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.