There’s a tiny “flying saucer” orbiting deep within Saturn’s rings, and a NASA probe has just gotten its most impressive look yet at the strange object.
When she first saw the new pictures of Pan, Cassini scientist Carolyn Porco thought they might be an artist’s representation.
“They are real! Science is better than fiction,” she later commented.
Named for the flute-playing Greek god of wild places, 21-mile-wide Pan is what’s called a shepherd moon. It lives within a gap in Saturn’s A ring, which is the farthest loop of icy particles from the planet. As it zips around Saturn, Pan continually clears debris from the gap by vacuuming up some ring particles and punting others away, like a little Roomba with a force field.
In fact, it’s this absence of ring junk that led scientists to predict Pan’s existence as early as the mid-1980s. But the small moon wasn’t officially discovered until 1990, when Mark Showalter and his colleagues took a good look at images returned by the Voyager 2 spacecraft and found the moon that is responsible for the gap’s existence.
Now, with the Cassini spacecraft zooming through the Saturnian system, scientists have gotten the chance to see Pan up close. Early images revealed its walnut shape, which Porco and her colleagues attributed to debris from the rings.
These more recent images show in detail that the moon is swaddled in what’s called an equatorial accretion disk, or a smooth, thin layer of ring particles that have been glued on to Pan’s waistline by the moon’s meager gravity.
"This is such a far cry from the nondescript 'dots' that I was tracking way back in 1990 in the Voyager images! It's very gratifying finally to see Pan's closeup,"says Showalter, now at the SETI Institute in California.
In a 2007 study published in Science, Porco suggested the thin disk formed long ago, before the moon had completely vacuumed out material from the gap.
"The shape, as others have also pointed out, is probably because it is always sweeping up fine dust from the rings," Showalter explains. "The rings are very thin compared to the size of Pan, so the dust accumulates around its equator."
Pan isn’t alone in its bizarre appearance: Another small moon, Atlas, bears a similar shape for similar reasons.