DPS 35th Meeting, 1-6 September 2003
Session 23. Planetary Bookends I
Oral, Chairs: W. B. McKinnon and W. M. Grundy, Thursday, September 4, 2003, 10:30am-12:00noon, DeAnza I-II

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[23.05] Nereid's Story

A. R. Dobrovolskis (Lick Observatory, U. C. Santa Cruz)

The photometric variability of Neptune's moon Nereid has been a source of controversy since 1988, when Schaefer and Schaefer (Nature 333, 436-438) reported that its brightness varied by a factor of 3 with a period of 8 to 24 hours. Subsequent observations (by the same authors as well as others) did not agree as to the amplitude, period, nature, or even existence of these variations.

In an effort to reconcile these discrepancies, Dobrovolskis (Icarus 118, 181-198, 1995) suggested that Nereid might be tumbling chaotically, like Hyperion. He found that this is possible only if Nereid has been despun to a rotation period on the order of a month. According to Goldreich et al. (Science 245, 500-504, 1989), Nereid was once a regular satellite of Neptune which was perturbed into its present eccentric and inclined orbit during the capture and subsequent tidal evolution of Triton. Tides could have despun Nereid if it originated with a semimajor axis of about 26 Neptune radii or less (Dobrovolskis 1995).

However, it now appears that Nereid is not in chaotic or even nonprincipal axis rotation. As an adjunct to a successful search for other small outer satellites of Neptune, Grav, Holman, and Kavelaars (Ap. J. 591, L71-L74, 2003) have obtained accurate photometry of Nereid with fine time resolution. Their data reveal a nearly sinusoidal brightness variation of 2.8 percent with a period of 7.76 +/- 0.07 hours. If this is due to albedo contrasts across Nereid's surface, it represents Nereid's rotation period; if this is due to an elongated shape, Nereid's spin period is twice as long, or 11.52 +/- 0.14 hours.

In either case, these results imply that Nereid never was despun, or has been despun only partially. In turn this means that Nereid could not have originated as a close satellite of Neptune. Constraints on Nereid's origin and history will be described.

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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.