AMALTHEA, one of Jupiter鈥檚 inner moons, may be a loose jumble of pieces rather than a solid object.
Planetary scientists already knew the volume of the 270-kilometre potato-shaped moon, but only measured its mass when NASA鈥檚 Galileo probe made a close pass on 5 November. The results show that the moon鈥檚 density is close to that of water ice, John Anderson of NASA鈥檚 Jet Propulsion Laboratory told the American Geophysical Union last week. The surface appears to be made of dense rock, so the low average density means the moon may be more than 50 per cent empty space, says Anderson. He suspects an impact shattered Amalthea early in its history, and the pieces have since clumped back together.
