PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
Contact: Franklin O'Donnell
FOR IMMEDIATE RELEASEJanuary 22, 1996
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
Information on the extent of water and clouds and on the
chemical composition of the Jovian atmosphere is particularly
revealing. Instruments on the descent probe 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.
"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 descent probe made the most difficult planetary
atmospheric entry ever attempted, according to probe manager
Marcie Smith of the Ames center. 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 main Galileo spacecraft more than 130,000
miles overhead for storage and transmission to Earth. The
Galileo 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
it's entire 600-kilometer 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 descent probe's measurements may necessitate a
reevaluation 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 156 kilometers
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 were 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
The latest analyses of data from the Voyager spacecraft that
flew by Jupiter in 1979 had suggested a water abundance for the
planet of twice the solar level (based on the Sun's oxygen
content). Atmospheric wave propagation data across cloud tops
from an analysis of 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 the oxygen and water are, and to reconsider
their interpretation of the comet 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. We know this because the winds extend
well below the level to which sunlight reaches," 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 atmospheric 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 addresses:
The Galileo Project is managed by NASA's Jet Propulsion
Laboratory, Pasadena, CA, which also built the Galileo orbiter.
The atmospheric probe 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.