Space / Solar System

En route to Saturn, NASA's Cassini spacecraft detoured past Jupiter, returning 26,000 images that are now prompting a top-to-bottom reassessment of the solar system's most massive planet. Scientists had long thought that Jupiter's distinctive dark belts were giant downdrafts and that the lighter zones were areas of rising air. The new snapshots prove the opposite, revealing upwelling storms within the dark belts. "Cassini's global coverage shows that the belts are rising overall. This changes our sense of the whole circulation between belts and zones," says Anthony Del Genio, an atmospheric scientist at NASA's Goddard Institute for Space Studies in New York City.

Poring over images from Cassini's ultraviolet cameras, planetary scientist Bob West of the Jet Propulsion Laboratory spotted another unexpected feature in Jupiter's atmosphere: a 16,000-mile-wide storm near the planet's north pole. This vortex, dubbed the Great Dark Spot, apparently lurks beneath the uppermost clouds most of the time, only rarely rising to the top. "I never imagined anyone would see anything so large, least of all in the polar stratosphere," West says. He suspects that Jupiter's powerful auroras contribute to the storm by breaking methane molecules into fragments, which combine with hydrogen to form the hydrocarbon particles that give the Great Dark Spot its distinctive coloration.

Cassini also revealed fresh details about Jupiter's environs. High-resolution images showed that particles in Jupiter's thin rings are irregularly shaped, not spherical as previously assumed. And the spacecraft's sensors detected a strange, doughnut-shaped cloud of charged gas around the planet. Scientists say high-speed subatomic particles from Jupiter's magnetic field probably blast water molecules off the surface of its icy moon Europa, creating a water-molecule ring that fills the moon's orbit.