SAN FRANCISCO--Ganymede, the largest of Jupiter's moons, probably harbors a deep salty sea entombed in thick layers of ice. The discovery, announced here on 16 December at a meeting of the American Geophysical Union (AGU), adds Ganymede to the list of two other Jovian moons--Europa and Callisto--that appear to host oceans beneath their pocked and fractured surfaces.
Evidence for Jovian seas comes from the Galileo spacecraft, which has orbited Jupiter since 1995 and periodically swoops near its moons. When Galileo gets close enough to the moons, it sometimes picks up their magnetic signatures. These weak fields are induced by Jupiter's own intense magnetic field, which extends far into space. As the tilted planet rotates, its powerful field changes direction, inducing electrical currents in any conducting layer within the moons. Those currents, in turn, spawn slight magnetic fields. Earlier this year, Galileo scientists confirmed an induced magnetic field around Europa, indicating that a salty ocean flows beneath its icy crust (ScienceNOW, 11 January). Even seemingly dead Callisto appears to host an internal sea (Science, 12 June 1998, p. 1694). However, Ganymede's core of molten iron and sulfur generates a magnetic field of its own, masking the subtle signatures that an ocean might produce.
Galileo's latest close encounter with Ganymede, in May, provided researchers with enough data to discern the faint signal. Planetary scientist Margaret Kivelson of the University of California, Los Angeles, and her colleagues found that Ganymede's internal field shuddered on its axis by 3.6 angular degrees as Jupiter's field rocked back and forth. That shift, Kivelson says, is consistent with a watery layer just a few kilometers thick, buried beneath about 170 kilometers of ice. The results are "compelling but not totally unambiguous," she notes.
Planetary theorist David Stevenson of the California Institute of Technology in Pasadena agrees that the best explanation for the magnetic field data is a deep layer of salty water. What makes that layer possible are temperatures that approach -20°C within Jupiter's outer icy moons at certain depths--exceeding the melting point of ice at high pressures. Some slushy ice may have oozed to the surface in Ganymede's past if the ocean was closer to the surface, Stevenson notes, creating smoother lanes of ice between crumpled fragments of the crust. But he says life isn't likely in the curious ice-water-ice "sandwiches" on Ganymede and Callisto.