Saturn Moon's Heat Glow Looks Just Like Pac-Man

As if dominating one 1980s pop icon wasn?t enough, Saturn?s moon Mimas ? which bears a striking resemblance to the Death Star in "Star Wars" ? has a heat glow that looks like Pac-Man chomping down on some video game chow.

New high-resolution images and a temperature map of Mimas taken by NASA's Cassini spacecraft revealed the surprising patterns on the surface of the small moon. The odd Pac-Man-like feature, along with striking bands of light and dark in crater walls, were seen by during its closest flyby of the moon on Feb. 13.

The new photos also take a look at the moon?s enormous scar, called Herschel Crater, and has often been said to give Mimas a Death Star look.

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

Scientists working with Cassini's composite infrared spectrometer, which mapped Mimas's temperatures, expected to see smoothly varying temperatures peaking in the early afternoon near the equator. Instead, the warmest region was in the morning, along one edge of the moon?s disk, making a sharply defined Pac-Man shape, with temperatures around minus 294 degrees Fahrenheit (minus 181 Celsius).

The rest of the moon was much colder, around minus 320 degrees Fahrenheit (minus 196 Celsius). A smaller warm spot ? the dot in Pac-Man's mouth ? showed up around Herschel, with a temperature around minus 310 degrees Fahrenheit (minus 190 Celsius).

The warm spot around Herschel makes sense because tall crater walls (about 3 miles, or 5 kilometers, high) can trap heat inside the crater. But scientists were completely baffled by the sharp, V-shaped, Pac-Man-like pattern.

"We suspect the temperatures are revealing differences in texture on the surface," said John Spencer, a Cassini composite infrared spectrometer team member based at Southwest Research Institute in Boulder, Colo. "It's maybe something like the difference between old, dense snow and freshly fallen powder."

Denser ice quickly conducts the heat of the Sun away from the surface, keeping it cold during the day. Powdery ice is more insulating and traps the Sun's heat at the surface, so the surface warms up.

Even if surface texture variations are to blame, scientists are still trying to figure out why there are such sharp boundaries between the regions, Spencer said.

It is possible that the impact that created Herschel Crater melted surface ice and spread water across the moon. That liquid may have flash-frozen into a hard surface. But it is hard to understand why this dense top layer would remain intact when meteorites and other space debris should have pulverized it by now, Spencer said.

Icy spray from the E ring, one of Saturn's outer rings, should also keep Mimas relatively light-colored, but the new visible-light images from the flyby paint a picture of surprising contrasts. Cassini imaging team scientists didn?t expect to see dark streaks trailing down the bright crater walls or a continuous, narrow pile of concentrated dark debris tracing the foot of each wall.

The pattern may appear because of the way the surface of Mimas ages, said Paul Helfenstein, a Cassini imaging team associate based at Cornell University in Ithaca, N.Y. Over time, the moon?s surface appears to accumulate a thin veil of silicate minerals or carbon-rich particles, possibly because of meteor dust falling onto the moon, or impurities already embedded in surface ice.

As the Sun's warming rays and the vacuum of space evaporate the brighter ice, the darker material is concentrated and left behind. Gravity pulls the dark material down the crater walls, exposing fresh ice underneath.

Although similar effects are seen on other moons of Saturn, the visibility of these contrasts on a moon continually re-paved with small particles from the E ring helps scientists estimate rates of change on other satellites.

"These processes are not unique to Mimas, but the new high-definition images are like Rosetta stones for interpreting them," Helfenstein said.

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