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FOR IMMEDIATE RELEASE
August 25, 2000
GALILEO EVIDENCE POINTS TO POSSIBLE WATER WORLD UNDER EUROPA'S
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
"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.