February 17, 1999

An Ocean Beneath Callisto's Icy Crust?

Callisto, the outermost of Jupiter's four Galilean satellites, as viewed by NASA's Voyager 2 spacecraft in 1979.

Closeup view of Callisto's Asgard impact region showing craters, inter-crater plains, and part of one of the many rings surrounding its center (the ridge feature in the lower left corner).

Cut-out of a model of Callisto (details not drawn to scale). Model calculations suggest that below a 120-mile-thick icy crust lies a liquid, salty ocean with a depth of six miles or greater. The interior is composed of rock and ice. Callisto's diameter is 3,000 miles.

Click on images for enlarged views and more detailed descriptions.

Image Credits: Jet Propulsion Laboratory and NASA.

Until recently, scientists thought Callisto was a dead, boring, Jovian satellite made of ice and rock. Its only distinguishing feature, they thought, was its surface--the most ancient, least changed surface of any planet or satellite in the solar system.

Data from NASA's Galileo spacecraft have changed this view. A magnetometer onboard Galileo has detected fluctuating magnetic fields surrounding Callisto. Interestingly, the direction and strength of the magnetic fields vary with Jupiter's rotation.

What could give rise to these magnetic fields? The simplest explanation is the presence of electric currents surging through Callisto.

To generate such currents requires an electric conductor. Callisto's icy crust is not a good conductor, nor is its core, which is believed to consist largely of rock and ice. However, a liquid, salty ocean, similar to the Earth's, would be a good conductor. Furthermore, theoretical calculations indicate that Jupiter's immense magnetic fields, which stretch well past Callisto into interplanetary space, could induce sufficiently strong electric currents in such an ocean to generate the magnetic fields observed by Galileo.

Thus, the Galileo magnetometer data and this line of reasoning led to the conclusion that Callisto may very well possess a salty ocean beneath its frozen surface. The heat that prevents this ocean from freezing is believed to come from the decay of radioactive elements embedded in the satellite's rocks.

Similar evidence for the existence of an ocean exists for Europa, Jupiter's second Galilean satellite. It, too, possesses a magnetic field whose fluctuations are synchronized with Jupiter's rotation. However, the evidence for an ocean beneath Europa's icy crust has been less of a surprise to scientists than one on Callisto. Europa orbits much closer to Jupiter than Callisto. This causes strong tides that perpetually distort Europa's structure, releasing ample heating for keeping a subsurface ocean from freezing.

For a more detailed explanation of why scientists think that Europa and Callisto have subsurface oceans, click here.

More Cool Stuff

For more information on Callisto's suspected subsurface ocean, go to press release 98-192 of NASA Headquarters and the Jet Propulsion Laboratory (JPL):

http://www.jpl.nasa.gov/galileo/news32.html

Additional information and images on Jupiter and its satellites can be obtained at:

http://photojournal.jpl.nasa.gov/cgi-bin/PIAGenPlanetPage.pl?Jupiter

The Project Galileo homepage is at:

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

As part of its Learning Technologies Project (LTP), NASA supports a number of educational Web sites that have excellent material on the space sciences:

Windows to the Universe is a site created by the University of Michigan that offers access to information about Callisto and the moons of other planets of the solar system.

http://www.windows.umich.edu/

Other LTP sites that are dedicated to space science can be found at:

http://learn.ivv.nasa.gov/education/topics/space_sci.html