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

by Dr Tony Phillips

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."

"High-energy electrons that hit Jupiter's atmosphere not only cause auroras, but also break apart methane (CH₄), which is more abundant on Jupiter than it is on Earth," says West. "Fragments of methane molecules combine with ambient hydrogen to form acetylene C₂H₂. That's the basic building block. Acetylene combines with other carbon- and hydrogen-containing molecules to build even more complex molecules, which eventually condense into dark droplets."

more The Great Red Spot is a long-lasting storm rooted deep in Jupiter's atmosphere.

If West is right, the Great Dark Spot is a haze of hydrocarbon-rich droplets floating in the uppermost layers of Jupiter's stratosphere. Such a haze would be prominent in UV images because hydrocarbon droplets are strong absorbers of UV radiation. Indeed, the Great Dark Spot is invisible to the human eye. "It can only be seen in UV light."

His idea fits the facts, but West remains puzzled: "There was no strong auroral display when the Great Dark Spot intensified in late 2000. What caused it to appear when Cassini was flying by? We don't know. This shows us that Jupiter's stratosphere is a more interesting place than we once thought."

Furthermore, it can teach us something about our own planet.

West explains: "This dark spot is trapped by a polar vortex - a jet stream that encircles Jupiter's north pole." Fast-moving winds in the vortex act like an atmospheric wall, keeping the Dark Spot corralled at high latitudes. Similar vortices encircle Earth's polar regions. Our planet's Arctic vortex is disrupted somewhat by northern land masses, but the Antarctic vortex is better organized. It plays a key role in confining the ozone hole - much as Jupiter's polar vortex confines the Great Dark Spot.

more A Hubble Space Telescope image of Northern Lights on Jupiter.

"Monitoring the Dark Spot could help us understand how planetary vortices work." For such studies, two planets are clearly better than one.

Meanwhile, West would be delighted just to see the Dark Spot again. "It's elusive," he says. But he's ready to be blown away ... any time.