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More than 400 years after its discovery by Italian astronomer Galileo Galilei (see Figure 3-2, p. 38), the largest moon in the Solar System – Jupiter’s moon Ganymede – has finally been fully mapped (see Section 7.1g(iii), pp. 175–176). Since its discovery in January 1610, Ganymede has been the focus of repeated observation, first by Earth-based telescopes, and later by the flyby missions and spacecraft orbiting Jupiter. These studies depict a complex, icy world whose surface is characterized by the striking contrast between its two major terrain types: the dark, very old, highly cratered regions, and the lighter, somewhat younger (but still very old) regions marked with an extensive array of grooves and ridges.

Credit: USGS Astrogeology Science Center/Wheaton/NASA/JPL-Caltech

Scientists have now produced the first global geologic map of Ganymede, Jupiter’s seventh moon. The map combines the best images obtained during flybys conducted by NASA’s Voyager 1 and 2 spacecraft (1979) and Galileo orbiter (1995–2003) and is now published by the U.S. Geological Survey as a global map. It technically illustrates the incredibly varied geologic character of Ganymede’s surface and helps planetary scientists to make sense of the apparent chaos of its complex surface, in order to decipher the icy world’s evolution. It will also enable researchers to compare the geologic characters of other icy satellite moons in the Solar System.

The European Space Agency’s Jupiter Icy Moons Explorer mission is slated to be orbiting Ganymede around 2032, with instrument contributions from NASA.

Earth-bound astronomers can observe Ganymede (with binoculars) in the evening sky this month, as Jupiter is in opposition and easily visible.

Links: JPL press release; a rotating Ganymede movie; the geologic map.

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