Douglas Isbell
Headquarters, Washington, DC                    March 18, 1996
(Phone:  202/358-1753)

Ann Hutchison
Ames Research Center, Mountain View, CA
(Phone: 415/604-4968)

RELEASE:  96-54

GALILEO SCIENTISTS REPORT CHANGING FINDINGS ABOUT JUPITER

    Scientists continuing to analyze information returned by
the Galileo atmospheric probe that plunged into Jupiter last
December report more surprises about the giant gas planet.

    Most significantly, the ratio of the elements that make up
99 percent of the Jovian atmosphere -- helium and hydrogen --
now closely matches that found in the Sun, suggesting that
Jupiter's bulk composition has not changed since the planet
formed several billion years ago.  Estimated amounts of key
heavy elements such as carbon and sulfur have increased, but
minimal organic compounds were detected, and estimates for
Jupiter's wind speeds have climbed still higher.

    Probe scientists are reporting these refined results today
at the Lunar and Planetary Science Conference in Houston, TX.

    The ratio of helium to hydrogen by mass is key to
developing theories of planetary evolution.  In the Sun, this
value is about 25 percent.  During a January 1996 press
conference, Galileo probe scientists estimated that this number
for Jupiter was 14 percent.  More comprehensive analysis of
results from the probe's helium abundance detector has raised
this estimate for Jupiter to 24 percent.

    "This increase implies that the amount of helium in the
Jovian atmosphere is close to the original amount that Jupiter
gathered as it formed from the primitive solar nebula that
spawned the planets," according to Galileo probe project
scientist Dr. Richard Young of NASA's Ames Research Center,
Mountain View, CA.

    "The revised helium abundance also indicates that
gravitational settling of helium toward the interior of Jupiter
has not occurred nearly as fast as it apparently has on Saturn,
where the approximate helium-to-hydrogen ratio is just six
percent," said Young.

    "This then confirms that Jupiter is much hotter in its
interior than its neighbor Saturn, the next largest planet in
the Solar System.  It also may force scientists to revise their
projections for the size of the rocky core believed to exist
deep in the center of Jupiter," he said.

    The new estimate of the helium-to-hydrogen ratio on Jupiter
is supported by analysis of complementary data from the Galileo
probe's neutral mass spectrometer.

    These new helium results are raising related estimates for
the abundances of other key compounds, such as methane.
Several heavy elements, including carbon, nitrogen and sulfur,
are significantly greater in abundance on Jupiter than in the
Sun.  "This implies that the influx of meteorites and other
small bodies into Jupiter over the eons since its formation has
played an important role in how Jupiter has evolved," said Young.

    However, minimal organic compounds were detected,
indicating that such complex combinations of carbon and
hydrogen are rare on Jupiter and that the chances of finding
biological activity on Jupiter similar to that found on Earth
are extremely remote.

    The strong Jovian atmospheric winds continue to exceed
expectations.  Wind speed estimates announced in January of up
to 330 mph have grown to more than 400 mph.  The winds
persisted far below the one cloud layer detected, strongly
suggesting that heat escaping from deep in the planet's
interior drives the winds, rather than solar heating.  Since
all the outer giant planets exhibit strong winds, scientists
hope that understanding Jupiter's winds will lead to important
new insights into their unusual meteorology, Young said.

    The scientists continue to report that the probe apparently
entered Jupiter's atmosphere near the southern edge of a so-
called infrared hot spot, which is believed to be a region of
reduced clouds. "The probe's nephelometer observed only one
distinct cloud layer, and it is tenuous by Earth standards.  It
is likely to be an ammonium hydrosulfide cloud," said Young.
Three distinct cloud layers (an upper layer of ammonia
crystals, a middle layer of ammonium hydrosulfide, and a thick
bottom layer of water and ice crystals) were expected.

    Further analysis of probe data has confirmed the
preliminary report that the Jovian atmosphere appears to be
relatively dry, with much less water than anticipated on the
basis of solar composition and predictions from data sent by
the Voyager spacecraft that flew by Jupiter in 1979.   These
studies predicted a water abundance for the planet of twice the
solar level (based on the Sun's oxygen content.)  Actual probe
measurements now suggest an amount of water less than that of
the Sun.

    Scientists confirmed that the probe's instruments found
much less lightning activity on Jupiter per unit area than on
Earth.  Lightning on Jupiter was found to be about 1/10th of
that found on Earth in an area of the same size.  "Although we
found much less lightning activity, the individual lightning
events are about ten times more energetic than similar events
on Earth," Young said.

    "This is the sort of unique and exciting information that
could not have been obtained in any way other than an
atmospheric entry probe," Young said.  Complete detailed
results of the Galileo probe data analysis will be reported in
the May 10 issue of Science  magazine.

    The cone-shaped Galileo probe entered the atmosphere of
Jupiter on Dec. 7, 1995, at a speed of over 106,000 mph and
survived deceleration forces of 228 times Earth's gravity.
After deploying a parachute, it relayed data to the Galileo
orbiter spacecraft overhead for 57 minutes.

    The Galileo orbiter is beginning a two-year, 11-orbit tour
of Jupiter and will have its first major encounter with a
Jovian moon on June 27 when it flies closely by Ganymede.  The
orbiter successfully conducted a key engine burn on March 14 to
prepare for this encounter.

    The Galileo probe project is managed by Ames.  Hughes Space
and Communications Co., El Segundo, CA, designed and built the
probe.  Lockheed Martin Hypersonic Systems (formerly General
Electric), Philadelphia, built the probe's heat shield.  NASA's
Jet Propulsion Laboratory, Pasadena, CA, built the Galileo
orbiter spacecraft and manages the overall mission.

                            -end-