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No. 59: Sep-Oct 1988

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Nereid: grotesque shape or two-faced?

Nereid, a satellite of Neptune, is peculiar in several ways:

  1. Its orbit is retrograde and highly elliptical (1.4 x 9.7 million kilometers)
  2. Its brightness changes by a factor of four as it rotates
  3. Its diameter, according to M.W. and B.E. Schaefer (Nature, 333:436, 1988) is thought to be at least 660 kilometers.

None of these facts taken alone is anomalous, but (2) and (3) taken together seem incompatible. If the large brightness changes are due to a highly irregular shape, Nereid's 660-kilometer size is too large, because astronomers agree that gravitational forces will sphericize all objects larger than 400 kilometers. On the other hand, if Nereid is two-faced, like Saturn's moon Iapetus (it's carbon-black on one side, light-colored on the other), astronomers are again faced with trying to explain how such a large solar-system object can acquire so much carbonaceous material on one side only. Also, Nereid's eccentric, retrograde orbit surely hints at a history of capture or orbit disruption. (Weisburd, S.; "Neptune's Nereid: Another Mysterious Moon," Science News, 133:374, 1988. Also: Veverka, J.; "Taking a Dim View of Nereid," Nature, 333:391, 1988.)

From Science Frontiers #59, SEP-OCT 1988. 1988-2000 William R. Corliss