Scientists Discover Ganymede Has A Lumpy Interior

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Carolina Martinez (818) 354-9382
Jet Propulsion Laboratory, Pasadena, Calif.

Donald Savage (202) 358-1727
NASA Headquarters, Washington

News Release: 2004-200 August 13, 2004

Scientists Discover Ganymede Has A Lumpy Interior

Scientists have discovered irregular lumps beneath the icy surface of
Jupiter's largest moon, Ganymede. These irregular masses may be rock
formations, supported by Ganymede's icy shell for billions of years.
This discovery comes nearly a year after the orchestrated demise of
NASA's Galileo spacecraft into Jupiter's atmosphere and more than
seven years after the data were collected.

Researchers at NASA's Jet Propulsion Laboratory, Pasadena, Calif., and
the University of California, Los Angeles, report their findings in a
paper that will appear in the Aug. 13 issue of the journal Science.

The findings have caused scientists to rethink what the interior of
Ganymede might contain.

The reported bulges reside in the interior, and there are no visible
surface features associated with them. This tells scientists that the
ice is probably strong enough, at least near the surface, to support
these possible rock masses from sinking to the bottom of the ice for
billions of years. But this anomaly could also be caused by piles of
rock at the bottom of the ice.

"The anomalies could be large concentrations of rock at or underneath
the ice surface. They could also be in a layer of mixed ice and rock
below the surface with variations in the amount of rock," said Dr.
John Anderson, a scientist and the paper's lead author at JPL. "If
there is a liquid water ocean inside Ganymede's outer ice layer there
might be variations in its depth with piles of rock at the ocean
bottom. There could be topographic variations in a hidden rocky
surface underlying a deep outer icy shell. There are many
possibilities, and we need to do more studies."

Dr. Gerald Schubert, co-author at UCLA, said "Although we don't yet
have anything definitive about the depth at this point, we did not
expect Ganymede's ice shell to be strong enough to support these lumpy
mass concentrations. Thus, we expect that the irregularities would be
close to the surface where the ice is coldest and strongest, or at the
bottom of the thick ice shell resting on the underlying rock. It would
really be a surprise if these masses were deep and in the middle of
the ice shell."

Ganymede has three main layers. A sphere of metallic iron at the
center (the core), a spherical shell of rock (mantle) surrounding the
core, and a spherical shell of mostly ice surrounding the rock shell
and the core. The ice shell on the outside is very thick, maybe 800
kilometers (497 miles) thick. The surface is the very top of the ice
shell. Though it is mostly ice, the ice shell might contain some rock
mixed in. Scientists believe there must be a fair amount of rock in
the ice near the surface. Variations in this amount of rock may be the
source of these possible rock formations.

Scientists stumbled on the results by studying Doppler measurements of
Ganymede's gravity field during Galileo's second flyby of the moon in
1996. Scientists were measuring the effect of the moon's gravity on
the spacecraft as it flew by. They found unexpected variations.

"Believe it or not, it took us this long to straighten out the anomaly
question, mostly because we were analyzing all 31 close flybys for all
four of Jupiter's large moons," said Anderson. "In the end, we
concluded that there is only one flyby, the second flyby of Ganymede,
where mass anomalies are evident."

Scientists have seen mass concentration anomalies on one other moon
before, Earth's, during the first lunar orbiter missions in the 1960s.
The lunar mass concentrations during the Apollo moon mission era were
due to lava in flat basins. However, scientists cannot draw any
similarities between these mass concentrations and what they see at
Ganymede.

"The fact that these mass anomalies can be detected with just flybys
is significant for future missions," said Dr. Torrence Johnson, former
Galileo project scientist. "With this type of information you could
make detailed gravity and altitude maps that allow us to actually map
structures within the ice crust or on the rocky surface. Knowing more
about the interior of Ganymede raises the level of importance of
looking for gravity anomalies around Jupiter's moons and gives us
something to look for. This might be something NASA's proposed Jupiter
Icy Moons Orbiter Mission could probe into deeper."

The paper was co-authored by Dr. Robert A. Jacobson and Eunice L. Lau
of JPL, with Dr. William B. Moore and Jennifer L. Palguta of UCLA.
JPL is a division of the California Institute of Technology in
Pasadena. JPL designed and built the Galileo orbiter, and operated the
mission.

For images and information about the Galileo mission, visit

http://galileo.jpl.nasa.gov/ .

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