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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:
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.