The first detailed analysis of the images of Jupiter recorded by the cameras aboard the Cassini spacecraft has shed light on its atmosphere, magnetosphere, rings and some of its moons.
By Dr David Whitehouse
BBC News Online science editor
After studying those images, some assembled into movies, Carolyn Porco and colleagues, writing in the journal Science, have obtained new insights into the atmospheric circulation that makes up Jupiter's weather.
Cassini took 26,000 images of Jupiter when it passed the gas giant in December 2000, en route to Saturn. The probe did not get as close to the planet as previous missions have, but this gave it some advantages.
It meant that the spacecraft could travel at a leisurely pace, recording images of Jupiter continuously for six months.
Cloudy belts and zones dominate Jupiter's appearance - the darker belts alternating with lighter zones. Scientists think the pale zones are areas of rising atmosphere, partly because many clouds on Earth form where atmosphere rises.
Conversely, the dark belts have been viewed as areas where the atmosphere descends. However, pictures from the Cassini spacecraft tell a different story.
The Jupiter results provide some hints of the spectacular new findings that await Cassini when it reaches Saturn
Larry Esposito, University of Colorado
They appear to show that individual cells of upwelling bright-white clouds emerge almost without exception in the dark belts.
"We have a clear picture emerging that the belts must be the areas of net rising atmospheric motion on Jupiter, with the implication that the net motion in the zones has to be sinking," says Tony Del Genio, an atmospheric scientist at the US space agency's (Nasa) Goddard Institute for Space Studies, New York.
"It's the opposite of expectations for the past 50 years," he adds.
Source of rings
Cassini may have also revealed the origin of Jupiter's thin ring made of very dark, fine material.
"The range of illumination angles at which Cassini viewed Jupiter's main ring gives insight about particles in the ring by the way they scatter sunlight. The particles appear to be irregularly shaped, not spheres," says Carolyn Porco of Southwest Research Institute, Boulder, Colorado.
The way two small moons, Metis and Adrastea, near the ring are inclined matches its vertical thickness. That points to those moons as sources of the ring particles, says Porco.
"They likely come from surfaces of one or more moons being eroded by micrometeoroid impacts," she adds.
Poles and plumes
Ultraviolet images of Jupiter's north polar region show a swirling dark oval of high-atmosphere haze the size of the planet's famous Great Red Spot.
"It's a phenomenon we haven't seen before, so it gives us new information about how stratospheric circulation works," says Robert West of Nasa.
Other discoveries reported include atmospheric glows of the large moons Io and Europa during eclipses, a volcanic plume over Io's north polar region, and the irregular shape of a small outer moon, Himalia.
"The Jupiter results provide some hints of the spectacular new findings that await Cassini when it reaches Saturn," says Larry Esposito of the University of Colorado.
Cassini will begin orbiting Saturn on 1 July, 2004, and will release its piggybacked Huygens probe about six months later for descent through the atmosphere of the planet's major moon Titan.