The Moon Ganymede: Laboratory
for Tectonic Motion

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 (more info).

Detailed Surface Features

The following two images show more detailed features on Ganymede. The left figure (Ref) shows tectonic features (evidence for horizontal crustal motion); the right figure shows details of old, darker terrain on Ganymede.

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 (more info).

In the left image below, new terrain overlays older terrain, which overlays still older terrain on the surface of Ganymede.

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 (Ref).

History and Structure of Ganymede

The figure above on the right shows a model of Ganymede (Ref). We believe that Ganymede, further from Jupiter and less heated by tidal effects, is frozen more solid than Europa, but its surface and subsurface is still somewhat plastic because of tidal heating. The past plate tectonic activity that we see evidence for may have resulted from the heat generated by earlier tidal coupling to Jupiter.