Nereid Has Complex Large-Amplitude Photometric Variability

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We report on 224 photometric measurements of Nereid, a small outer satellite of Neptune with a 360-day orbit of high eccentricity (0.751). Our photometry covers 64 nights from 1987 to 1997 and is primarily in the V-band, although we also have 20 measurements in the U, B, R, and I bands. (1) Nereid displays large-amplitude brightness variations with a total amplitude of 1.83 mag on time scales ranging from a few hours to roughly a year. (2) During the 12 days of the Voyager encounter with Neptune, Nereid did not display any short-term variations; however, large-amplitude long-term variations could easily be hidden by the large phase effects and the short duration of observation. (3) Nereid's variability is caused by high contrast albedo features, i.e., a dark hemisphere, along with rotational modulation. (4) The character of the brightness variations changed around 1991 from fast and large-amplitude to comparatively slow and low-amplitude. This demonstrates that the direction and magnitude of Nereid's rotational angular momentum vector is changing on time scales comparable to its orbital period. (5) Large changes in the magnitude and direction of Nereid's angular momentum vector are predicted to arise from chaotic rotation during every periapse passage provided that Nereid is more than ~1% nonspherical and is spinning slowly. The match between prediction and observation could be taken as strong evidence for chaotic rotation of Nereid. However, the intranight variability cannot be readily explained by chaotic rotation. (6) The colors of Nereid are U-V=0.84±0.05, B-V=0.71±0.04, V-R=0.44±0.03, V-I=0.72±0.05, and V-K=1.6, indicating a nearly flat reflectance spectrum from 0.36 to 2.2 μm. We identify asteroids, inner satellites, and centaurs with similar spectra. (7) Nereid is likely either an inner moon of Neptune kicked to its current orbit or a captured Kuiper Belt object or centaur, with the latter possibility being strongly preferred. © 2000 Academic Press.

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