An ocean for Callisto?

Of the four large moons of Jupiter, Callisto lies farthest from its mother planet and has always been regarded as the least interesting--a geologically dead chunk of ice and rock. New evidence from the Galileo spacecraft hints that, like its sister satellite Europa, Callisto may harbor an ocean beneath its icy crust.

During two recent passes by Callisto, Galileo found that the moon modified Jupiter's magnetic field by different amounts. The measurements suggest that Callisto does not possess a fixed magnetic field of its own. Instead, the Jovian field seems to induce a magnetic field within the moon, says Margaret G. Kivelson of the University of California, Los Angeles.

As Callisto orbits Jupiter, it moves through a Jovian magnetic field of varying intensity and direction. According to Kivelson, those variations set up electric currents within Callisto, which in turn generate a magnetic field. The currents could only be produced if Callisto possesses an electrically conducting medium, such as salt water, she notes.

To generate an appreciable field at Callisto's surface, the salt water can't be buried too deeply. Gravity maps of Callisto suggest that its icy shell is about 100 kilometers thick. If a layer of water lies within that shell and is as salty as the oceans on Earth, it would need to be 10 to 20 km deep, Kivelson says. She and her colleagues, including Krishan K. Khurana of UCLA, describe their findings in the Oct. 22 Nature.

"We haven't proven that Callisto contains water, but now everything hangs together," says Kivelson.

Researchers had proposed such a model to explain magnetic measurements recorded near Europa (SN: 8/9/97, p. 90). Both the appearance of Europa's surface, scored with cracks and wedges, and its abundance of salt compounds suggest that it harbors a layer of salt water or slushy ice that seeps up from below. A gravitational tug-of-war between Europa, Jupiter, and two inner moons flexes Europa's surface and could generate enough heat to melt ice beneath its surface.

Callisto lies too far from Jupiter to experience the same type of heating. The heat could come instead from the slow release of energy from radioactive isotopes that helped form the moon, says Thomas B. McCord of the University of Hawaii in Honolulu. He also notes that carbon dioxide found on Callisto's surface could have bubbled up from a watery layer below.

Even if Callisto has an ocean, it probably lacks enough organic compounds for life, Kivelson says.