Douglas Isbell
Headquarters, Washington, DC                 January 22, 1996
(Phone:  202/358-1547)              Embargoed until 1 p.m. EST

David Morse
Ames Research Center, Mountain View, CA
(Phone:  415/604-4724)

RELEASE:  96-10

GALILEO PROBE SUGGESTS PLANETARY SCIENCE REAPPRAISAL

    Preliminary analysis of early data returned by NASA's
historic Galileo probe mission into Jupiter's atmosphere has
provided a series of startling discoveries for project scientists.

    Information on the extent of water and clouds and on the
chemical composition of the Jovian atmosphere is particularly
revealing.  Probe instruments found the entry region of Jupiter
to be drier than anticipated, and they did not detect the
three-tiered cloud structure that most researchers had
postulated.  The amount of helium measured was about one-half
of what was expected.

    These initial findings are encouraging scientists to
rethink their theories of Jupiter's formation and the nature of
planetary evolution processes, according to probe project
scientist Dr. Richard Young of NASA's Ames Research Center,
Mountain View, CA.

    "The quality of the Galileo probe data exceeds all of our
most optimistic predictions," said Dr. Wesley Huntress, NASA
Associate Administrator for Space Science.  "It will allow the
scientific community to develop valuable new insights into the
formation and evolution of our solar system, and the origins of
life within it."

    The probe made the most difficult planetary atmospheric
entry ever attempted, according to probe manager Marcie Smith
of NASA Ames.  Entering Jupiter's atmosphere on Dec. 7, 1995,
it survived entry speeds of over 106,000 mph, temperatures 
twice those on the surface of the Sun and deceleration forces 
up to 230 times the strength of gravity on Earth.  It relayed 
data obtained during its 57-minute descent mission back to the 
Galileo orbiter more than 130,000 miles overhead for storage 
and transmission to Earth.  The orbiter is now embarking on a 
two-year mission to study Jupiter and its moons.

    "The probe detected extremely strong winds and very intense
turbulence during its descent through Jupiter's thick
atmosphere.  This provides evidence that the energy source
driving much of Jupiter's distinctive circulation phenomena is
probably heat escaping from the deep interior of the planet,"
Young said.  "The probe also discovered an intense new
radiation belt approximately 31,000 miles above Jupiter's cloud
tops, and a veritable absence of lightning," he noted.

    The composition of Jupiter's atmosphere offered some
surprises, according to project scientists.  It contains
significantly lower than expected levels of helium, neon, and
certain heavy elements, such as carbon, oxygen and sulfur.

    The issue of the colors of Jupiter's atmosphere has been
much-debated, but no consensus has developed from probe data to
date.  The probe encountered no solid objects or surfaces
during its entire 373-mile (600 km) journey.  This was as
expected for a gas-giant planet such as Jupiter.

    What are the implications of these findings?  Most
scientists believe that Jupiter has a bulk composition similar
to that of the gas and dust cloud of the primitive solar nebula
from which the planets and our Sun were formed, with added
heavy elements from comets and meteorites.  The probe's
measurements may necessitate a re-evaluation of existing views
of how Jupiter evolved from the solar nebula.  For example, the
lower-than-expected helium and neon levels on Jupiter relative
to the Sun influence scientific understanding of the process of
fractionation, the "raining out" of helium and neon during
planetary evolution.

    During the probe's high-speed, atmospheric-entry phase,
deceleration measurements high in the atmosphere showed
atmospheric density to be much greater than expected.
Corresponding temperatures were also much higher than
predicted.  The high temperatures appear to require an
unidentified heating mechanism for this region of the atmosphere.

    Following probe parachute deployment, six science
instruments on the probe collected data throughout 97 miles
(156 km) of the descent.  During that time, the probe endured
severe winds, periods of intense cold and heat and strong
turbulence.  The extreme temperatures and pressures of the
Jovian environment eventually caused the probe communications
subsystem to terminate data transmission operations.

    Earth-based telescopic observations suggest that the probe
entry site may well have been one of the least cloudy areas on
Jupiter.  At this location, the probe did not detect the three
distinct layers of clouds (a topmost layer of ammonia crystals,
a middle layer of ammonium hydrosulfide, and a final, thick
layer of water and ice crystals) that researchers had anticipated.

    Some indication of a high-level ammonia ice cloud was
detected by the net flux radiometer.  Evidence for a thin cloud
which might be the postulated ammonium hydrosulfide cloud was
provided by the nephelometer experiment.  There was no data to
suggest the presence of water clouds of any significance.  The
vertical temperature gradient obtained by the atmospheric
structure instrument was characteristic of a dry atmosphere,
free of condensation.  Only the one, distinctive cloud
structure was identified, and that was of modest proportion.

    The latest analyses of data from the Voyager spacecraft
that flew by Jupiter in 1979 have suggested a water abundance
for the planet of twice the solar level (based on the Sun's
oxygen content).  Observations of the propagation of
atmospheric waves across Jupiter's cloud tops from the Comet
Shoemaker-Levy 9 impacts implied that Jupiter might have a
water content of ten times the solar level.  Actual probe
measurements, while subject to scientific debate, suggest a
level near that of the Sun.  Scientists are left to wonder,
"where is the oxygen?," "where is the water?," and to
reconsider their interpretation of the S-L 9 impacts.

    Scientists had expected to find severe winds on Jupiter
ranging up to 220 mph.  However, the probe appears to have
detected winds far greater, perhaps up to 330 mph.  The winds
remained fairly constant as the probe descended deep into the
Jovian atmosphere.  This suggests that Jupiter's winds are not
caused by differential sunlight at the equator versus the poles
or by heat released by water condensation as on Earth,
according to project scientists.

    "The origin of Jupiter's winds appears to be the internal
heat source which radiates energy up into the atmosphere from
the planet's deep interior," Young said.  "This impacts Jupiter's
climate and circulation patterns, and suggests a jet stream-like
mechanism rather than swirling hurricane or tornado-like storms."

    The probe found that lightning occurs on Jupiter only about
one-tenth as often as on Earth.  This is puzzling, but
consistent with the absence of water clouds.  A virtual absence
of lightning reduces the probability of finding complex organic
molecules in Jupiter's atmosphere, particularly given its
hostile, predominantly hydrogen composition.

    Scientists caution that results obtained to date, while
dramatic and exciting, are only preliminary and subject to much
further analysis and refinement.  Data transmission problems
associated with solar conjunction between the Earth and
Jupiter, the need to refine estimates based on probe and
orbiter trajectories, the presence of higher than anticipated
instrument temperatures, and the need for improved calibration
all require a cautious approach to these early findings.

    Additional information will be forthcoming over the next
few months as scientists continue to evaluate the wealth of
data obtained by the probe and to cross-compare results of
individual scientific instruments.  Further information and
images about the Galileo mission to Jupiter can be accessed on
the Internet through the following three URLs:

              http://ccf.arc.nasa.gov/dx
              http://ccf.arc.nasa.gov/galileo_probe/
              http://www.jpl.nasa.gov/galileo

    The Galileo probe project is managed by NASA's Ames
Research Center, Mountain View, CA.  Hughes Aircraft Co., El
Segundo, CA, designed and built the probe; General Electric,
Philadelphia, PA, built the probe's heat shield.  NASA's Jet
Propulsion Laboratory, Pasadena, CA, built the Galileo orbiter
spacecraft and manages the overall mission.

                            -end-