Callisto, Jupiter's second largest moon, seemed to be one of the solar system's more boring worlds. Its surface is so heavily pockmarked, says UCLA space physicist Margaret Kivelson, that "it looks as if nothing has changed for billions of years. People really thought it was just a frozen block of ice and rock." But Kivelson has found that Callisto may prove to be a far more intriguing place than astronomers had thought. Like its sister moon Europa, Callisto may harbor an ocean beneath an icy surface.
Kivelson began to suspect that Callisto might have an ocean after receiving a tip from David Stevenson, a planetary scientist at Caltech. Stevenson told her that the Galileo space probe had detected what seemed to be a Jupiter-induced magnetic field around Europa. Jupiter creates Europa's field, researchers believe, when its own huge magnetic field sets up electric currents in the salty ocean that many astronomers suspect sloshes some 60 miles below Europa's surface. The electric currents would in turn generate the magnetic field detected by Galileo.
Kivelson decided to take a look at data from November 1996 and June 1997 when Galileo flew by Callisto. To Kivelson's surprise, she found that Callisto apparently has a magnetic field induced by Jupiter as well. And that implies a current-carrying material within the frigid Jovian moon.
The most plausible explanation for Callisto's field, says Kivelson, is the existence of a salty ocean. Callisto's atmosphere doesn't have enough charged particles to generate the field detected by Galileo, she says. But a 6-mile-deep body of water as salty as Earth's oceans could (or a 60-mile-deep ocean one-tenth as salty as Earth's). Some 90 miles below Callisto's surface, radioactive heat from rocks and tremendous pressure might create temperatures approaching the melting point of water.
Kivelson suspects that one other Jovian moon-Ganymede-may also have an ocean beneath miles of ice. "It's also an icy moon," she says. "So it seems if Callisto can do it, one would expect there to be quite a good chance that Ganymede can do it."