solar system map.

More than 400 years after its discovery by Galileo, the largest moon in the solar system has finally claimed a spot on the map.

A team of scientists led by Wes Patterson of the Johns Hopkins Applied Physics Laboratory (APL), Laurel, Md., and Geoffrey Collins of Wheaton College, Norton, Mass., has produced the first global geologic map of Ganymede, a Galilean moon of Jupiter. Published by the U.S. Geological Survey, the map technically illustrates the varied geologic character of Ganymede's surface, and is the first complete global geologic map of an icy, outer-planet moon.

Patterson, Collins and colleagues used images from NASA's Voyager and Galileo missions to create the map. It's only the fourth of its kind covering a planetary satellite; similar maps exist for Earth's moon as well as Jupiter's moons Io and Callisto.

"By mapping all of Ganymede's surface, we can more accurately address scientific questions regarding the formation and evolution of this truly unique moon," says Patterson, a planetary scientist.

Since its discovery in January 1610, Ganymede has been the focus of repeated observation, first by Earth-based telescopes, and later by 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 ancient) regions marked with an extensive array of grooves and ridges.

With a diameter of 3,280 miles (5,262 kilometres), Ganymede is larger than both planet Mercury and dwarf planet Pluto. It's also the only satellite in the solar system known to have its own magnetosphere. The map details geologic features of the moon that formed and evolved over much of our solar system's history. These features record evidence of Ganymede's internal evolution, its dynamical interactions with the other Galilean satellites, and the evolution of the small bodies that have impacted Ganymede's surface.

The new chart will be a valuable tool for researchers to compare the geologic characters of other icy moons, since almost any type of feature that is found on other icy satellites has a similar feature somewhere on Ganymede. And with a surface over half as large as all the land area on Earth, Ganymede offers a wide variety of locations to observe. "Ganymede also shows features that are ancient alongside much more recently formed features, adding historical diversity in addition to geographic diversity," Collins says.

Along with Collins and Patterson, the Ganymede mapping team also includes Louise Prockter of APL; James Head, Brown University, Providence, R.I.; Robert Pappalardo, NASA's Jet Propulsion Laboratory, Pasadena, Calif.; Baerbel Lucchitta, USGS, Flagstaff, Ariz.; and Jonathan Kay, University of Idaho. NASA funded the project through its Outer Planets Research and Planetary Geology and Geophysics programs

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