Jupiter's Callisto has been called the "ugly duckling" of the four large Jovian moons because it is geologically dead, but new Galileo spacecraft images make the moon look a bit more attractive to planetary scientists.
Images taken by the Jupiter orbiter in May from a super-close distance of 86 miles (138 kilometers) show signs of slow but active erosion on the surface of the planet's farthest-flung large moon.
"We haven't seen terrain like this before," said James Klemaszewski of the Academic Research Lab in Arizona. "It looks like erosion is still going on, which is pretty surprising."
Klemaszewski has been analyzing the new Callisto imagery along with David Williams and Ronald Greeley of Arizona State University. The images were released by NASA's Jet Propulsion Laboratory (JPL), which manages the Galileo mission.
Callisto's surface of ice and rock is the most heavily cratered of any moon in the solar system, signifying that it is geologically inactive. Volcanic activity or tectonic shifting has erased most craters on Jupiter's other large satellites -- Io, Ganymede and Europa.
Europa and possibly Ganymede are thought to harbor liquid oceans underneath their icy crusts. By comparison with those moons and volcanically volatile Io, Callisto has looked scientifically dull to scientists.
Interpreting the images
The jagged hills in the new Callisto images are thought to be icy material thrown outward from large impacts billions of years ago, or possibly the highly eroded remains of a single large impact structure, Williams said.
Each bright formation was revealed by Galileo to be surrounded by darker dust slumping off the peaks.
One theory for an erosion process is that, as some of the ice sublimes away into vapor, it leaves the darker material behind. The accumulating dark material then absorbs enough of the Sun's heat to melt adjacent ice and keep the process going.
The new images support this idea by showing portions of the surface where the sharp knobs have apparently eroded away and left a plain blanketed with the dark material.
"They are continuing to erode and will eventually disappear," Klemaszewski said.
Mysteries remain
The scientists also plan to count the number of small craters crowded onto Callisto's surface so as to estimate the age of the moon's surface. Since Callisto has been so undisturbed by other geological processes, its cratering density is useful in calibrating age estimates for Jupiter's other moons.
So far, the close-up images have revealed numerous craters, though not as many as some predictions had anticipated.
Galileo, orbiting Jupiter since 1995, has provided the highest resolution views ever seen of any of Jupiter's satellites. The smallest details shown have measured as small as 10 feet (3 meters) across.
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