U.S. Water News Online
LOS ANGELES, Calif. -- NASA researchers have the strongest
evidence yet that one of Jupiter's most mysterious moons hides a
fermenting 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, one of five co- authors at
the University of California, Los Angeles (UCLA).
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 the 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
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, CA.
"This magnetometer data is the only indication we have that there's
an ocean there now, rather than in the geological past," Johnson
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.
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