Press Release

Galileo Flyby Reveals Callisto’s Bizarre Landscape

By SpaceRef Editor
August 22, 2001
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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, AZ. 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 of dust 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 turns 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 areas where the sharp knobs have
apparently eroded away, leaving a plain blanketed with dark

The close-up images show craters as small as about 3 meters across,
or about 10 feet, 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, CA, manages Galileo for NASA’s
Office of Space Science, Washington, DC.

Additional information about Galileo’s mission is available online

The close-ups and the first complete Callisto global color picture
from Galileo are available on the Internet at:

SpaceRef staff editor.