Amalthea's Density Is Less Than That of Water

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Science  27 May 2005:
Vol. 308, Issue 5726, pp. 1291-1293
DOI: 10.1126/science.1110422

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Radio Doppler data from the Galileo spacecraft's encounter with Amalthea, one of Jupiter's small inner moons, on 5 November 2002 yield a mass of (2.08 ± 0.15) × 1018 kilograms. Images of Amalthea from two Voyager spacecraft in 1979 and Galileo imaging between November 1996 and June 1997 yield a volume of (2.43 ± 0.22) × 106 cubic kilometers. The satellite thus has a density of 857 ± 99 kilograms per cubic meter. We suggest that Amalthea is porous and composed of water ice, as well as rocky material, and thus formed in a cold region of the solar system, possibly not at its present location near Jupiter.

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