LOS ANGELES - The United States-European Cassini spacecraft has found that lightning in Saturn's atmosphere is occurring much differently now than when Nasa's Voyagers flew by in the early 1980s, scientists said. It has also discovered a new radiation belt around the ringed planet.

SATURN'S southern polar region exhibits concentric rings of clouds which encircle a dark spot at the pole as seen in this Cassini spacecraft image released by the National Aeronautics and Space Administration (Nasa) on Wednesday. To the north and towards the right, wavy patterns are evident, resulting from the atmosphere moving with different speeds at different latitudes. The Cassini mission is a cooperative project of Nasa, the European Space Agency and the Italian Space Agency.

The discoveries announced on Thursday are part of an early wave of information about the Saturn system from Cassini, which arrived on June 30 on a US$3.3-billion (S$6-billion) exploration expected to last four years. The mission, managed by the Jet Propulsion Laboratory, is a joint project of the US National Aeronautics and Space Administration, the European Space Agency and the Italian Space Agency.

'This is exactly the point of doing a mission like this,' said Mr Bill Kurth of the University of Iowa, the deputy principal investigator for Cassini's radio and plasma wave instrument, which detects 'cracks' and 'pops' from lightning like those that can be heard on an AM radio during a thunderstorm on Earth.

'Cassini now has evidence for changes in the thunderstorms that occur on Saturn over more than 20 years since we first started making measurements of these,' Mr Kurth said.

One difference is in the patterns in which lightning is occurring. 'Currently, Cassini is seeing lightning on a very sporadic basis. It comes and goes,' said Mr Kurth. 'Some days we see no lightning at all; other days we see evidence perhaps of more than one storm. Back in the early 1980s on Voyager such storms were detected extremely regularly.'

The other major difference is that the thunderstorms observed by Cassini are taking a longer time to rotate around the giant gas planet - about 10 hours and 45 minutes, compared to the 10 hours and five minutes in the 1980s. -- AP


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