Galileo at Europa

NASA Press release 00-131

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 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, CA. “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

“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, Pasadena.

– end –

Related Links

° 25 August 2000:

Galileo Magnetometer Measurements: A Stronger Case for a Subsurface Ocean at Europa, Science, [summary – can be viewed for free once registered. A subscription fee is required for full access.]

“On 3 January 2000, the Galileo spacecraft passed close to Europa when it was located far south of Jupiter’s magnetic equator in a region where the radial component of the magnetospheric magnetic field points inward toward Jupiter. The Galileo magnetometer measured changes in the magnetic field predicted if a current-carrying outer shell, such as a planet-scale liquid ocean, is present beneath the icy surface. The evidence that Europa’s field varies temporally strengthens the argument that a liquid ocean exists beneath the present-day surface. “

° 25 August 2000: Europa’s Ocean–the Case Strengthens, Science, [summary – can be viewed for free once registered. A subscription fee is required for full access.]

“The possibility of a subsurface ocean on Jupiter’s moon Europa has been suggested on the basis of theoretical, geological,
and spectroscopic arguments. But, as Stevenson explains in his Perspective, none of these arguments were compelling. In
contrast, the magnetic field data obtained by the Galileo spacecraft and presented in the report by Kivelson et al., provide
persuasive evidence for a conducting layer–most likely a global water ocean–near Europa’s surface. “

Background Information

° Research papers on “Europa” at NASA’s Astrophysics Data System (ADS)

° Galileo Provides Further Evidence of an Ocean on Europa, SpaceRef

° Galileo Findings Boost Idea of Other-Worldy Ocean, NASA press release

° NASA Releases High Resolution Images of Io and Europa, SpaceRef

° Galileo Swoops Flies Past Europa; Galileo Project to be Extended, SpaceRef

° Galileo Mission Extended, NASA JPL

° Galileo Website, NASA JPL

° Europa Orbiter Mission

° A Science Strategy for the Exploration of Europa Committee on Planetary and Lunar Exploration, National Research Council, 1999.