Physical Characteristics
Nereid is Neptune's third-largest satellite and has an average radius of about 170 kilometres (110 mi). It is rather large for an irregular satellite. The shape of Nereid is not known.
Since 1987 some photometric observations of Nereid have detected large (by ~1 of magnitude) variations of it 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. As of 2010 there is no credible explanation of the variations, but, if they exist, they are likely related to the rotation of Nereid. This moon due to its highly elliptical orbit can be either in the state of forced precession or even chaotic rotation (like Hyperion). In any case its rotation should be rather irregular.
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 the outer-Solar-System minor planets, centaurs Pholus, Chiron and Chariklo, suggesting that Nereid formed around Neptune rather than being a captured body.
Halimede, which has similar colors, may be a fragment of Nereid that was broken off during a collision.
Read more about this topic: Nereid (moon)
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