Nereid (moon)

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).

Physical characteristics

Nereid is third-largest of Neptune's satellites, and has a mean radius of about 180 km, similar to Saturn's moon Mimas. It is by far the largest normal irregular moon known, having about two-thirds the mass of all irregular moons combined. (Triton is much larger, but is very unusual for an irregular satellite.) Nereid also accounts for about 98% of the mass of Neptune's entire irregular satellite system altogether (excluding Triton), which is similar to the situation of Phoebe at Saturn (the second-largest normal irregular moon in the Solar System).

The shape of Nereid is unknown. Since 1987 some photometric observations of Nereid have detected large (by ~1 magnitude) variations of its brightness, which can happen over years and months, but sometimes even over a few days. They persist even after a correction for distance and phase effects. On the other hand, not all astronomers who have observed Nereid have noticed such variations. This means that they may be quite chaotic. To date, there is no credible explanation of the variations, but if they exist, they are likely related to the rotation of Nereid. Nereid's rotation could be either in the state of forced precession or even chaotic rotation (like Hyperion) due to its highly elliptical orbit.

In 2016, extended observations with the Kepler space telescope showed only low-amplitude variations (0.033 magnitudes). Thermal modeling based on infrared observations from the Spitzer and Herschel space telescopes suggests that Nereid is only moderately elongated with a maximum aspect ratio of 1.3:1, which disfavors forced precession of the rotation. The thermal model also indicates that the surface roughness of Nereid is very high, likely similar to the Saturnian moon Hyperion.

Spectrally, Nereid appears neutral in colour and water ice has been detected on its surface. Its spectrum appears to be intermediate between Uranus's moons Titania and Umbriel, which suggests that Nereid's surface is composed of a mixture of water ice and some spectrally neutral material. The spectrum is markedly different from minor planets of the outer solar system, centaurs Pholus, Chiron and Chariklo, suggesting that Nereid formed around Neptune rather than being a captured body.

Halimede, which displays a similar gray neutral colour, may be a fragment of Nereid that was broken off during a collision.

Orbit and rotation

Nereid orbits Neptune in the prograde direction at an average distance of 5504000 km, but its high eccentricity of 0.749 takes it as close as 1381500 km and as far as 9626500 km.

In 1991, a rotation period of Nereid of about 13.6 hours was determined by an analysis of its light curve. In 2003, another rotation period of about 11.52 ± 0.14 hours was measured. However, this determination was later disputed, and other researchers for a time failed to detect any periodic modulation in Nereid's light curve from ground-based observations. In 2016, a clear rotation period of 11.594 ± 0.017 hours was determined based on observations with the Kepler space telescope.

Exploration

The only spacecraft to visit Nereid was Voyager 2, which passed it at a distance of 4700000 km between 20 April and 19 August 1989. Voyager 2 obtained 83 images with observation accuracies of 70 km to 800 km. Prior to *Voyager 2'''s arrival, observations of Nereid had been limited to ground-based observations that could only establish its intrinsic brightness and orbital elements. Although the images obtained by Voyager 2 do not have a high enough resolution to allow surface features to be distinguished, *Voyager 2'' was able to measure the size of Nereid and found that it was grey in colour and had a higher albedo than Neptune's other small satellites.

If selected, the Arcanum mission would do a flyby of Nereid before its primary purposes of orbiting Neptune and observing Triton.

Notes

References

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|access-date = 2011-10-26 |archive-url = https://web.archive.org/web/20100527091333/http://ssd.jpl.nasa.gov/?sat_phys_par |archive-date = 2010-05-27 |url-status = live}}

| author-link = Bradley Schaefer

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References

1. {{OED. Nereid
2. {{OED. nereidian, nereidean
3. Denk, Tilmann. (2024). "Outer Moons of Saturn". Tilmann Denk.
4. (March 2020). "Orbits and resonances of the regular moons of Neptune". Icarus.
5. (2024). "Concept of operations for the Neptune system mission Arcanum". The Aeronautical Journal.
6. (2021). "An L-class Multirole Observatory and Science Platform for Neptune".