Saturday, April 24, 1999 Published at 10:24 GMT 11:24 UK
Astronomers moonstruck by Hubble pictures
Io and its shadow were captured by the Hubble space telescope
By BBC News Online Science Editor Dr David Whitehouse
The Hubble Space Telescope has captured these fabulous images of the volatile moon Io sweeping across the giant face of Jupiter.
The pictures were taken to try to see volcanic plumes on one of Jupiter's major moons.
Only a few weeks before these images were taken, the orbiting telescope snapped a portrait of one of Io's volcanoes spewing sulphur dioxide "snow".
These stunning images of Io and Jupiter have been released to commemorate the ninth anniversary of the Hubble telescope's launch on April 24, 1990. All of these images were taken with the Wide Field and Planetary Camera Two.
"Other observations have inferred the presence of sulphur dioxide snow in Io's plumes, but this image offers direct observational evidence for it," explains John R Spencer of Lowell Observatory in Flagstaff, Arizona.
A trip around Jupiter
The three snapshots of the volcanic moon rounding Jupiter were taken over a 1.8-hour time span. Io is roughly the size of Earth's moon.
The black spot on Jupiter is Io's shadow and is about 3,700 km (2,300 miles) across. The shadow sails across the face of Jupiter at 17km per second (38,000mph). The smallest details visible on Io and Jupiter measure 150km (94 miles) across.
The images were further sharpened through image reconstruction techniques. The view is so crisp that one would have to actually be standing on Io to see this much detail on Jupiter with the naked eye.
The bright patches on Io are regions of sulphur dioxide frost. On Jupiter, the white and brown regions distinguish areas of high-altitude haze and clouds; the blue regions depict relatively clear skies at high altitudes.
A volcanic moon
In the close-up picture of Io, the mound rising from the surface is an eruption from Pillan, a volcano that had previously been dormant.
Pillan's plume is very hot and its ejecta are moving extremely fast. Based on information from the Galileo spacecraft currently in orbit around the planet, Pillan's outburst is at least 1,200 degrees Celsius (2,240 Fahrenheit). The volcano is spewing material at speeds of 2,880km/h (1,800mph).
The hot sulphur dioxide gas expelled from the volcano cools rapidly as it expands into space, freezing into snow.
Io is well known for its active volcanoes. It has hundreds of active volcanoes, but only a few, typically eight or nine, have visible plumes at any given time.