Ice volcanoes resurface Jupiter moon

A 3-D reconstruction of Ganymede's surface

By BBC News online science editor Dr David Whitehouse

Jupiter's major moon, Ganymede, may be being resurfaced by water seeping on to its surface from the satellite's interior. This so-called cryomagma freezes out to form vast plains of ice that cover over rough terrain or old craters.

Ganymede: A world of rock and ice

This is what Dr Paul Schenk, from the Lunar and Planetary Institute in Houston, US, and colleagues think is happening on Ganymede after having studied stereo maps of the icy moon's surface.

Ganymede is the largest moon in the Solar System. It is bigger than Mercury and almost the same size as Mars. If it orbited the Sun on its own, it would be classified as a planet.

Its surface is covered by two types of terrain. Most of the moon is composed of icy plains; other regions are heavily cratered and composed of rock and ice.

Stereo pairs

Since it was first visited in 1979 by the Voyager spaceprobe, the moon's surface has been something of a puzzle. It seems to be a hybrid world, half-way between two other moons of Jupiter.

Flat and rough: Ganymede's two types of terrain

Some of it looks like Callisto, which is dark, rocky and heavily cratered. The rest looks like Europa, which is bright, fresh and young.

To find out more about the two types of terrain, Dr Schenk combined pictures taken by the past Voyager and current Galileo missions. He created "stereo pairs" of images that allowed the height and relief of the terrain to be measured.

The researchers found that the rough terrain was higher, and that the lower, smoother regions seemed to be formed when water from the interior covered the surface and then froze out.

Close pass

But to the astronomer's frustration they have yet to see the parts of the surface from where the cryomagma erupts. Also a mystery is why only parts of Ganymede are covered in ice.

"If such a plentiful source of icy magma exists at shallow depth, then why is it so shy and fails to reveal itself," says Louise Prockter of Johns Hopkins University.

The Galileo spacecraft took its most detailed images of Ganymede during a close pass in May 2000.

Those images have yet to be analysed in detail but the researchers are hoping they might show evidence for ice volcanoes.

The research is published in the journal Nature.