23:00 28 June 2007
NewScientist.com news service
David Shiga

Jupiter's cloud patterns are undergoing dramatic changes, reveal new images by the Hubble Space Telescope. Similar transformations of the giant planet's clouds have been witnessed before, but never in such detail – and they have never been explained.

Hubble has been keeping an eye on Jupiter to provide context for close-up observations made by NASA's New Horizons spacecraft, which flew by the solar system's largest planet in February on its way to Pluto.

Some changes were already evident in January, when Jupiter became observable again after a period when it was too close to the Sun in the sky for Hubble to image. At that time, cloud bands around its equator that had been whitish for the past 15 years or so were noticeably darker.

Then, between 25 March and 5 June, a white band in the planet's northern hemisphere turned brown, while a gap in the cloud layer produced a serpent-shaped dark streak in the same area. White bands in the southern hemisphere seem to be changing colour now, too, says Amy Simon-Miller of NASA's Goddard Space Flight Center in Greenbelt, Maryland, US.

"This is the first time that we've been able to watch the sequence with high-resolution spacecraft images," she told New Scientist.

Fresh ice
Similar transformations happened in the 1980s and early 1990s. Although Hubble was in orbit during the second transformation, its initially flawed vision had not been corrected yet by a space shuttle servicing mission.

This time around, the planet has been observed during its transformation not only by Hubble and New Horizons but also at infrared wavelengths by several ground-based telescopes that were also providing context observations for New Horizons.

The reasons for the changes are not clear. White clouds are thought to be higher up in the planet's atmosphere than darker clouds and made of fresher ammonia ice. But exactly what gives the lower-altitude brownish clouds their distinctive colour is one of the biggest mysteries about Jupiter.

Scientists still do not understand what drives these major shifts in the distribution of these clouds, but the new observations could be the key. "This is the first time that we really have a complete set of data, so we hope that we can use this and figure out what's going on," Miller says.

http://space.newscientist.com/article/dn12159-jupiter-changes-its-stripes.html