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Best pics of jovian moon show odd spires

The knobby hills on Callisto are unlike anything seen on Jupiter's moons.
The knobby hills on Callisto are unlike anything seen on Jupiter's moons.  


By Richard Stenger
CNN

(CNN) -- While never seen before, eerie expanses of icy knobs come into clear focus in the highest-resolution pictures ever of a moon around Jupiter, taken by the Galileo probe when it flew within 86 miles (138 km) of the surface of Callisto.

Callisto, the outermost of four large jovian moons, boasts the heaviest concentration of crater impacts in the solar system, evidence that it is geologically inactive.

But the new images reveal bright ice and dark patches around the jagged hills, suggesting that erosion shapes the surface, according to planetary scientists.

"We haven't seen terrain like this before. It looks like erosion is still going on, which is pretty surprising," said James Klemaszewski, a Callisto imaging scientist at Arizona State University.

Icy knobs on Callisto will eventually erode, producing smoother terrain similar to this region of the moon.
Icy knobs on Callisto will eventually erode, producing smoother terrain similar to this region of the moon.  

The knobs reach about 300 feet (90 meters) in height in the images, which NASA released Wednesday. The smallest recognizable features are only 10 feet (3 meters) across.

Volcanic and tectonic pressures likely continue to sculpt the exteriors of Callisto's sibling satellites, Io, Ganymede and Europa, erasing most or all signs of crater impacts over time, according to researchers. In contrast, the rock and ice surface of Callisto seems to have changed little over billions of years.

But the new pictures, taken in May, suggest that at least minor erosion forces are at work, albeit slowly. The knobby terrain could be icy debris ejected from a major crater impact.

Darker material seems to slump from each peak. In time, the knobs will vanish, creating a much smoother moonscape, similar to that in the second image, researchers theorize.

First global color picture of Callisto taken by Galileo
First global color picture of Callisto taken by Galileo  

"They are continuing to erode and will eventually disappear," Klemaszewski said.

During the May encounter, Galileo took its first global color image of Callisto. The many crater scars on the otherwise dark exterior make known an ancient legacy of intense impacts.

Galileo has orbited the Jupiter system since 1995. Earlier this month, the bus-sized spacecraft flew near Io, the most volcanic body in the solar system. Managed the NASA's Jet Propulsion Laboratory, the probe is scheduled to make a fatal plunge into the thick atmosphere of Jupiter in 2003.






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RELATED SITES:
• Galileo Project Home
• Jet Propulsion Lab
• NASA Home Page

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