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Neptune: a watery planet at heart.

Neptune may indeed merit its aquatic name. On the basis of key measurements taken by Voyager 2 and a laboratory experiment that simulated the extreme conditions at the distant planet's interior, scientists say they believe Neptune has a core composed almost entirely of liquids--resembling the interior of Uranus.

Most researchers had assumed that rock accounts for 25 percent of the mass of Neptune's core, making it more like Earth's interior than Uranus'. But scientists based that belief on models never fully tested by observations, says William B. Hubbard of the University of Arizona in Tucson. Two years ago, Voyager 2 passed close by Neptune, measuring the planet's rate of rotation and mapping its gravitational field. Results of the flyby, Hubbard says, strongly suggest that Neptune's core has a relatively low density--about that of water--rather than the high density associated with iron, silicon and magnesium.

This interpretation, however, assumes that researchers can accurately model the equilibrium behavior of fluids at the extreme temperatures and pressure found inside Neptune and Uranus. To verify his model, Hubbard collaborated with experimenters at the Lawrence Livermore National Laboratory in Livermore, Calif. These researchers created "a planet in a bottle" -- a mixture of water, ammonia and alcohol that approximated the postulated composition of Uranus' core.

Using shock waves generated by firing a cylindrical bullet at the container, the Livermore team created pressures in the bottled planet up to 2.2 million times that of Earth's atmosphere and temperatures as hot as 4,100 kelvins. The simulation confirmed that a fluid core could produce the density deduced from the Voyager measurements. Alternatively, but less likely, says Hubbard, the cores of Uranus and Neptune might consist of dense metals interspersed with materials less dense than water, such as hydrogen. Overall, such a core would still have a density equal to that of water. The investigators report their work in the Aug. 9 SCIENCE.
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Title Annotation:evidence that Neptune has a core composed of liquids
Publication:Science News
Date:Sep 7, 1991
Words:315
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