Douglas Isbell
Headquarters, Washington, DC                   May 21, 1996
(Phone:  202/358-1753)

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



RELEASE:  96-103

GALILEO PROBE DATA SPURS NEW CONCEPTS 
FOR JUPITER'S CIRCULATION AND FORMATION

    Measurements returned by NASA's Galileo probe into Jupiter 
have provided dramatic new evidence about circulation 
processes within the planet's atmosphere and prompted 
scientists to propose radical new theories about Jupiter's 
original formation.

    The new concepts arise from the probe's successful 
parachute-borne descent into Jupiter on Dec. 7, 1995.  The 
probe made the first quantitative measurements of the Jovian 
atmosphere below its outer clouds, reaching a region below 
where heat from the Sun can penetrate.  This means the probe 
sampled the upper part of what is believed to be Jupiters 
well-mixed, relatively uniform interior atmosphere.

    Several members of the probe scientific team announced new 
mid-term findings today at a meeting of the American 
Geophysical Union in Baltimore.  "The returns from the probe's 
scientific instruments have sparked a lively worldwide 
scientific debate about theories of planetary formation and 
about internal mechanisms in the huge Jovian atmosphere," 
according to 
Dr. Richard Young, Galileo probe scientist at NASA's Ames 
Research Center, Mountain View, CA.  

    Prior to the probe mission, the leading theory of Jovian 
weather assumed that, like on Earth, most action occurs in the 
thin, cloudy, solar-heated exterior region -- the so-called 
"skin of the apple."  Winds within Earth's 100-mile-deep 
atmosphere are primarily the result of differential sunlight 
at the poles versus the equator, and heat released due to 
water condensation.  

    According to mission scientists, Galileo probe data 
strongly suggest that circulation patterns in Jupiter's cloud 
tops and its interior (which runs 10,000 miles deep) are part 
of one continuous process.  Dr. David Atkinson of the 
University of Idaho continues to report persistent Jovian wind 
velocities of over 400 mph.  The probe detected no reduction 
in wind speed, even at its deepest levels of measurement, 
approximately 100 miles below Jupiter's clouds.

    Galileo scientists regard this finding as confirmation 
that the main driving force of Jupiter's winds is internal 
heat radiating upward from the planet's deep interior.  The 
strength of the Jovian winds and the fact that they do not 
subside with depth is very significant, according to Dr. 
Andrew Ingersoll of the California Institute of Technology, 
Pasadena, CA.

    "This may be evidence that Jupiter has high-speed wind 
currents extending thousands of miles deep into its hot, dense 
atmosphere," Ingersoll said.  Such interior currents are 
believed by probe scientists to be the source of the dramatic 
banded appearance of Jupiter's cloud tops.

    The most difficult probe finding for scientists to explain 
continues to be the extreme lack of water detected in the 
Jovian atmosphere.  Pre-probe mission scientific estimates 
based on planetary formation theories, data from the earlier 
NASA Voyager spacecraft flybys of Jupiter and observations 
from the impacts of the fragments of Comet Shoemaker-Levy 9 
with Jupiter forecast Jovian water levels at or well above 
those found in the Sun.  However, probe scientists report that 
Jupiter is extremely dry -- with water levels (based on oxygen 
content) at one-fifth to one-tenth of the solar amount.

    This finding is now well established, having been 
confirmed by analysis of data from five of the probe's science 
instruments.  For example, the virtual absence of Jovian water 
clouds and the low relative frequency of lightning are all 
consistent with dry atmospheric conditions.  Where is the 
water that should remain from Jupiter's formation in the same 
primitive nebula of gas and dust that spawned the Sun and the 
other planets?  Several theories have been proposed.

    According to one theory, Jupiter's true total water levels 
are probably at or above solar, with the bulk of Jovian water 
trapped in the planet's deep interior.  According to this 
view, Jupiter began as a solid, rocky/icy proto-planet that 
grew to 8-10 times the mass of the Earth by gathering up ice 
grains and dust in the original primordial cloud.  This 
process may well have concentrated water ice in the solid 
body, trapping it in the core while drying out surrounding 
regions.

