The Legacy of Galileo - Keck Institute for Space Studies - Caltech

Challenging the Paradigm: The Legacy of Galileo
(Figure 10). Gravity data taken during the close flybys also indicated a low density (i.e., ~1000
kg/m-3) layer about 100 km deep overlying a denser rock and iron interior. These
observations were all consistent with models showing that a liquid water ocean, with a
volume twice that of all Earth’s oceans, could be maintained under an icy crust by tidal
dissipation.
Figure 10: Europa’s blocks of ice.
Crucial evidence for a subsurface ocean at not only Europa, but also Ganymede and Callisto
came from magnetic field data. Only Ganymede, as described earlier, exhibits a strong, fixed,
dipole-like magnetic field indicative of an internal dynamo. However, significant magnetic
deflections were observed during close flybys of each of the icy satellites, with the pattern of
the deflection changing from encounter to encounter. Correlating these magnetic
perturbations with the direction and timing of Jupiter’s magnetic field, the magnetometer
investigators found that the perturbations exactly matched calculations for an induced
magnetic field produced by changes in Jupiter’s field as the planet rotates. The satellites are
embedded in the magnetosphere, and Jupiter’s dipole field is tilted about ten degrees to the
planet’s rotation axis. Thus, the magnetic field seen by each satellite rocks back and forth,
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