"Rings around planets are supposed to stay close to home, as any Astro 101 textbook will tell you. Once they venture too far afield from their gravitational overlord, conventional astronomical wisdom dictates that they will collapse and form new satellites.
"That does a really good job of explaining rings—except for this one," says Douglas Hamilton, an astrophysicist at the University of Maryland. Hamilton and colleagues describe Saturn's biggest, strangest, most recently-discovered ring in a study published today in Nature. The so-called Phoebe ring is not only bigger than the researchers thought, it appears to be made of unusually fine particles—particles that continually collide with Saturn's moon Iapetus as it circles the planet, turning the moon's leading face black.
Hamilton was part of the team that discovered the Phoebe ring in 2009, with help from the Spitzer Space Telescope. They wouldn't have found it except for that weird pattern on Iapetus' surface: one face an icy white, the other inky black. Like Earth's own moon, Iapetus keeps one constant face toward its planet. So one side—the darkened one—is forever leading the charge through space. Phoebe, a dark-as-coal moon further away from Saturn than Iapetus, was the second piece of the puzzle.
"You can imagine something big smashed into Phoebe a billion years ago, and all of this debris was flung out," Hamilton says. When he calculated how long it would take for this debris to be cleaned up (either by collapsing into a new satellite or to fall to Saturn) he found that it would take a shockingly long time: about 10 billion years. That's longer than the solar system has existed. "We realized that all of the debris coating the face of Iapetus, that didn't happen long ago. It's going on now," he says. So his team started searching for the source of the material painting the moon black. "Finding it was really gratifying."
That was six years ago. Back then, when his team first wrote about their findings, the only available data on this vanishingly faint ring came in the form of a small cross-section. For Hamilton the picture felt "not much bigger than a postage stamp." Now, using data from NASA's Wide-field Infrared Survey Explorer spacecraft launched in 2009, they've gathered a complete view of Phoebe's enormous ring. "It was immense beforehand, and we just made it even bigger,"…"
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Amicalement
Armand
replaces his mysteriously missing frigates.
