Posted: Tue, Dec 10, 2002, 7:42 AM ET (1242 GMT)
Amalthea, a small inner moon of Jupiter, has an unusually low density which means that it is likely an agglomeration of smaller pieces, scientists said Monday. The conclusion is based on data collected during Galileo's flyby of the small moon on November 5, when the spacecraft passed within 160 kilometers of the moon. While Jupiter's strong radiation caused the spacecraft to go into safe mode shortly after closest approach, ground controllers were able to track the spacecraft during the flyby and determine how its trajectory was altered by Amalthea's gravity, permitting scientists to measure the moon's mass and gravity. The results show that the moon's density, about the same as water ice, is much lower than expected. Scientists believe that since the moon is not likely to be made mostly of ice, it is instead made of chunks of denser rock with large gaps between them; the empty space may make up more than half of the moon's volume. Amalthea may have started out as a single solid body, scientists speculate, that was later broken up through collisions only to reform in its current state.

Related Links:

•  NASA press release
•  SPACE.com article
•  BBC article
•  Astronomy.com article