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.

  • 13 August 2004: Discovery of Mass Anomalies on Ganymede, 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/ .