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Guy Webster, JPL, (818) 354-6278
James Hathaway, Arizona State University, (480) 965-6375

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.

The pictures taken by Galileo's camera on May 25 from a distance of less than 138 kilometers, or about 86 miles, above Callisto's surface give the highest resolution view ever seen of any of Jupiter's moons.

"We haven't seen terrain like this before. It looks like erosion is still going on, which is pretty surprising," said James Klemaszewski of Academic Research Lab, Phoenix, Ariz. Klemaszewski is processing and analyzing the Galileo Callisto imagery with Dr. David A. Williams and Dr. Ronald Greeley of Arizona State University, Tempe.

Callisto, about the same size as the planet Mercury, is the most distant of Jupiter's four large moons. Callisto's surface of ice and rock is the most heavily cratered of any moon in the solar system, signifying that it is geologically "dead." There is no clear evidence that Callisto has experienced the volcanic activity or tectonic shifting that have erased some or all of the impact craters on Jupiter's other three large moons.

The jagged hills in the new images may be icy material thrown outward from a large impact billions of years ago, or the highly eroded remains of a large impact structure, Williams said. Each bright peak is surrounded by darker dust that appears to be slumping off the peak.

"They are continuing to erode and will eventually disappear," Klemaszewski said. One theory for an erosion process is that, as some of the ice sublimes away into vapor, it leaves behind dust that was bound in the ice. The accumulating dark material may also absorb enough heat from the Sun to warm the ice adjacent to it and keep the process going. The new images show portions of the surface where the sharp knobs have apparently eroded away, leaving a plain blanketed with dark material.

The close-up images show craters as small as about 3 meters (10 feet) across, though not as many as some predictions anticipated. One scientific goal from the high-resolution images is to see how many small craters are crowded onto the surface. Crater counts are one way to estimate the age of a moon's surface, and since Callisto has been so undisturbed by other geological processes, its cratering density is useful in calibrating the estimates for Jupiter's other moons.

NASA's 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. Additional information about the mission is available online at: http://galileo.jpl.nasa.gov .

The close-ups and the first complete Callisto global color picture from Galileo are available on the Internet at: http://www.jpl.nasa.gov/images/callisto .

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Note to broadcasters: A video file with these images and an animation of the Callisto flyby will air on NASA Television today (Aug. 22) at 6 p.m., 9 p.m. and midnight, Eastern Daylight Time, and on Aug. 23 at noon, 3 p.m., 6 p.m., 9 p.m. and midnight, Eastern Daylight Time. The schedule is subject to change for the space shuttle landing. NASA Television is broadcast on GE-2, transponder 9C, C-Band, located at 85 degrees west longitude. The frequency is 3880.0 MHz. Polarization is vertical and audio is monaural at 6.8 MHz. For questions about the NASA Video File, contact Fred Brown, (202) 358- 0713.

08/22/01 GW
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