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 August
28, 2000 -- NASA researchers have the strongest evidence
yet that one of Jupiter's most mysterious moons hides a fermenting
ocean of water beneath its icy coat. This evidence comes
from magnetic readings by NASA's Galileo
spacecraft, reported in the Friday, August 25, edition of
the journal Science.
Europa, the fourth largest satellite of Jupiter, has long been
suspected of harboring vast quantities of water. Because life
as we know it requires water, this makes the moon a prime target
in the search for exobiology, or life beyond Earth.
Above: This false color Galileo image of Europa highlights
color differences in the moon's predominantly water-ice crust.
"The direction that a magnetic compass on Europa would point
to flips around in a way that's best explained by the presence
of an 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.
"We have good reason to believe that 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 layers.
"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
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
there.
Left: Fragmented chunks of ice
on Europa, similar in appearance to those seen in Earth polar
seas during a springtime thaw. [more
information]
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. "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 that there really
is a salty ocean," he said. "A definitive answer could
come from precise measurements of gravity and altitude to check
for the effects of tides."
NASA is planning a Europa Orbiter 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 currents that flow
through a conducting layer near Europa's surface. The currents
create a secondary magnetic field at Europa that the magnetometer
on Galileo is able to detect.
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 earlier 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.
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 the giant planet.
Galileo will be inside Jupiter's magnetic field while Cassini
is just outside it. Scientists plan to take advantage of that
positioning to learn more about how the solar wind affects the
giant planet's magnetic field.
Left: 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.
Galileo completed its original mission nearly three years ago,
but has been given a three-year extension. It has survived three
times the amount of radiation it was designed to endure.
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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, Pasadena. |
