Two small regions on the surface of Callisto, to the south of the Asgard impact basin. The icy hills are clearly visible, with small impact craters seen in the lower picture. The smallest features discernible are just 3 meters (10 feet) across. (Image credit: NASA/JPL/Arizona State University.) New images from the Galileo spacecraft give the sharpest-ever view of Jupiter's moon Callisto and reveal a bizarre landscape of ice and dust on one of the oldest surfaces in the Solar system.

NASA's probe passed by Callisto at an altitude of just 138 km (86 miles), making its closest approach on May 25 this year. Callisto is one of Jupiter's four largest moons, was discovered in the seventeenth century by the astronomer Galileo Galilei and is easily visible in small telescopes. With a diameter of 5000 km it is about the size of the planet Mercury and has one of the most heavily cratered surfaces of any moon in the solar system, indicating that it long ceased any geological activity. In contrast, resurfacing on the other large Jovian satellites – Io, Europa and Ganymede – has erased many or all of their surface features.

The new images show bright jagged hills or knobs of ice surrounded by darker dust which appears to be slumping off the peaks. They are between 80 and 100 metres tall and may be icy material thrown outward from a large impact billions of years ago. 'We haven't seen terrain like this before. It looks like erosion is still going on, which is pretty surprising', commented James Klemaszewski of Arizona's Academic Research Lab.

One theory for the erosion process is that the Sun's heat turns some of the ice to vapour. Dust that was bound in the icy spires is left behind, slides down and collects in low-lying areas. This darker material then absorbs more heat from the Sun and warms adjacent ice, keeping the process going. In some areas, the sharp knobs have eroded away completely, leaving a plain of dust behind.

The high-resolution images will also be used to count the number of small craters – this enables an estimate to be made of the age of the surface. As Callisto is relatively undisturbed by subsequent geological processes, a reliable age would give astronomers a benchmark for the other Jovian moons.

These results mark the latest success for the Galileo probe which was originally expected to complete its work in December 1997. NASA has now announced a further extension of the mission to make five more close passages of the Galilean moons and the inner moon Amalthea. Galileo's final demise will be in August 2003, when it will be directed to burn up in the Jovian atmosphere.