Hubble finds stormy weather above Jupiter.

More than twice as massive as all the other planets combined, Jupiter is nonetheless too small to pass muster as a star. But in one respect, this heavyweight could still be a stellar contender. A new study suggests that Jupiter's atmosphere is just as turbulent and stormy as that of a bona fide star.

Using a Hubble Space Telescope spectrograph, researchers have deduced that hydrogen atoms in Jupiter's upper atmosphere are moving at supersonic speeds.

Moreover, the velocities fluctuate wildly on a timescale shorter than 10 minutes, note Claude Emerich of the Institute of Spatial Astrophysics in Orsay, France, and the Institute of Astrophysics in Paris, John T. Clarke of the University of Michigan in Ann Arbor, and their colleagues. "This behavior, unexpected in a planetary atmosphere, is evidence of the particularly stormy Jovian upper atmosphere, not unlike a star's atmosphere," the team writes in the Aug. 23 Science.

"To find velocities that high is really very surprising," agrees Donald E.

Shemansky of the University of Southern California in Los Angeles.

The light analyzed by Hubble, known as Lyman-alpha emission, originates from hydrogen atoms well above Jupiter's cloud tops. The spectrograph revealed that the emission forms an unusually wide spectral line, indicating that the atoms have velocities as high as 100 kilometers per second. Although another craft, the International Ultraviolet Explorer, had already hinted at such velocities, Hubble's higher sensitivity provides more compelling evidence, notes Clarke.

The hydrogen radiation forms part of the Lyman-alpha bulge, a ring of ultraviolet light that girdles Jupiter's magnetic equator. First observed by a short-flight rocket 18 years ago and seen by the Voyager spacecraft a few months later, the ring has eluded explanation.

The Hubble data can't identify what powers the ring or produces the high velocities. One explanation points to Jupiter's auroras, oval-shaped regions above the planet's north and south magnetic poles where charged particles crash and emit light. Jets of gas emanating from each magnetic pole might meet and collide near the magnetic equator, accounting for the Lyman-alpha bulge and the turbulent, high-speed motion of the hydrogen atoms.

Fast-moving hydrogen represents only one percent of the hydrogen in Jupiter's upper atmosphere. It's too soon to say whether the mechanism behind these high velocities could also be the solution to another mystery. The Voyager craft found that Jupiter-like Saturn, Uranus, and Neptune-has upper atmospheric temperatures too high to be accounted for by the sun's heat. The same force might power the hydrogen and heat the atmosphere.