By BBC News Online Science Editor Dr David Whitehouse

The Galileo spacecraft has captured images of an active volcano on Jupiter's moon Io.

The volcano is shooting a plume of lava a couple of kilometres above the moon's surface.

"We've finally caught a close-up of a massive volcanic eruption in action on Io," said Galileo project scientist Dr Torrence Johnson. "The erupting lava was so hot and bright, it over-exposed part of the camera picture and left a bright blur in the middle."

In fact, the lava fountains were hot enough and tall enough to be observed by the Nasa Infrared Telescope situated on top of Mauna Kea in Hawaii.

The Galileo images show a region of giant calderas, or crater depressions, in Io's northern latitudes. They came from two of the spacecraft's onboard instruments, the visible wavelengths camera and near-infrared mapping spectrometer.

On Earth, lava fountains also provide a spectacular volcanic display, although the fountains found in Hawaii and elsewhere rarely exceed a few hundred metres in height.

"Catching these fountains on Io was a one-in-500-chance observation," said Galileo scientist Dr Alfred McEwen, from the University of Arizona in Tucson.

Huge craters

Galileo also observed Loki, the most powerful volcano in the Solar System, which is also on Io. The observations, as well as those from Earth-based telescopes, show large changes in the output of heat at Loki over time, with huge portions of the lava surface appearing to be of a uniform temperature.

The observations show that Loki began a series of major eruptions in early September, and Galileo caught the eruption in full force during its October flyby.

While observing Loki's 193-km (120 mile) wide caldera, one Galileo instrument found a sharply-defined region that was much hotter than the rest.

Galileo's cameras were also targeted to provide the first close-up view of a chain of huge calderas. They are some of the largest on Io and they dwarf other craters elsewhere in the Solar System. At 290 by 100 km (180 by 60 miles), one chain of calderas covers an area seven times larger than the largest caldera on the Earth.

Galileo's images also show some of the curious mountains found on Io. By measuring the lengths of the shadows, scientists can estimate the height of the mountains at 4,000 m (13,000 feet).

These mountains, like others imaged during a previous Galileo flyby of Io, seem to be in the process of collapsing. Huge landslides have left piles of debris at their bases. The ridges that parallel their margins are also indicative of material moving down the mountainsides due to gravity.