Aug 22, 2001

The Strange Spires of Callisto




NASA's Galileo spacecraft has spotted bizarre icy knobs jutting out of Callisto's cratered terrain.


Marshall Space Flight Center


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August 22, 2001: Three months ago when NASA's adventurous Galileo spacecraft skimmed a mere 138 km above the surface of Jupiter's moon Callisto, onboard cameras captured the sharpest pictures ever of that moon's mysterious landscape. Scientists have since examined the images, and what they found is surprising. Callisto is peppered with strange icy features -- spires that seem to be slowly eroding on a world long considered changeless and dead.


"We haven't seen terrain like this before. It looks like erosion is going on, which is pretty surprising," said James Klemaszewski (Academic Research Lab) who, along with David Williams and Ronald Greeley (Arizona State University), is analyzing Galileo's latest pictures of Callisto.

Above: Bright scars on a darker surface testify to a long history of impacts on Jupiter's moon Callisto in this image of Callisto from NASA's Galileo spacecraft. The picture, taken in May 2001, is the only complete global color image of Callisto obtained by Galileo. [more information]




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Callisto's icy surface is the most heavily cratered place in the Solar System. There are no volcanoes or winds and rain to obliterate landforms. So, craters that form as the result of occasional impacts with meteorites are very long-lasting. Planetary scientists call terrains like Callisto's "old." Earth's much-weathered surface, on the other hand, is very "young." For billions of years, little has changed on Callisto other than the relentless accumulation of craters -- or so researchers thought.

Galileo's latest images are painting a different picture. Callisto's knobby terrain is unlike any seen before on Jupiter's moons --and it's not entirely unchanging.

During the Callisto flyby, Galileo's camera saw spire-like "knobs" jutting 80 to 100 meters (260 to 330 feet) high, consisting perhaps of material thrown outward from a major impact billions of years ago. The knobs are very icy, but they also harbor some darker dust. The dark material seems to be sliding down the knobs and collecting in low-lying areas.

"They are continuing to erode and will eventually disappear," Klemaszewski said. One theory for an erosion process is that, as some of the ice turns into vapor, it leaves behind dust that was bound in the ice. The accumulating dark material could absorb enough heat from the distant Sun to warm the surrounding ice and keep the process going.


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Right: The top inset shows some of Callisto's many icy knobs. The bottom image shows a dark-plained region where erosion has essentially ceased, allowing impact craters to persist and accumulate. [more information]


The eroding spires of Callisto are just one of the moon's riddles. Indeed, with a diameter of 4,800 km -- nearly the size of Mercury -- Callisto is a bona fide world of its own with mysteries befitting a full-fledged planet. For example, magnetic readings picked up by Galileo when it encountered Callisto in 1996 and 1997 suggest that the pock-marked satellite harbors one of the solar system's biggest salty oceans. But the water, if it's really there, does not lie atop the frigid surface. Instead, Callistan oceans would be hidden deep below the moon's spikey, slowly eroding crust -- another tantalizing puzzle for future spacecraft and explorers.

For more information about Jupiter's moons and the ongoing Galileo mission please visit



The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages Galileo for NASA's Office of Space Science, Washington, D.C.

Web Links


Galileo's Flyby Reveals Callisto's Bizarre Landscape - A spiky landscape of bright ice and dark dust shows signs of slow but active erosion on the surface of Jupiter's moon Callisto in new images from NASA's Galileo spacecraft. JPL press release.

The Galileo Mission - read all about NASA's intrepid Galileo spacecraft at the mission's home page from JPL

Callisto -- a collection of web links and facts about Jupiter's 4th Galilean satellite

JPL's Jupiter Photojournal -- This wonderful site offers a comprehensive collection of Callisto imagery.


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