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-