The Galileo spacecraft, which was launched 13 years ago, is scheduled to crash into Jupiter in September 2003. On its final flyby of the planet last month, the probe came within 160 kilometers of Amalthea, one of Jupiter's many moons. Findings from that encounter, reported this week at the fall meeting of the American Geophysical Union, indicate that the strangely-shaped inner moon is full of empty space. "The density is unexpectedly low," notes John D. Anderson of NASA's Jet Propulsion Laboratory (JPL). "Amalthea is apparently a loosely packed pile of rubble."

Previous Galileo images of Amalthea allowed researchers to determine the volume of the 270-kilometer-wide moon. With the new data, the NASA team calculated its density and determined that overall, the moon is about as dense as ice. But the conditions surrounding Jupiter do not suggest an icy composition. Instead, the scientists posit that the Jovian moon consists of a large collection of solid chunks with empty gaps between them. "It's probably boulder-size or larger pieces just touching each other, not pressing hard together," Anderson says. The results indicate that Jupiter's inner moons survived some tumultuous times. According to Torrence Johnson of JPL, "Amalthea may have formed originally as one piece, but then was busted to bits by collisions."