The Great Dark Spot on Jupiter

"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.