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Amalthea - moon full of holes - data from Galileo's last mission

Avi Blizovsky

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Amalthea, Jupiter's moon, was visited by the Galileo spacecraft about a month ago. This moon is full of holes and has a surprisingly low density, says Dr. John Anderson of NASA's Jet Propulsion Laboratory (JPL) in California. "Amalthea looks more like a loose pack of rocks."

The space between the fragments of rock and ice appears to occupy more of the moon's volume than the solid components. The solid materials in it also do not fit some of the theories about Jupiter's moons.
Dr. Torrance Johnson, the chief scientist on the Galileo project says that Amalthea appears to be made mostly of light rock and little ice rather than a compact mass of ice and rocks. The diameter of the moon Amalthea is about 270 kilometers in the wide area and less than that in the narrow area.
Its mass, as estimated from the effect of its gravity on the Galileo spacecraft, 160 miles away on November 5, is much smaller than previously predicted. Its density is very close to the density of ice, said Dr. Anderson.

Amalthea, Jupiter's inner moon appears to have been heavily bombarded and disintegrated. From its irregular shape it appears that it has previously broken up into small parts and now these parts are reuniting by their united gravity. In between there are spaces where the parts did not fit. "There are several large rocks that seem to barely touch each other.
Dr. Johnson adds that: "These findings support the idea that the inner moons of Jupiter were bombarded and collapsed. It is possible that Amalthea was originally formed as a single unit, but was disintegrated due to collisions. It also doesn't seem to have enough mass to recombine and become a solid spherical body like Earth's moon or Jupiter's four large moons.

Scientists are surprised that the density is so low that even solid parts of Amalthea appear less dense than Io, a larger moon orbiting Jupiter in an orbit twice as distant as Amalthea's. This is a puzzle because the preferred model for the formation of Jupiter's moons holds that the closer moons are made of denser materials than the more distant ones.
The flyby of Amalthea brought Galileo the closest it has ever been to Jupiter since it began orbiting the planet in 1995. After more than thirty flybys of the four major moons, this flyby was the last of Galileo, which is now on a collision course with Jupiter on September 21, 2003.

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