Douglas Isbell
Headquarters, Washington, DC     June 5, 1997
(Phone: 202/358-1753)

Jane Platt
Jet Propulsion Laboratory, Pasadena, CA
(Phone: 818/354-5011)


RELEASE:  97-123

GALILEO FINDS WET SPOTS, DRY SPOTS AND
NEW VIEW OF JUPITER'S LIGHT SHOW

     Jupiter has both wet and dry regions, just as Earth has
tropics and deserts, according to new images and data from the
Galileo spacecraft released today.  The data may explain why
Galileo's atmospheric probe found much less water than scientists
had anticipated when it dropped into the Jovian atmosphere in
December 1995.

     "We had suspected that the probe landed in the 'Sahara Desert of
Jupiter,'" said Dr. Andrew Ingersoll, a professor at the California Institute
of Technology, Pasadena, CA, and member of the Galileo science team.
"But the new data show there is moisture in the surrounding areas.
Jupiter is not as dry overall as we thought."

     The area where the probe entered was a clearing in the clouds
-- a dry spot through which deeper, warmer layers can be seen.  By
studying various areas, including those resembling the probe entry
site, the Galileo orbiter has helped scientists understand the
probe results.  In fact, the air around a dry spot has 100 times
more water than the dry spot itself, according to Dr. Robert
Carlson of NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA,
principal investigator for the imaging spectrometer instrument
onboard Galileo.

     Such dry spots cover less than one percent of the Jovian
atmosphere, and they appear to be regions where the winds converge
and create a giant downdraft, according to Cal Tech graduate
student Ashwin Vasavada.  In fact, the water content of the giant,
gaseous planet varies at least as much as the moisture varies from
place to place on Earth.

     "Winds rise from the deep atmosphere and lose water and
ammonia," explained Dr. Glenn Orton, a Galileo interdisciplinary
scientist at JPL and Photopolarimeter-Radiometer co-investigator.
"At the top, when they converge and drop back down, nothing is
left to condense back into clouds, and a dry clearing is created.
These dry spots may grow and diminish, but they recur in the same
places, possibly because of the circulation patterns on Jupiter."

     Ingersoll said the dry spots are found in a northern
hemisphere band at five to seven degrees latitude.  When the
Galileo probe was released near the tops of the clouds, it found
dry air underneath.  But at other locations, the weather might be
rather Earth-like.

     In the months since the probe's descent, Galileo mission
scientists have debated whether the dry conditions it encountered
were due to the downdraft concept, or whether Jupiter's water had
somehow been concentrated deep in the gas planet's interior as it
formed and evolved four billion years ago.  "There was a cosmo-
chemical explanation and a meteorological explanation, and our
latest analysis clearly favors the idea that the dry spots are a
consequence of weather-related activity," Ingersoll said.

     "Fifty miles below the cloud tops, we could expect
thunderstorms, lightning and rain," Ingersoll added.  "But in
contrast to Earth, individual Jovian storms and weather systems
sometimes last for months, years or even centuries. The Great Red
Spot, for example, has existed for at least the 300 years that
we've been aware of it."

     Despite the relatively warm temperatures and the presence of
water on Jupiter, Ingersoll said it is "highly unlikely" that the
planet could sustain life in its thick, gaseous environment
without any solid surface.  He expressed the opinion that any
Jovian life forms would have to hover, and "while we might imagine
an advanced life form that could adapt, pre-biotic compounds would
not survive in that environment and, therefore, evolution could
not take place there."

     The latest data from Galileo also shed new light on the
auroras which glow in a narrow ring around the poles of Jupiter.
The auroras, essentially a light show stretching like a ribbon
around the planet, are created when charged particles collide with
atmospheric particles, causing them to light up.  The new images
show the nightside aurora for the first time.

     Dr. Scott Bolton of JPL, co-investigator for the Galileo
plasma and plasma wave instruments, said the latest findings show
"Jupiter's auroras are a lot like the auroras we see on Earth as
the northern lights. However, we now know that the auroral arc on
Jupiter is thin and patchy, and we can also estimate its altitude
is between 300 and 600 kilometers."

     These auroras had previously been viewed in ultraviolet light
by the Hubble Space Telescope, and in infrared light by Earth-
based telescopes, but Galileo was able to capture images of the
auroras in visible light and from a closer vantage point.

     Galileo was launched in 1989 and entered orbit around Jupiter
on Dec. 7, 1995.  The Galileo mission is managed by JPL for NASA's
Office of Space Science, Washington, DC.

     Images and other data received from Galileo are posted on the
Galileo home page on the World Wide Web at URL:

               http://www.jpl.nasa.gov/galileo