Pan, moon of Saturn, looks like a cosmic ravioli (or maybe a walnut)

In a stunning set of close-ups, Pan, a diminutive moon of Saturn, looks like a floating ravioli lost in space, or a wrinkled flying saucer.

NASA’s Cassini spacecraft took the photographs Tuesday, passing within 15,268 miles of the moon, which has a diameter of about 20 miles, roughly the size of New York City.

Carolyn C. Porco, a visiting scholar at the University of California, Berkeley, who leads Cassini’s imaging team, was asleep when Cassini’s shots were transmitted to Earth. She first saw an image on Twitter.

“I saw this picture, and I thought, that’s an artist’s conception,” she said. “Then I realized it was real.”

These are the clearest images ever seen of Pan, named after the flute-playing Greek god of hunters and shepherds. They depict a ridge around the moon’s equator that rises perhaps a couple of miles.

“The detailing in the last images is just astounding,” Porco said. “You can see where this ridge actually curves like a ribbon and where it’s not completely smooth.”

Pan is one of Saturn’s shepherd moons, clearing out a 200-mile-wide space in Saturn’s rings known as the Encke Gap.

“To see it with that detail, to be able to count craters on it,” said Mark R. Showalter, an astronomer at the SETI Institute in Mountain View, California, who discovered Pan in 1990, “I could remember when it was not even a pixel.”

In the mid-1980s, astronomers noticed that the ring edges along the gap had a scalloped appearance, almost like waves of water, possibly the wake of a small moon orbiting within the gap.

By refining the calculations, Showalter, then at the NASA Ames Research Center, figured out an orbit for the unseen moon. He went back to images taken by the Voyager 2 spacecraft during its flyby of Saturn in 1981.

“In the first image I looked at, there was Pan,” Showalter said.

The ridge is most likely a pile of ring dust that fell on the moon as it cleared out the Encke Gap.

“It looks like some of it is very steep,” said Peter Thomas, a visiting scientist at the Cornell Center for Astrophysics and Planetary Science who helped plan the Pan encounter.

Cassini, in orbit around Saturn since 2004, has only a few months left. With its maneuvering fuel almost exhausted, the spacecraft will be sent on a swan dive into Saturn in September. But before that, it will make a few more orbits from a different angle, enabling it to study Saturn and the moons in new ways.

For the Pan flyby, the spacecraft was pointed in the right direction, capturing a sequence of images.

“If you see it edge on,” Porco said, “it really looks like a walnut, except the equatorial ridge is more extended.”

In January, during a flyby of Daphnis, another of the shepherd moons, Cassini was able to successfully take only one photograph.

In April, it will aim at Atlas, yet another shepherd moon, which, in the blurrier images seen to date, also resembles a flying saucer.

“It’s built in the same way,” Porco said.