Surf's Up on Europa?

Jan 10, 2000: 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 Galileo flew just 351 kilometers (218 miles) above the icy moon on January 3, its magnetometer detected directional changes in Europa's magnetic field. Such changes are consistent with fluctuations that could occur if Europa contains a shell of electrically conducting material, such as a salty, liquid ocean.

Right: Fragmented chunks of ice on Europa, similar in appearance to those seen in Earth's polar seas during a springtime thaw. Measurements of Europa's magnetic field, obtained during a close flyby on January 3, 2000, suggest that there may be a salty ocean underneath this jumbled, icy terrain. [more information]

"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 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."

Parents and Educators: Please visit Thursday's Classroom for lesson plans and activities related to this story.

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.

Right: Two models consistent with images of Europa's surface include a subsurface layer of liquid water or perhaps warmer, convecting ice. Image credit: JPL and the SETI Institute. [more information]

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.

Left: Europa might not be the only moon of Jupiter with a salty ocean beneath the surface. Magnetic measurements made during Galileo encounters with Callisto also reveal a variable magnetic field. A plausible explanation is that Callisto has a subsurface liquid layer. If the liquid were salt water it could easily carry electrical currents and produce the changing magnetic field. This cutaway view of Callisto shows a whitish 200 kilometer thick band of ice just beneath the moon's surface. The hypothetical ocean - indicated by the underlying light blue stripe - is potentially a salty layer of liquid water up to 10 kilometers thick, while the rest of the interior is seen as a jumble of rock and ice.

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

Galileo has been orbiting Jupiter and its moons since December 1995, beaming to Earth unprecedented images and other information. JPL, a division of the California Institute of Technology, Pasadena, CA, manages the Galileo mission for NASA's Office of Space Science, Washington, DC.

Web Links

Galileo Mission Home Page -from NASA's Jet Propulsion Laboratory

Divining Water on Europa -Science@NASA feature story about the possibility of water on Jupiter's icy moon

Thursday's Classroom -educational lesson plans and classroom activities about Europa and life in extreme environments.

Topics:

Surf's Up on Europa?

Science News