January 2000
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GALILEO FINDINGS BOOST IDEA OF OTHER-WORLDLY OCEAN

     When NASA's Galileo spacecraft swooped past Jupiter's moon
Europa a week ago, it picked up powerful new evidence that a liquid
ocean lies beneath Europa's icy crust.

      As the spacecraft flew 218 miles (351 kilometers) above the
icy moon on January 3, its magnetometer instrument studied changes
in the direction of Europa's magnetic field.  Galileo's
magnetometer observed directional changes consistent with the type
that would occur if Europa contained a shell of electrically
conducting material, such as a salty, liquid ocean.

     "I think these findings tell us that there is indeed a layer
of liquid water beneath Europa's surface," said Dr. Margaret
Kivelson, principal investigator for the magnetometer.  "I'm
cautious by nature, but this new evidence certainly makes the
argument for the presence of an ocean far more persuasive."

    It appears that the ocean lies beneath the surface somewhere in
the outer 60 miles (about 100 kilometers), the approximate
thickness of the ice/water layer, according to Kivelson, a
researcher at the University of California, Los Angeles (UCLA).

     "Jupiter's magnetic field at Europa's position changes
direction every 5-1/2 hours," Kivelson explained.  "This changing
magnetic field can drive electrical currents in a conductor, such
as an ocean.  Those currents produce a field similar to Earth's
magnetic field, but with its magnetic north pole -- the location
toward which a compass on Europa would point -- near Europa's
equator and constantly moving.  In fact, it is actually reversing
direction entirely every 5-1/2 hours."

     On previous Europa flybys, Galileo identified a magnetic north
pole, but did not determine whether its position changes with time.
"We wondered, 'Was it possible that the north pole did not move?' "
Kivelson said.

     The new evidence was gathered during a flyby specially
planned so that the observed position of Europa's north pole would
make it clear whether or not it moves.  In fact, Monday's data
showed that its position had moved, thus providing key evidence
for the existence of an ocean.

     It is not likely that the electric currents on Europa flow
through solid surface ice, Kivelson explained, because ice is not a
good carrier of currents.  "But melted ice containing salts, like
the sea water found on Earth, is a fairly good conductor," she
said.

     There is no other likely current-carrying material near
Europa's surface, Kivelson added.  "Currents could flow in
partially melted ice beneath Europa's surface, but that makes
little sense, since Europa is hotter toward its interior, so it's
more likely the ice would melt completely.  In addition, as you get
deeper toward the interior, the strength of the current-generated
magnetic field at the surface would decrease."

      These latest findings are consistent with previous Galileo
images and data showing a tortured surface seemingly formed when
Europa's surface ice broke and rearranged itself while floating on
a sea below. Further theoretical work is under way to analyze the
fluid layer and its properties.
 
     "It will be interesting to see whether this same type of
phenomenon occurs at Jupiter's moon Ganymede," Kivelson said.
Galileo is tentatively scheduled to fly by Ganymede twice this
year.

     Kivelson is joined in her magnetometer studies by Drs. Krishan
Khurana, Christopher Russell, Raymond Walker, Christophe Zimmer,
Martin Volwerk of UCLA, as well as Steven Joy and Joe Mafi, also of
UCLA, and Dr. Carole Polanskey of NASA's Jet Propulsion Laboratory
(JPL), Pasadena, CA.

     Additional information and pictures taken by Galileo are
available at

                    http://galileo.jpl.nasa.gov
 
 

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