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

Challenging the Paradigm: The Legacy of Galileo
As it swung back through the inner part of the Jovian system, Galileo’s first close encounter
with a Galilean satellite was, appropriately enough, a fly-over of Galileo Regio, a dark region
named for the moon’s discoverer. High-resolution images of nearby Uruk Sulcus, more than
ten times better than Voyager pictures, quickly resolved the origin of the bright, apparently
smooth lanes that Voyager had seen. Galileo’s images revealed that, seen close up, they were
not smooth frozen flows of icy “lava,” as had been suggested after Voyager, but rather a
tectonically reworked region where dark material had slumped off the steep slopes of a set of
faulted and fractured ridges, shown in Figure 6.
Figure 6: Faulted and fractured ridges.
While Galileo’s cameras and spectrometers were probing the surface of Ganymede, space
physics instruments were providing startling evidence about the moon’s deep interior. The
spacecraft’s magnetometer recorded a strong deflection of the magnetic field while Galileo
was passing close to the moon. At the same time the plasma wave analyzer recorded bursts of
radio noise indicative of electrons interacting with a magnetosphere. Taken together the data
suggested that Ganymede was a “moon with magnetism” and possessed its own internal,
dipole, magnetic field strong enough to counteract the local field from Jupiter, creating a
magnetic “bubble” in Jupiter’s magnetic environment⎯a magnetosphere-within-amagnetosphere.
This result makes Ganymede the only planetary satellite known to have an
actively generated internal field at the present time (the Earth’s Moon may have had such a
field very early in its history). Prior to Galileo’s arrival at Jupiter, Io was believed to be the most
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