In the search for extraterrestrial life, Jupiter's moon Ganymede, the largest in the solar system, is no Europa. The salty subsurface ocean it likely harbors is much farther below its surface than is Europa's  probable liquid ocean, and it's sandwiched between layers of ice, leading most scientists to conclude that the prospects for life on—or inside—Ganymede are dim. Little else has been known about this gigantic moon, despite the fact that it could be key to understanding the history of the Jovian system, thereby unlocking the geologic and biological secrets of its icy Galilean satellites. At 5,262 kilometers in diameter, it is larger than Mercury and dwarf planet Pluto, and would qualify as a planet itself, save for the primary exception that it orbits a planet rather than a star. A joint NASA/European Space Agency (ESA) mission to both Europa and Ganymede is proposed to launch in 2020.

So it's a big deal that planetary scientists have just announced finishing the first-ever global geologic map of Ganymede, one that reveals strong evidence for how tectonic forces formed the most prominent features on the moon's surface. Ganymede's surface is characterized primarily by covering of dark, heavily cratered primordial material, along with icier light areas that are younger. Much of the lighter material is covered with what geologists have only previously been able to describe as "grooves."

View the full map of Ganymede here

The new map supports the theory that the grooves likely formed as a result of tidal heating generated by orbital resonances among Ganymede and two of Jupiter's three other Galilean satellites—Europa and Io. (Today, for every revolution of Jupiter made by Ganymede, Europa orbits the planet twice and Io circles it four times.) The map provided Wes Patterson of the Johns Hopkins University Applied Physics Laboratory (APL), Geoffrey Collins of Wheaton College in Massachusetts, and their colleagues, with observational evidence that had been collected by the Voyager and Galileo spacecraft against which to test this theory.

A decade in the making, the map represents every known tectonic feature on the surface of Ganymede—60,000 total, including 4,000 craters more than 10 kilometers in diameter—and their relative ages, Collins says. The dark material is akin to a dirty snow bank—it is a thin lag deposit overlying ice, accumulated by slow sublimation at the surface and leaving non-ice impurities behind. As the snow melts, a crust of dirt forms over the snow. The light material is so either because in forming grooves it has shed the lag deposit through landslides as the surface was being torn apart by tectonic forces or because the surface has been covered by bright, icy volcanic flows. "When you look at really close-up pictures of these grooves, a lot of them look like huge normal faults, a geological feature produced by pulling the surface apart, like the Hudson River Valley or the Basin and Range [Province] in Nevada," Collins says. "We really want to know what forces pulled the surface apart."

Collins and his colleagues calculated the pattern and sequence of forces on the surface necessary to make those faults and were able to match the orbital resonance theory with the data. "So far it is looking promising," Collins says. But, Patterson adds, chuckling: "Nothing's ever clear-cut." The results were presented earlier this month at the 2009 European Planetary Science Congress in Potsdam, Germany. The paper is also under review at a journal, the authors say. Further analysis could reveal not only how the faults and light material in general formed but also when, Collins says. "We're trying to figure out when in Ganymede's history did the grooves form," he says. "So far we have no hard numbers but it looks like it happened in one giant hiccup." That is, the evidence shows that Ganymede underwent a short but intense episode of activity when tectonic forces ripped it apart, but that activity is now long over.

Collins and Patterson also are trying to learn more about the history and evolution of the inner Galilean satellites' orbits, specifically, how and when the resonance was organized among the orbits of Ganymede, Io and Europa.

Ganymede is a strange moon in some ways, aside from now being one of only three moons in the solar system to be completely mapped (Earth's moon and Jupiter's Callisto are the others). Unlike a lot of moons—but like Earth—it's a differentiated body with a liquid metallic core surrounded by a rocky mantle. Above that is a water ice "sandwich" that's 800 to 900 kilometers thick. The "bread" is a shell of ice, some 200 kilometers thick at the surface, and within is a liquid water ocean with dissolved ions that has no contact with the irradiated surface of Ganymede or the silicate rocks in its mantle.

The orbital resonance of Jupiter's inner three Galilean moons is what fuels volcanic eruptions on Io and makes Europa a potentially hospitable place for life. Ganymede, however, is the only satellite in this resonance with a surface that bears historical evidence of the system's history, avoiding erasure by continuous tectonic and volcanic activity as is the case on the other moons. For that reason understanding Ganymede's geology could open the door on the history of the entire Jovian system, and even other icy satellites such as Saturn's Enceladus, answering such questions as when, how and where the moons formed, and how they have interacted with each other. In fact, Ganymede bridges a gap in terms of its history and geology. "Ganymede is an archetype model for a lot of midsize icy satellites in our solar system and maybe in other solar systems, as well," Patterson says.

The NASA–ESA Europa Jupiter System Mission is currently proposed to include two orbiters—one would circle Jupiter, then Europa; the other would circle Jupiter, then Ganymede with the aim of detailing the geophysical, compositional, geologic and other processes that affect icy satellites. Patterson and Collins presented their paper at the German conference this month in hopes of helping ESA scientists gathered there to move the mission planning along for the Jupiter–Ganymede orbiter. "We came here and brought this work here to get them excited," Patterson says. "You can't understand any of these moons in isolation. Ganymede is a link between the currently active bodies, such as Europa, and things that have happened in the past. It preserves a history of the Jupiter system like no other satellite."