The Great Dark Spot on Jupiter

The Cassini spacecraft has photographed an extraordinary dark cloud on Jupiter twice as big as Earth itself.

For more than a century astronomers thought that the Great Red Spot was the biggest thing on Jupiter. Not anymore. Images from NASA's Cassini spacecraft have revealed something at least as large.

The Great Dark Spot.

"I was totally blown away when I saw it - a dark cloud twice as big as Earth swirling around Jupiter's north pole," says Bob West, a planetary scientist at the Jet Propulsion Laboratory.

West has been chasing this cloud for some time. He first saw it - "just a glimpse," says West - in an ultraviolet (UV) picture of Jupiter taken by the Hubble Space Telescope in 1997. But it only appeared in one image out of many spanning a period of years. "I didn't know what to make of it," he recalls.

Now he knows. "The Cassini spacecraft was en route to Saturn in 2000 when it passed by Jupiter and had a good view of the planet's north pole," says West. "At first there was nothing unusual - just ordinary polar clouds. Then the Dark Spot emerged." For weeks Cassini's UV-sensitive cameras watched as the cloud grew into an oval the size of the Great Red Spot itself. It swirled, darkened and changed shape until, as Cassini was departing, it began to fade again.

more This composite of Cassini ultra-violet (UV) images reveals the "Great Dark Spot" swirling near Jupiter's north pole. Jupiter's auroral zone is denoted by the blue curve.

"The Dark Spot is ephemeral," says West. That's probably why Hubble saw it only once. And if Cassini had arrived a month or two later, it might not have seen the Dark Spot at all. Instead, Cassini's cameras monitored the cloud for 11 straight weeks, and those data have allowed West to draw some conclusions.

"The Great Dark Spot and the Great Red Spot are entirely different," he says. The Great Red Spot is deep. "It's a high-pressure storm system rooted in Jupiter's troposphere far below the cloudtops. The Great Dark Spot is apparently shallow and confined to Jupiter's high stratosphere."

West believes the Dark Spot is a curious side-effect of auroras on Jupiter.

"Jupiter has Northern Lights just as Earth does, although on Jupiter they are hundreds to thousands of times more powerful," says West. Auroras happen when electrons and ions rain down on the polar atmosphere and cause the air to glow where they hit. Here on Earth, auroras are usually sparked by solar wind gusts. The solar wind can also trigger auroras on Jupiter, but it's not necessary: On Jupiter, the planet itself energizes Northern Lights. "Jupiter's magnetic field is a huge reservoir of charged particles," explains West. "These particles are accelerated poleward by the 11-hour rotation of Jupiter and its magnetic field. Thus, auroras on Jupiter are almost always active."