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. (See the movie.)

“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.”

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.

“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.