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Cassini finds video arcade relic on Saturn moon

Posted: March 29, 2010

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The Cassini spacecraft touring Saturn found an odd arrangement of hot spots resembling the iconic Pac-Man arcade character on the heavily-cratered moon Mimas during a February flyby.

Bright orange represents warmer temperatures and deep blue and violet colors show cooler temperatures in this map of Mimas. Credit: NASA/JPL/GSFC/SWRI/SSI
Cassini's composite infrared spectrometer stared at Mimas as the probe flew past the moon Feb. 13, finding a distinct V-shaped pattern of relatively warm temperatures in one hemisphere of the rocky body.

Another warm spot near Herschel crater appears in the temperature map as a dot being consumed by the jaws of Pac-Man. Herschel is a massive impact crater that dominates the scarred surface of the 246-mile diameter moon.

Scientific interest in Mimas usually takes a back seat to Saturn's more studied moons, such as cloud-shrouded Titan and geyser-spewing Enceladus.

"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."

The warmer regions making up the V-shaped jaws of Pac-Man aren't exactly a paradise on Mimas. The average temperatures in the yellow-orange region of the image are about 92 Kelvin, or -294 degrees Fahrenheit. Bluer areas of the temperature map are a bit colder, around 77 Kelvin, or -320 degrees Fahrenheit.

Cassini's narrow-angle camera captured this false-color image of Mimas during the closest ever flyby of the moon. Cassini was about 10,000 miles from Mimas when this image was taken. The 80-mile-wide Herschel crater is visible in the lower part of the picture. Credit: NASA/JPL/Space Science Institute
Scientists are not sure of the cause of the variations or the sharp boundaries between temperatures.

"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."

One hypothesis is heat from the impact that carved Herschel crater could have melted surface ice that spread across a wide area of Mimas. When the water refroze, it may have a smoother, colder surface than the rest of the moon, according to a NASA press release.

Visible imagery from the Feb. 13 flyby also showed unexpected stark contrasts of light and dark material, including streaks of debris pulled down crater walls by gravity. Scientists have found similar features on other Saturn moons, but Mimas offers a unique opportunity to measure how fast surface changes occur.

That's because Mimas is bombarded by ice spray from one of Saturn's rings, continuously supplying bright new material on the moon's surface. The constant regeneration of Mimas's surface allows scientists to pin down time scales of change as darker material appears.

"These processes are not unique to Mimas, but the new high-definition images are like Rosetta stones for interpreting them," said Paul Helfenstein, a Cassini imaging team associate from Cornell University in Ithaca, N.Y.