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Weak at heart. Jupiter's fifth-largest moon, shown in this artist's conception with the Galileo spacecraft, is surprisingly porous.

Jupiter's Spongy Moon

SAN FRANCISCO--In its final encounter before incinerating in the atmosphere of Jupiter next year, NASA's Galileo spacecraft has shown that one of Jupiter's innermost moons is a lightweight agglomeration of icy rocks. The discovery, reported here on 9 December at a meeting of the American Geophysical Union, startled Galileo scientists, who expected that moons close to the giant planet would consist of much denser solid rock.

Galileo has orbited Jupiter for 7 years, far longer than planned. Now the end is in sight. Radiation near the planet has fried some of the robot's systems, and the spacecraft will plunge into Jupiter in September 2003. But on 5 November, scientists steered Galileo to within 160 kilometers of Jupiter's fifth-largest moon, Amalthea, for its last scientific flyby. During the encounter, radio antennas in NASA's Deep Space Network tracked Galileo as the moon's gravity tugged it. Slight changes in Galileo's motions let the team gauge Amalthea's mass. Previous data on the moon's irregular shape--270 kilometers long and half as wide--yielded its density.

Amalthea's overall heft is surprisingly similar to water ice: about 1 gram per cubic centimeter. However, the moon is too big and too close to Jupiter to be a solid hunk of ice, says planetary scientist John Anderson of NASA's Jet Propulsion Laboratory in Pasadena, California. Rather, models suggest that less than half of Amalthea's volume consists of chunks of low-density, carbon-rich rocks and ice, while the rest is empty space. "It's apparently a loosely packed rubble pile," Anderson says. "It's full of holes, lots of holes." Volcanic sulfur particles from the nearby moon Io have bombarded Amalthea and tinted its pockmarked surface a deep red over time.

Amalthea probably arose in Jupiter's youth as a solid body near its current orbit, says planetary formation theorist Robin Canup of the Southwest Research Institute in Boulder, Colorado. Then, Jupiter's gravity attracted so many smaller objects that Amalthea was battered repeatedly for eons. After the moon was reduced to rubble, its gravity was too weak to squeeze it back into a coherent ball but strong enough to keep the fragments from drifting apart, Canup says.

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