A new temperature map of Saturn's moon Mimas has revealed a sharply defined region of warmer surface area shaped like the 1980s video game icon Pac-Man, according to a team of scientists that includes a Boulder researcher.

Scientists were surprised by the bizarre temperature pattern on Mimas -- one of the larger of Saturn's 60-plus moons -- which was discovered after NASA's Cassini spacecraft made its closest-ever flyby of the moon on Feb. 13.

The researchers had expected to find temperature variations across the moon's surface, but they thought they would be gradual, peaking at the Mimas equator in the early afternoon. Instead, the "composite infrared spectrometer" onboard Cassini captured abrupt transitions.

"We certainly didn't expect that. We kind of fell out of our chairs when we saw that," said John Spencer, a scientist at the Southwest Research Institute in Boulder and a member of the infrared spectrometer team. "In terms of measuring the heat radiation, we'd gotten a couple of views, but they were very distant."

Scientist don't know much about icy Mimas -- where "warm" means that the temperature is only 294 degrees below zero compared to minus 320 degrees -- except that its surface is marred by a gigantic crater, which has earned the moon comparisons to the Death Star in "Star Wars" movies.

"We'd gotten some pretty good pictures of it before, and we new that its surface was covered in craters," Spencer said. "And apart from having this one spectacular crater, it didn't seem that interesting -- we hadn't paid it much attention."

Scientists are paying attention now, and they have a host of new questions they hope to answer: How far does the cold region extend? What about the warm region? What does the surface temperature do at night?

And while Cassini, which began orbiting Saturn in 2004, will never again get as close to Mimas as it did in February, scientists hope to answer some of these questions by pointing their instruments at the moon while the spacecraft passes at a greater distance.

"Other moons usually grab the spotlight, but it turns out Mimas is more bizarre than we thought it was," Linda Spilker, a scientist at NASA's Jet Propulsion Laboratory in California, said in a news release. "It has certainly given us some new puzzles."
