In Greek mythology, Hyperion was the father of Helios the sun, Selene the moon, and Eos the dawn. In planetary science, Hyperion is a moon of Saturn—one, it seems, that does not live up to its lofty name.
New images from the Cassini space probe show what astronomers are calling "a rubble-pile moon." The satellite has been pounded by so many meteors for such a long period of time that astronomers suspect it is one of the oldest surfaces in Saturn's system. But researchers aren't yet certain why it appears fluffy and porous, like a wasp's nest or a paper carnation.
"Hyperion looks very different from any of the satellites we've seen so far," says Cassini deputy project scientist Linda Spilker of NASA's Jet Propulsion Laboratory in Pasadena, California.
Hyperion is small—on average only 165 miles across. It seems to be made mostly of ice, with a dark, dusty coating. Bright white rims surround some craters within craters, suggesting that the dark material is not very deep. Its interior appears to be a series of voids, not unlike a loosely compacted snowball. Little moons, battered by meteors, tend to lose pieces to space. Hyperion may be unusual, says Peter Thomas, planetary geologist at Cornell University, in that it somehow retains those knocked-off bits on its surface.
The origin of the moon is also a mystery. Its peculiar nonspherical shape suggests that it could be a fragment left over from a giant collision or bits and pieces from a bigger moon, says Spilker. Its rotation is odd. It wobbles like a poorly thrown football in an orbit close to its much larger neighbor, Titan. The strange spin happens because it "periodically gets a gravitational kick from Titan," Spilker says.
By the end of the year, Cassini will have provided similar close-ups of six other Saturnian moons—Titan, Enceladus, Iapetus, Tethys, Dione, and Rhea. Says Spilker, "This is the season of the satellites."