Nereid, or Neptune II, is the third-largest moon of Neptune. It was the second moon of Neptune to be discovered, by Gerard Kuiper in 1949. It is notable for its unusually eccentric orbit and relatively large size for an irregular satellite. Nereid also exhibits large brightness variations over long periods of time, the origin of which has yet to be conclusively explained.
Observational history
Discovery and naming
Nereid was discovered on 1 May 1949 by Gerard P. Kuiper using photographic plates taken with the 82-inch telescope at the McDonald Observatory. He proposed the name in the report of his discovery. It is named after the Nereids, sea-nymphs of Greek mythology and attendants of the god Neptune. It was the second moon of Neptune to be discovered, and the last before the arrival of Voyager 2 (not counting a single observation of an occultation by Larissa in 1981). In his original report, Kuiper estimated its apparent magnitude at 19.5, which stood as the only photometric information on Nereid until 1987.
Brightness variations Since 1987 some photometric observations of Nereid have detected large variations of its brightness. Variations over a few days or within the same night have been observed, but the short term behaviour also varies as a long term trend over years and months. They persist even after a correction for distance and phase effects, and are likely related to the rotation of Nereid. Multiple independent observing groups have detected these variations, both within a single observing run and between separate runs, and the variations are too large to have been instrumental errors. On the other hand, not all astronomers who have observed Nereid have noticed any variation at all. Short-term variations of up to ~1.5 magnitudes have been reported by observers, and across all studies, the brightness of Nereid has been seen to deviate up to −1.0 to +1.5 magnitudes away from the average. When data collection started in 1987, Nereid seemed to have very large brightness variations until 1991. This was followed by a gap of a few years with no observations, in which this active period seems to have ended. Nereid then entered an inactive state, and only showed low amplitude variations for a while until another observation gap. After the second gap, Nereid showed variations in between that of the active and inactive periods.
Rotation measurements The rotation period was similarly in dispute. In 1991, a rotation period of Nereid of about 13.6 hours was determined, reporting a very large variation in brightness of 1.3 magnitudes. A 1997 study found no rotation period at all, with a data uncertainty of around ~0.04 mag. In 2003, a rotation period of about 11.52 ± 0.14 hours was measured, with a low peak-to-peak amplitude of only 0.029 ± 0.003 mag. This determination was later questioned by a study in 2008, who were skeptical due to their poor sampling of the light curve. Examining a data set spanning 20 years' worth of ground-based observations, they failed to detect any periodic modulation above 0.08 mag in Nereid's light curve, though their data precision did not allow them to confirm or deny the 2003 rotation period. In 2013, a period of 11.50 ± 0.10 hours was obtained, as well as a 0.031 ± 0.001 mag variation, very similar to the 2003 results. Most recently, in 2016 a study observed Nereid with the Kepler space telescope. Scientists found it useful to measure light curves of distant objects in the Solar System for its ability to gather data uninterrupted for several weeks at a time. Closely matching the previous determinations in 2003 and 2013, Nereid's rotation period was found to be 11.594 ± 0.017 hours, and again showed only low-amplitude variations (0.033 magnitudes).
Physical characteristics
… excerpt ends here. Continue reading the full article.
