A recent image from NASA’s Galileo spacecraft adds
evidence to a theory that Callisto, the outermost of Jupiter’s
four large moons, may hold an underground ocean.

The image shows a part of Callisto’s surface directly
opposite from the Valhalla basin where Callisto was punched by
a major collision. The opposition point shows no effect from
the impact. Points opposite major impact features on some
similar-size worlds, such as Mercury and Earth’s Moon, show
lumpy terrain attributed to seismic shocks from the distant
impacts.

The new image is consistent with a 1990s model proposing
that a liquid layer could be acting as a shock absorber inside
Callisto, said planetary geologist Dr. David A. Williams of
Arizona State University, Tempe.

“Although there is a lot of uncertainty in the computer
modeling of Callisto, it’s good that this image supports the
hypothesis presented a decade ago. But it’s not a smoking gun,
and a lot more evidence needs to be uncovered before we will
know for sure whether Callisto has a subsurface ocean,”
Williams said.

“Galileo has given us indications, primarily from
magnetometer data, of the possibility that Europa, Ganymede
and Callisto — three of Jupiter’s four large moons — have
liquid-water layers,” said Dr. Torrence Johnson, project
scientist for Galileo at NASA’s Jet Propulsion Laboratory,
Pasadena, Calif.

“Liquid water is of interest not only for what it may
tell us about the evolution of these bodies, but also for
biological implications,” Johnson said. Life relies on liquid
water, but an ocean on Callisto would not draw as much
interest in a search for life as one on Europa. An ocean on
Callisto would be much farther below the surface than Europa’s
ocean. It would also be trapped between two layers of ice
rather than sitting on top of a warm rocky layer, as models
suggest for Europa.

Images taken of Valhalla’s opposite point, or antipode,
during a May 25, 2001, flyby of Callisto by Galileo, show the
same type of cratered surface seen all over Callisto. In
contrast, regions opposite large impact basins on the Moon and
Mercury have grooved and hilly features known as “antipodal
terrains” and attributed to shocks from the impacts.

“The Valhalla antipodal region on Callisto is cratered,
but definitely not grooved and hilly,” Williams said. He is
processing and analyzing the Galileo Callisto imagery with
James E. Klemaszewski and Dr. Ronald Greeley, also of Arizona
State University. Williams presented a preliminary analysis
today at the annual meeting of the American Astronomical
Society’s Division for Planetary Sciences, being held in New
Orleans.

Earlier computer modeling of Callisto by Greeley and his
student Allison Watts suggested that if Callisto had a liquid
water layer in its interior, this layer would have dispersed
the seismic shock waves from the ancient Valhalla impact.
These shock waves might otherwise have produced grooved and
hilly terrain at the antipode.

Callisto is about the same size as Mercury. Its surface
of ice and rock is the most heavily cratered of any moon in
the solar system, signifying that it is geologically “dead.”
There is no clear evidence that Callisto has experienced the
volcanic activity or tectonic shifting that have erased some
or all of the impact craters on Jupiter’s other three large
moons. So, if the Valhalla impact billions of years ago had
reshaped the landscape on the opposite side of Callisto, those
effects would likely still be detectable.

NASA’s Jet Propulsion Laboratory, a division of the
California Institute of Technology in Pasadena, manages
Galileo for NASA’s Office of Space Science, Washington, D.C.
Additional information about the mission is available online
at: http://galileo.jpl.nasa.gov .

The new Callisto image is available online at
http://www.jpl.nasa.gov/images/callisto .

Close-up–High Resolution JPEG
Close-up–Full Resolution TIFF
Full image–High Resolution JPEG

The heavily cratered portion of the surface of Jupiter’s moon Callisto, seen in this image recorded by NASA’s Galileo spacecraft, resembles most of Callisto that’s been seen in high resolution. This adds evidence to a theory that Callisto may hold an underground ocean.

The area in the image is the opposite point, or antipode, of Callisto’s Valhalla impact basin. The antipode of any point on a sphere is the opposite point on a line through the center of the sphere. Antipodes of major impact sites on some other worlds similar in size to Callisto, such as Mercury and Earth’s moon, show a grooved and hilly terrain attributed to seismic shocks focusing on those points from the distant impacts. The antipode of Mercury’s Caloris impact site is one example. When Galileo flew near Callisto on May 25, 2001, scientists sought an image to check the Valhalla antipode for similar signs of disruption.

Computer modeling has suggested that if Callisto had a water layer in its interior, that layer would have dispersed the seismic shock waves from the ancient Valhalla impact. The absence of grooved and hilly terrain at the Valhalla antipode is consistent with that possibility.
Magnetic-field measurements have previously suggested that Callisto has a layer of liquid water deep below its surface.

This image, taken from a distance of 32,000 kilometers (about 20,000 miles) shows details at the Valhalla antipode down to a size of about 330 meters (about 1,250 feet) across. Callisto is the outermost of Jupiter’s four large moons. Its surface of ice and rock is the most heavily cratered of any moon in the solar system.

The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Galileo mission for NASA’s Office of Space Science, Washington, D.C. Additional information about the spacecraft and its discoveries is available on the Galileo home page at http://galileo.jpl.nasa.gov .

Credit: NASA/JPL/Arizona State University