15 Jan 04

An intense jet-stream in Jupiter's atmosphere very similar to one on Earth has been observed by an international team of scientists including researchers from Oxford University.

Researchers were able to make an exceptionally close study of Jupiter when the Cassini spacecraft went past and took measurements of unprecedented detail. Cassini took 26,000 images of Jupiter when it passed the gas giant in December 2000, en route for 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 and making other measurements of Jupiter continuously for six months.

The team on the Composite Infrared Spectrometer instrument reported their analysis of the results from the Cassini encounter in this week's edition of Nature. The substantial jet-stream they discovered appears to be a manifestation of a phenomenon found in the stratosphere on Earth where east-west winds reverse direction and the temperatures change cyclically roughly every two years, known as the quasi-biennial oscillation.

Ground-based observations already suggested that similar temperature oscillations, with a 4-5 year cycle, occurred on Jupiter. The Cassini swing-by allowed the researchers to produce temperature and wind maps with high spatial resolution, confirming the oscillations. They found an intense, high-altitude jet of air, and wave activity in the stratosphere which appears analogous to that occurring on Earth.

'This was valuable to us as atmospheric physicists because it tests our understanding of what happens on Earth in an extremely unfamiliar environment' said Professor Peter Read of Oxford's department for Atmospheric Physics. 'Our findings are of as much interest, or more, to meteorologists as to astronomers'

To find this similarity between Earth and Jupiter is surprising because the two planets are so different. 'Jupiter's atmosphere is driven by differential heating from the sun, but also by heat coming from within the planet itself. It's actually a net exporter of heat – you might describe it as a failed star – and in that sense is totally different from Earth,' said Professor Read.