Looping from Moon to Moon

Abstract

Six months after becoming a part of the Jovian system, Galileo began a slingshot journey that took it from moon to moon. The first attraction came on June 17, 1996, when the orbiter flew within 835 kilometers of Ganymede, the largest planetary moon (at 5,270 kilometers in diameter) in the Solar System. It is larger than the planet Mercury, but its density is so low (1.9 grams per cubic centimeter) that it must contain large amounts of ice. The “geology” that emerged in the Voyager and Galileo photographs consisted not of rock but of frozen water! The Voyager pictures had already revealed a long and turbulent history of the ice ball, including plate movements of the crust and the formation of mountains. Ganymede’s outer shell seemed to be broken up into blocks that were then shifted dozens of kilometers laterally. Long parallel mountain ranges and valleys cut across other regions, as if a gigantic rake had been drawn across the surface, scratching “grooves,” as experts called them, into the surface. It was probably in undergoing a general expansion that the surface broke up into the pieces that had drifted apart. That could have occurred, to name one possibility, when the relatively light ice separated from the heavy constituents of the moon’s progenitor body.