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Thursday, March 6, 2003
Pictures of Jupiter, taken by a NASA spacecraft on its
way to Saturn, are flipping at least one long-standing notion
about Jupiter upside down.
Stripes dominate Jupiter's appearance. Darker "belts"
alternate with lighter "zones." Scientists have long
considered the zones, with their pale clouds, to be areas of
upwelling atmosphere, partly because many clouds on Earth
form where air is rising. On the principle of what goes up
must come down, the dark belts have been viewed as areas
where air generally descends.
However, pictures from the Cassini spacecraft show that
individual storm cells of upwelling bright-white clouds, too
small to see from Earth, pop up almost without exception in
the dark belts. Earlier spacecraft had hinted so, but not
with the overwhelming evidence provided by the new images of
43 different storms.
"We have a clear picture emerging that the belts must be the
areas of net-rising atmospheric motion on Jupiter, with the
implication that the net motion in the zones has to be
sinking," said Dr. Tony Del Genio, an atmospheric scientist
at NASA's Goddard Institute for Space Studies, New York.
"It's the opposite of expectations for the past 50 years," he
said.
Del Genio is one of 24 co-authors from America and Europe
reporting diverse results from the Cassini imaging of Jupiter
in Friday's edition of the journal Science. Cassini's camera
took about 26,000 images of Jupiter, its moons and its faint
rings over a six-month period as the spacecraft passed nearby
two years ago.
"The range of illumination angles at which Cassini viewed
Jupiter's main ring gives insight about particles in the ring
by the way they scatter sunlight. The particles appear to be
irregularly shaped, not spheres," said camera-team leader Dr.
Carolyn Porco of Southwest Research Institute, Boulder, Colo.
"They likely come from surfaces of one or more moons being
eroded by micrometeoroid impacts," she said.
Spherical particles would suggest an origin as melted
droplets, not erosion. In addition, Cassini imaging shows the
degree to which the orbits of two small moons near the ring,
Metis and Adrastea, are inclined matches the vertical
thickness of the ring. That points to those moons as sources
of the ring particles said Porco.
One surprise in ultraviolet images of Jupiter's north polar
region is a swirling dark oval of high-atmosphere haze the
size of the planet's famous Great Red Spot. "It's a
phenomenon we haven't seen before, so it gives us new
information about how stratospheric circulation works," said
Dr. Robert West of NASA's Jet Propulsion Laboratory (JPL),
Pasadena, Calif. The results show the winds and the life
cycle of clouds in the stratosphere.
Also, movies of infrared images reveal persistent bands of
globe-circling winds extend north of the conspicuous dark and
light stripes. "The planet's appearance at high latitudes is
like leopard spots, but when you see it in motion, it's
interesting that all the spots at one latitude move in one
direction and all the spots at adjacent latitudes move the
opposite direction," said Dr. Andrew Ingersoll of the
California Institute of Technology (Caltech), Pasadena.
Other discoveries reported include atmospheric glows of the
large moons Io and Europa during eclipses, a volcanic plume
over Io's north polar region, and the irregular shape of a
small outer moon, Himalia.
"The Jupiter results provide some hints of the spectacular
new findings that await Cassini when it reaches Saturn," Dr.
Larry Esposito of the University of Colorado, Boulder,
principal investigator for Cassini's ultraviolet-imaging
spectrograph instrument, predicts in a separate commentary in
Science about the Cassini camera results at Jupiter. Cassini
will begin orbiting Saturn July 1, 2004, and will release its
piggybacked Huygens probe about six months later for descent
through the atmosphere of the moon Titan.
Cassini is a cooperative venture of NASA, the European and
Italian Space Agencies. JPL manages it for NASA's Office of
Space Science, Washington. Other co-authors include
scientists from Cornell University, Ithaca, N.Y.; Free
University of Berlin, Germany; Queen Mary, University of
London, United Kingdom; University of Arizona, Tucson;
University of Paris, France; German Aerospace Center, Berlin;
and University of California, Los Angeles.
Images and mission information are available on the Internet
at:
http://saturn.jpl.nasa.gov/gallery/jupiter-flyby/index.cfm,
and
http://ciclops.lpl.arizona.edu/ciclops/images_jupiter.html
