s equatorial plane. A tilt of a thousandth of a degree would suffice. "It’s a little shift, but it affected the structure of the rings in a pretty profound way," Hedman says.
In other words, the entire planet Saturn suddenly lurched in the mid-1980s. A change in the planet’s spin axis is out of the question. Saturn is, in effect, a gyroscope with a radius of 60,000 kilometers and the mass of 95 Earths. It isn’t going to tilt over with anything short of an apocalyptic event. What must have happened instead is that mass shifted within the planet, reorienting the effective gravitational equator. But how? "It’s not obvious how to shift Saturn’s internal gravitational distribution fast enough to do this," Hedman says. Saturn modelers have clearly got their work cut out for them.
Corrugations in Saturn’s C and D rings, as seen by the Cassini spacecraft, courtesy of NASA/JPL/Space Science Institute