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Wednesday, 27 June, 2001, 08:23 GMT 09:23 UK
Strange glows on Jupiter moon
Io Nasa
The white dots seen near the equator are volcanoes
By BBC News Online science editor Dr David Whitehouse

Strange glows have been seen dancing over Jupiter's moon Io. They were recorded during an eclipse of Io in January, witnessed by the Cassini spacecraft that was on its way to Saturn.

Cassini's camera captured images of the eclipsed Io in several colours, ranging from the near-ultraviolet to the near-infrared.

The colour of the diffuse glows tells scientists what elements are responsible; oxygen and sulphur are the best candidates.

Some of the glows are associated with volcanic eruptions; others are auroral in nature being caused by Jupiter's titanic magnetic field wafting over the tiny moon.

Sulphur volcanoes

In the multi-spectral image the white dots seen near the equator are volcanoes, some of which are much brighter than the faint atmospheric glows.

Io Nasa
Io as seen from Galileo in 1996
The diffuse atmospheric glows - emissions of light at wavelengths of 595 to 645 nanometres - probably come from a tenuous atmosphere of oxygen and sulphur thrown above Io by the sulphur-spewing volcanoes.

The diffuse blue glows seem to be restricted to areas deep down in the atmosphere near the surface of Io, while the red glows are much more extensive, reaching heights of up to 900 km (560 miles).

Researchers say this is to be expected if the blue glows are produced by sulphur dioxide. The molecule, being heavier than oxygen atoms, would be more closely bound to the moon's surface by gravity.

Volcanic plume

The blue and red glows near the equator of Io appear to move over the moon, possibly due to the changing orientation of Jupiter's magnetic field.

This motion allows researchers to study the relationship between Io's auroras and the electric currents that excite them.

Also visible in the image is a faint blue emission near the north pole of Io, which is believed to be due to a volcanic plume erupting from the mountain known as Tvashtar.

This eruption was observed by both Cassini, which was just passing through, and by Galileo, which has been in orbit around Jupiter since 1995.

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See also:

26 Jan 01 | Sci/Tech
Jupiter's moons spied
31 Dec 00 | Sci/Tech
Spacecraft gets close to Jupiter
19 May 00 | Sci/Tech
Io's wandering volcanoes
10 Oct 00 | Sci/Tech
...and here it is in colour
06 Oct 00 | Sci/Tech
Cassini approaches Jupiter
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