Nereid, or Neptune II, is the third-largest
moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
of
Neptune. It has the most eccentric orbit of all known moons in the
Solar System
The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
.
It was the second
moon of Neptune to be discovered, by
Gerard Kuiper
Gerard Peter Kuiper (; ; born Gerrit Pieter Kuiper; 7 December 1905 – 23 December 1973) was a Dutch astronomer, planetary scientist, selenographer, author and professor. He is the eponymous namesake of the Kuiper belt.
Kuiper is ...
in 1949.
Discovery and naming
Nereid was discovered on 1 May 1949 by
Gerard P. Kuiper
Gerard Peter Kuiper (; ; born Gerrit Pieter Kuiper; 7 December 1905 – 23 December 1973) was a Dutch astronomer, planetary scientist, selenographer, author and professor. He is the eponymous namesake of the Kuiper belt.
Kuiper is co ...
on photographic plates taken with the 82-inch telescope at the
McDonald Observatory
McDonald Observatory is an astronomical observatory located near unincorporated community of Fort Davis in Jeff Davis County, Texas, United States. The facility is located on Mount Locke in the Davis Mountains of West Texas, with additional faci ...
. He proposed the name in the report of his discovery. It is named after the
Nereid
In Greek mythology, the Nereids or Nereides ( ; grc, Νηρηΐδες, Nērēḯdes; , also Νημερτές) are sea nymphs (female spirits of sea waters), the 50 daughters of the ' Old Man of the Sea' Nereus and the Oceanid Doris, sisters ...
s, sea-nymphs of
Greek mythology
A major branch of classical mythology, Greek mythology is the body of myths originally told by the ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the origin and nature of the world, the lives and activities ...
and attendants of the god
Neptune.
It was the second and last moon of Neptune to be discovered before the arrival of ''
Voyager 2
''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, '' Voyager 1'', o ...
'' (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 .
It is rather large for an
irregular satellite
In astronomy, an irregular moon, irregular satellite or irregular natural satellite is a natural satellite following a distant, inclined, and often eccentric and retrograde orbit. They have been captured by their parent planet, unlike regular s ...
.
The shape of Nereid is unknown.
Nereid is around half the size and mass of
Saturn's moon
Mimas, though Nereid is slightly denser.
Since 1987 some photometric observations of Nereid have detected large (by ~1 of 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
Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In oth ...
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 suggest that Nereid is only moderately elongated with an
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 Water ice could refer to:
* Ice formed by water (as opposed to other substances)
*The alternate term for various similar frozen fruit-flavoured desserts:
** Italian ice primarily in Philadelphia and the Delaware Valley
**Sorbet
Sorbet (), also ...
has been detected on its surface.
Its spectrum appears to be intermediate between
Uranus
Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus ( Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars), grandfather of Zeus (Jupiter) and father of ...
'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 planet
According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''mino ...
s of the outer solar system,
centaurs
Pholus,
Chiron
In Greek mythology, Chiron ( ; also Cheiron or Kheiron; ) was held to be the superlative centaur amongst his brethren since he was called the "wisest and justest of all the centaurs".
Biography
Chiron was notable throughout Greek mythology ...
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
orbit
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...
s
Neptune in the
prograde direction at an
average distance of , but its high
eccentricity
Eccentricity or eccentric may refer to:
* Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal"
Mathematics, science and technology Mathematics
* Off-Centre (geometry), center, in geometry
* Eccentricity (g ...
of 0.749 takes it as close as and as far as .
The unusual orbit suggests that it may be either a captured
asteroid or
Kuiper belt object, or that it was an inner moon in the past and was perturbed during the capture of Neptune's largest moon
Triton.
If the latter is true, it may be the only survivor of Neptune's original (pre-
Triton capture) set of
regular satellite
In astronomy, a regular moon is a natural satellite following a relatively close and prograde orbit with little orbital inclination or eccentricity. They are believed to have formed in orbit about their primary, as opposed to irregular moons, whi ...
s.
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 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
The Kepler space telescope is a disused space telescope launched by NASA in 2009 to discover Earth-sized Exoplanet, planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocen ...
.
Exploration
The only spacecraft to visit Nereid was ''
Voyager 2
''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, '' Voyager 1'', o ...
'', which passed it at a distance of
between 20 April and 19 August 1989.
''Voyager 2'' obtained 83 images with observation accuracies of to .
Prior to ''Voyager 2s 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
Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refl ...
than Neptune's other small satellites.
See also
*
Moons of Neptune
The planet Neptune has 14 known moons, which are named for minor water deities in Greek mythology. By far the largest of them is Triton, discovered by William Lassell on October 10, 1846, 17 days after the discovery of Neptune itself; over a ce ...
References
{{DEFAULTSORT:Nereid (Moon)
Moons of Neptune
Irregular satellites
19490501
Discoveries by Gerard Kuiper
Moons with a prograde orbit