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January 10, 2000
Galileo Findings Boost Idea of Other-Wordly 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 351 kilometers (218 miles) 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
It appears that the ocean lies beneath the surface somewhere in the outer
100 kilometers (60 miles), 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
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
The Galileo mission is managed for NASA's Office of Space Science,
Washington, D.C. by JPL, a division of the California Institute of
Technology, Pasadena, CA.