New
Evidence for an Alien Ocean
from science@NASA
Fluctuations in the magnetic
field surrounding Jupiter's moon Europa are a telltale
sign of salty liquid water beneath the moon's icy
crust. Europa could harbor the solar system's largest
ocean.
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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.
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.
Related Links
Galileo
Home Page - from the NASA Jet Propulsion
Laboratory
Divining
Water on Europa -- Sept 9, 1999, Science@NASA
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