Galileo took pictures of Jupiter's thin, disrupted ice crust.

The hint of liquid water is also driving exploration towards Europa, one of Jupiter's four major moons. Four times farther from the heat of the Sun than Earth is, Europa wears a global coat of ice. But the gravity of giant Jupiter exerts tidal tugging that warms Europa's insides, possibly enough to keep a layer of water melted under its frozen surface.

Water clues appeared in pictures taken by NASA's Galileo spacecraft in 1996 as it orbited Jupiter. The pictures supported earlier theories about a hidden Europan ocean. On some parts of Europa's surface, for example, blocks of ice appear to have broken apart and rearranged themselves as if by floating, like Arctic ice floes, on a fluid underlayer. However, the pictures that build evidence for a hidden ocean do not reveal whether it might still exist or had perhaps frozen solid sometime after the surface features formed.

Europa's fractured surface shows signs of liquid water, ice or slush.

Galileo's magnetometer instrument has sent home the strongest indication that a layer of saltwater remains melted under Europa's crust today. As Europa moves through different parts of Jupiter's strong magnetic field, its own weaker magnetic field changes direction, indicating that the moon has a layer of electrically conducting material. Since ice would not conduct electricity well enough, saltwater is the best candidate. Similar magnetic evidence from Galileo indicates that two of Jupiter's other large moons, Ganymede and Callisto, may have liquid saltwater layers, too. The prospect that worlds so far from the Sun might have niches suitable for life has broadened ideas about where conditions might be right for life on planets of other stars.

The Europa Orbiter, being designed at JPL, will one day orbit Europa to confirm the presence or absence of an ocean. It will also map the thickness of surface ice to guide selection of landing sites for possible follow-up missions.

Martian Clues

Moon and Comets