1980s Video Icon Glows on Saturn Moon

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Jia-Rui C. Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jia-rui.c.cook@jpl.nasa.gov

NEWS RELEASE: 2010-103 March 29, 2010

1980S VIDEO ICON GLOWS ON SATURN MOON

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2010-103&cid=release_2010-103

PASADENA, Calif. -- The highest-resolution-yet temperature map and images of
Saturn's icy moon Mimas obtained by NASA's Cassini spacecraft reveal surprising
patterns on the surface of the small moon, including unexpected hot regions that resemble
"Pac-Man" eating a dot, and striking bands of light and dark in crater walls.

"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."

Cassini collected the data on Feb. 13, during its closest flyby of the moon, which is
marked by an enormous scar called Herschel Crater and resembles the Death Star from
"Star Wars."

Scientists working with the composite infrared spectrometer, which mapped Mimas'
temperatures, expected 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 92
Kelvin (minus 294 degrees Fahrenheit). The rest of the moon was much colder, around 77
Kelvin (minus 320 degrees Fahrenheit). A smaller warm spot – the dot in Pac-Man's
mouth – showed up around Herschel, with a temperature around 84 Kelvin (minus 310
degrees Fahrenheit).

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

The Cassini-Huygens mission is a cooperative project of NASA, the European Space
Agency and the Italian Space Agency. JPL, a division of the California Institute of
Technology in Pasadena, manages the mission for NASA's Science Mission Directorate,
Washington, D.C. The Cassini orbiter and its two onboard cameras were designed,
developed and assembled at JPL. The imaging operations center is based at the Space
Science Institute in Boulder, Colo. The composite infrared spectrometer team is based at
NASA's Goddard Space Flight Center, Greenbelt, Md., where the instrument was built.

More information and images are available at http://www.nasa.gov/cassini and
http://saturn.jpl.nasa.gov.

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