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No. 66: Nov-Dec 1989

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Neptune Spins Too Fast And Its Magnetic Field Is Awry

Some pre-Voyager theories about Neptune have been severely tried by the data trickling back to earth across the great gulf separating us from what is now the most distant planet.

Before Voyager, Neptune's spin period was believed to be about 17 hours. This was just the spin rate needed by theorists to explain why Neptune radiates much more heat than Uranus. It seems that spin rate is related to the mixing of a planet's molten innards, which in turn affects the rate at which heat reaches the surface where it is radiated away. With Neptune's period now pegged at 16 hours by Voyager's measurements, the mixing-cooling theory is in trouble.

The magnetic-field situation is in even worse shape. When planetary scientists found that Uranus' magnetic field was tilted 60 from the axis of rotation, they worried a bit but didn't think that this one exception would overthrow the favored dynamo theory of field generation. After all, the magnetic fields of Jupiter, Saturn, and earth are reasonably well-behaved. But Neptune's field is now found to be misaligned by 50! The confidence of the planetologists has now been shaken. What, if anything, is different about Neptune and Uranus? It may just be that we don't really know how the magnetic field of any planet is generated.

(Kerr, Richard A.; "The Neptune System in Voyager's Afterglow," Science, 245:1450, 1989.)

Reference. The anomalies of Neptune and the other planets are cataloged in our book: The Moon and the Planets. To order, visit: here.

From Science Frontiers #66, NOV-DEC 1989. 1989-2000 William R. Corliss