In 1989 the space shuttle Atlantis launched the Galileo spacecraft on a mission to Jupiter where it arrived in 1995. Well over a decade after its launch, the probe is still orbiting Jupiter, exploring our solar system's largest planet and its varied collection of moons.

Data returned from Galileo tentatively indicate that two of the larger moons, Europa and Callisto, have a global subsurface ocean. The icy moon Europa is the strongest candidate. Since Galileo is orbiting Jupiter, not any of its satellites, data must be gathered on infrequent close passes to Europa. Data for Kivelson's study is from passes closer than 2000 km to the moon's surface.

The probe's magnetometer measures fields as it swings from around Jupiter, largely under the strong magnetic influence of the large planet. Near the moon, however, the magnetic signature changes. The signature is distint from Jupiter's field. It indicates a conducting material just beneath the surface of the moon. The field is best modeled to that of a perfectly conducting sphere.

The case for a moon wide conductor was confirmed on subsequent passes of Europa.

The most likely substance for this conductor is salty water. This hypothesis is consistent with photographs of the surface. The surface of Europa most closely resembles the pack ice of Earth's arctic regions. Although geological studies of the visual evidence cannot determine whether the icy surface covers an existant or extinct global ocean, the magnetometer shows that is likely that liquid water persists today.