Add Jupiter’s moon Ganymede, which is bigger than two of
the solar system’s nine planets, to the growing list of worlds
with evidence of liquid water under the surface.
A thick layer of melted, salty water somewhere beneath
Ganymede’s icy crust would be the best way to explain some of
the magnetic readings taken by NASA’s Galileo spacecraft during
close approaches to Ganymede in May 2000 and earlier, according
to one new report.
In addition, the types of minerals on parts of Ganymede’s
surface suggest that, in the past, salty water may have emerged
from below or melted at the surface, according to a study of
infrared reflectance measured by Galileo.
Third, new Galileo images of Ganymede hint how the water or
slushy ice may have surfaced through the fractured crust,
reminiscent of linear features on Europa, a neighboring moon
believed likely to have a deep ocean beneath its ice.
Several of the new images, prepared by researchers at Brown
University, Providence, R.I., and the German Aerospace Center
(DLR), Berlin, Germany, are available from NASA’s Jet Propulsion
Laboratory, Pasadena, Calif., at
http://www.jpl.nasa.gov/pictures/jovianmoons .
They include the most detailed photos ever taken of
Ganymede and an animated virtual flyover of an area where a
smooth, bright swath resembling parts of Europa cuts across
older, more heavily cratered terrain.
The new information about Ganymede is being presented at
the fall meeting of the American Geophysical Union, beginning
today (Dec. 15) in San Francisco. Ganymede is the biggest moon
in the solar system and bigger than the planets Mercury and
Pluto. It was named for a boy in Greek mythology who was so
beautiful that Jupiter, king of the gods, had him brought to
Olympus by an eagle.
The magnetic clues to a possible saltwater layer at
Ganymede are more complicated than earlier magnetic evidence of
hidden oceans on two other moons of Jupiter, Europa and
Callisto, said Dr. Margaret Kivelson, a planetary scientist at
the University of California, Los Angeles, and principal
investigator for Galileo’s magnetometer instrument. That’s
because Ganymede has a strong magnetic field of its own, instead
of just a secondary field induced by Jupiter’s magnetism.
But the indications of an induced field at Ganymede are
“highly suggestive” of a salty ocean on Ganymede, too, Kivelson
said. “It would need to be something more electrically
conductive than solid ice,” she said.
A melted layer several kilometers or miles thick, beginning
within 200 kilometers (120 miles) of Ganymede’s surface would
fit the data if it were about as salty as Earth’s oceans,
Kivelson said.
Ganymede is covered with lots of ice and frost, both in the
older, dark terrains and younger, bright terrains, said Dr.
Thomas McCord, a geophysicist at the University of Hawaii,
Honolulu, who has been using Galileo’s infrared spectrometer
instrument to identify surface materials on Ganymede. Portions
of the moon appear to have types of salt minerals that would
have been left behind by exposure of salty water near or onto
the surface, he said.
“They are similar to the hydrated salt minerals we see on
Europa, possibly the result of brine making its way to the
surface by eruptions or through cracks,” McCord said. The
infrared evidence does not indicate whether or not an ocean
persists at Ganymede today, he said.
Photos Galileo took as it passed within 809 kilometers (503
miles) of Ganymede on May 20 display details of a tumultuous
past, according to Drs. James Head III and Robert Pappalardo,
planetary scientists at Brown.
“Bright broken swaths, disrupted dark plains and the
astounding Arbela Sulcus suggest Ganymede may be more similar to
Europa than previously believed,” Pappalardo said. Arbela Sulcus
is a relatively smooth, bright band interrupting a more
cratered, older landscape. The new images show subtle striations
along its length. “It is possible that Arbela Sulcus has formed
by complete separation of Ganymede’s icy crust, like bands on
Europa, but unusual for Ganymede,” he said.
Natural radioactivity in Ganymede’s rocky interior should
provide enough heating to maintain a stable layer of liquid
water between two layers of ice, about 150 to 200 kilometers (90
to 120 miles) below the surface, said Dr. Dave Stevenson,
planetary scientist at the California Institute of Technology,
Pasadena. That’s a difference from Europa, where interior
flexing from tidal effects of Jupiter’s gravity provides much of
the internal heat, he said.
“I would have been surprised if Ganymede had not had an
ocean, but the issue of whether it’s there is different than the
issue of whether you can expect to see it clearly in the data,”
Stevenson said.
Galileo has been orbiting Jupiter since Dec. 7, 1995. It
will fly past Ganymede again on Dec. 28, but will not come as
close as it did in May. Additional information on Galileo is
available at http://galileo.jpl.nasa.gov . The Galileo mission
is managed for NASA’s Office of Space Science, Washington, D.C.
by JPL, a division of the California Institute of Technology.
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