    As the solid body of the proto-Jupiter became larger, it 
attracted the already-dried-out surrounding lighter gases, 
mixing them with its existing atmosphere.  This atmosphere 
would contain carbon and other gases that were originally 
locked in the core but had escaped as methane, ammonia, 
hydrogen sulfide and other volatiles as the core heated up.  
This process would produce a gas mixture similar to that found 
by the Galileo probe.  It also would explain the enhanced 
carbon, sulfur and nitrogen levels found on Jupiter, which are 
significantly enriched relative to their abundance on the Sun.

    In fact, this water-locked-in-the-Jovian-interior theory 
explains many of the measurements made by the probe.  However, 
"there are problems with this new view, as there are with all 
the other current theories," said Dr. Tobias Owen of the 
University of Hawaii.  "The primary one being, how does the 
ice stay in the hot planetary core while carbon-containing 
gases escape?"

    An alternative theory suggests that the probe entered the 
Jovian atmosphere in an area comparable to the Earth's desert 
regions.  This theory is supported by Earth-based telescopes 
and other spacecraft that observed extreme dryness at the 
probe's entry point on Jupiter's north equatorial belt.  This 
theory holds that, like on Earth, Jupiter's atmosphere is 
heated by the Sun at the equator, causing air to rise until 
clouds form and water is lost.  The dry air then may flow 
north and south, descending in "desert" regions.  If a large 
enough downdraft exists, it might be sufficient to explain the 
dryness that the Galileo probe encountered.

    However, several scientists find fault with this "huge 
downdraft" theory, doubting that such a massive downdraft and 
continued dryness could exist at the depth and pressure levels 
to which the probe descended.  While such a downdraft might 
explain the observed dryness, its persistence down to 20 times 
Earth's atmospheric pressure is very hard to explain, 
according to Ingersoll.

    "This explanation is particularly difficult when 
considering that Jupiter emits more heat from its interior 
than it receives from the Sun," he said.  "This up-flowing 
interior heat should block a huge, deep downflow of dry air.  
It should evenly mix Jupiter's atmospheric water vapor at this 
pressure level, preventing the existence of a very dry region 
such as that found by the probe."

    One possibility, Owen responds, is that "perhaps Jupiter's 
interior heat comes out only in certain regions where 
ascending currents bring up hot material from the planet's 
interior, like the heat escaping from the Earth's interior" in 
volcanoes and mid-ocean floor spreading zones.

    A variation on the dry-region theory has been advanced by 
Young and others.  "Jovian water distribution may vary 
radically over large latitude regions, with much of Jupiter's 
water being concentrated at high latitudes where most of the 
planet's lightning has been detected," he said.   "More of 
Jupiter's interior heat is also emitted at high latitudes.  
Unfortunately, at the moment, we can't put all of this into a 
mechanism to explain how major parts of Jovian water could be 
concentrated uniquely at these high latitudes."

    The Galileo probe successfully accomplished the most 
difficult planetary atmospheric entry ever attempted.  It 
relayed a total of  61 minutes of unique science data to the 
Galileo orbiter passing 100,000 miles overhead for subsequent 
transmission to Earth.  The probe descended about 400 miles 
into the Jovian atmosphere, taking  measurements down to a 
level corresponding to 20 times Earth's atmospheric pressure.  
The Galileo orbiter has since embarked on a two-year tour of 
Jupiter and its moons.

    Additional information on the Galileo probe, including a 
discussion of the craft's science instruments and a non-
technical summary of the first scientific papers on the probe 
mission that were published in the May 10 issue of Science 
magazine, can be found on the Internet at the following URL:

        http://ccf.arc.nasa.gov/galileo_probe/

    The Galileo probe is managed by NASA's Ames Research 
Center, Mountain View, CA.  Hughes Space and Communications 
Co., El Segundo, CA, designed and built the probe.  Lockheed 
Martin Hypersonic Systems (formerly 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 -


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