This Echo is READ ONLY !   NO Un-Authorized Messages Please!
 ~~~~~~~~~~~~~~~~~~~~~~~~   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

NASA Science News for March 12, 2003

The Great Dark Spot
===================

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

March 12, 2003: 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.

Credits & Contacts
Authors: Dr. Tony Phillips 
Responsible NASA official: Ron Koczor 
Production Editor: Dr. Tony Phillips 
Curator: Bryan Walls 
Media Relations: Steve Roy

-END OF FILE-
=============

---