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An artist's depiction of what the scene must have looked like when the NASA’s Galileo spacecraft approached Amalthea, moon of Jupiter.
Click on image for full size
Courtesy of NASA

# A Jupiter moon that has more holes than Swiss cheese! News story originally written on December 11, 2002

Amalthea, a small reddish moon of Jupiter, is not made of Swiss cheese, but it does seem to be full of holes making its density surprisingly low according to new information from the Galileo spacecraft.

To figure out the density of Amalthea, scientists needed to know the mass and the volume of the moon (because, as you may know, density=mass/volume). They calculated the mass of the moon from its gravitational affect on the Galileo spacecraft, which passed 160 km (99 miles) from the moon. The volume of the moon was calculated from images like the one on the left.

They found that Amalthea has very low density. Scientists at the NASA Jet Propulsion Laboratory suggest that the rock that makes up Amalthea is broken into many pieces that are about the size of large boulders. The pieces are just barely touching each other leaving many empty gaps in the moon. “This finding supports the ideas that the inner moons of Jupiter have undergone intense bombardment and breakup. Amalthea may have formed originally as one piece, but then was busted to bits by collisions,” said Dr. Torrance Johnson, project scientist for Galileo.

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