The figure shown to the right is a Galileo Spacecraft image of the third
Galilean moon, Ganymede (Ref), which
has the distinction of being the largest moon in the Solar System. Its
diameter is 5270 km, much larger than our own Moon (3476 kilometers).
This moon is also largely covered by a frozen water ocean. Portions of
its surface look young geologically and portions look old. It has large
grooves in places, the crust is broken into fragments, and there is evidence
for past plate tectonic activity. This represents the first conclusive
evidence of plate tectonics in the Solar System beyond the Earth.
Young Surface and Old Surface
In the adjacent image the dark areas are older, more heavily cratered regions and the light areas are younger, tectonically deformed regions. The brownish-gray color is believed due to mixtures of rocky materials and ice. Geologically recent impact craters and their ejecta are indicated by bright regions.Detailed Surface Features
The following two images show more detailed features on Ganymede. The left figure shows tectonic features (evidence for horizontal crustal motion); the right figure shows details of old, darker terrain on Ganymede.
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| Tectonic features on the surface of Ganymede | The Galileo Regio region on Ganymede |
The right figure is a mosaic of four Galileo images of the Galileo Regio
region on Ganymede overlayed on the data obtained by the Voyager 2 spacecraft.
These Galileo images show fine details of the dark terrain that makes up about
half of the surface. Ancient impact craters indicate the great age of the
terrain, dating back several billion years.
In the left image below, new terrain overlays older terrain, which overlays still older terrain on the surface of Ganymede.
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| New and old terrain | The structure of Ganymede |
The images making up this mosaic were obtained by NASA's Galileo
spacecraft when it flew past Jupiter's moon Ganymede for the second time on
September 6, 1996.