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Friday, 19 May, 2000, 04:37 GMT 05:37 UK
Io's wandering volcanoes
Io Nasa
A dark lava lake at the summit of a volcano
By BBC News Online science editor Dr David Whitehouse

Astronomers have obtained their best view yet of the most volcanically active world in our Solar System: Jupiter's moon, Io.

Io, roughly the size of our own Moon, has a yellow and ochre surface covered in sulphur chemicals deposited by giant volcanic plumes that reach hundreds of kilometres in height.

Io Nasa
A ribbon of light marks the edge of a lava lake
Its volcanism is driven by the extreme tidal energy it gets from Jupiter.

The satellite was first studied in detail in 1979 by the Voyager fly-by mission. Now, new images, taken from the Galileo spacecraft show Io's surface to be marked by volcanic calderas, plains, and fissures.

It is awash with fluid lava thrown up by fountains and pooled in lakes.

Ribbon of light

A lava lake was seen in the central caldera of Pele, a volcano 1200 km (746 miles) across that sends a gaseous plume 400 km (250 miles) into space. Viewed at night-time, the lava lake is seen as a glowing ribbon of light.

Io Nasa
Prometheus: A wandering volcano
The most powerful volcano in the Solar System is located on Io and is called Loki Patera. Galileo's images show it to have a huge, dark lava lake cooling in its summit depression.

The images also reveal evidence of lava flows that may have streamed across the surface at hundreds of metres a second.

Analysing the spectra of the lava allows scientists to deduce its composition. Changes between the images show how the eruptions change over time, and how they help to create mountains and other landforms.

Scientists believe that Io's surface is very young, being continuously covered by sulphur deposits. The fact that no impact crater was seen in the images anywhere on Io means that the world must be completely resurfaced every million years or so.

Wandering volcanoes

One volcano even appears to have moved.

Galileo and the 1979 Voyager spacecraft observations show that one large volcanic plume called Prometheus may have been caught in the act of migrating across Io's surface.

Unlike any volcano on Earth, Prometheus appears to have wandered as much as 95 km (59 miles) over a volcanic plain in just a few decades.

Scientists think that the two vents are connected by a dark lava flow running over a so-called snowfield of sulphur dioxide. The pressure and heat of the lava flow from Prometheus melts the sulphur snowfield that makes it way to the surface through a wandering pipe.

The research is published in the journal Science

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See also:

11 Oct 99 | Sci/Tech
Galileo sweeps over Io's volcanoes
20 Dec 99 | Sci/Tech
Io's fountains of fire
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