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NASA researchers have the strongest evidence yet that one of Jupiter's most mysterious moons hides an ocean of water underneath its icy coat. This evidence comes from magnetic readings by NASA's Galileo spacecraft, reported in the Friday, Aug. 25, edition of the journal Science.
Europa, the fourth largest satellite of Jupiter, has long been suspected of harboring vast quantities of water. Since life as we know it requires water, this makes the moon a prime target for the search of exobiology - life beyond Earth.
"The direction that a magnetic compass on Europa would point to flips around in a way that's best explained by the presence of a layer of electrically conducting liquid, such as saltwater, beneath the ice," explained Dr. Margaret Kivelson of the University of California, Los Angeles (UCLA), principal investigator for Galileo's magnetometer instrument, and co-author of the Science paper.
Kivelson announced that conclusion when she first received telltale readings from the Galileo magnetometer after the veteran spacecraft flew near Europa in January. Her team details its theory about the liquid layer in this week's formal report.
"We have good reason to believe the surface layers of Europa are made up of water that is either frozen or liquid," Kivelson said, pointing out that earlier gravity measurements show a low density, such as water's, for the moon's outer portions. "But ice is not a good conductor, and therefore we infer that the conductor may be a liquid ocean."
Galileo has flown near Europa frequently since the spacecraft began orbiting Jupiter and its moons in December 1995. Pictures from those flybys show patterns that scientists see as evidence of a hidden ocean. In some, rafts of ice appear to have shifted position by floating on fluid below. In others, fluid appears to have risen to the surface and frozen.
However, those features could be explained by a past ocean that has subsequently frozen solid, said Galileo's project scientist, Dr. Torrence Johnson of NASA's Jet Propulsion Laboratory, Pasadena, Calif. This magnetometer data is the only indication we have that there's an ocean there now, rather than in the geological past," Johnson said.
Johnson said the case for liquid water on Europa is still not clinched. "The evidence is still indirect and requires several steps of inference to get to the conclusion there is really a salty ocean," he said. "A definitive answer could come from precise measurements of gravity and altitude to check for effects of tides."
NASA is planning a Europa Oribiter mission to carry instruments capable of providing that information. Magnetic evidence for an ocean is possible because Europa orbits within the magnetic field of Jupiter. That field induces electric current to flow through a conductive layer near Europa's surface, and the current creates a secondary magnetic field at Europa, the new report explains.
Key evidence that the magnetic readings near Europa result from this type of secondary effect, implying a saltwater layer, relies on timing. The direction of Jupiter's magnetic field at Europa reverses predictably as the moon's position within the field changes. During Galileo's flyby in January, the direction of Jupiter's field at Europa was the opposite of what it had been during passes in 1996 and 1998. Kivelson's team predicted how that would change the direction of Europa's magnetic polarity if Europa has a saltwater layer, and Galileo's measurements matched their prediction.
"It makes a very strong case that the source of the magnetic signature is a conducting layer near the surface," Kivelson said. Galileo's magnetometer is also expected to play an important role this fall and winter in joint studies of Jupiter while NASA's Saturn-bound Cassini spacecraft passes near Jupiter. Galileo will be inside Jupiter's magnetic field while Cassini is just outside it, in the solar wind of particles streaming away from the Sun. Scientists plan to take advantage of that positioning to learn more about how the solar wind affects the magnetic field.
Galileo completed its original mission nearly three years ago, but has been given a three-year extension and has survived three times the amount of radiation it was designed to endure.
Kivelson's UCLA co-authors are Drs. Krishan Khurana, Christopher Russell, Martin Volwerk, Raymond Walker, and Christopher Zimmer. The Galileo mission is managed for NASA's Office of Space Science, Washington, DC, by JPL, a division of the California Institute of Technology in Pasadena.