Moons of Neptune
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Neptune has thirteen known moons. The largest by far is Triton, discovered by William Lassell just seventeen days after the discovery of Neptune itself. It took about one hundred years to discover the second natural satellite, Nereid.
Triton is massive enough to have achieved hydrostatic equilibrium, and so would be considered a dwarf planet if it were in direct orbit about the Sun. Triton has a very unusual orbit that is circular but retrograde and inclined. Inward of Triton are six regular satellites, which all have prograde orbits that are not greatly inclined with respect to Neptune's equatorial plane. Some of these orbit among Neptune's rings.
Neptune also has six outer irregular satellites, including Nereid, whose orbits are much farther from Neptune, have high inclinations, and are mixed between prograde and retrograde. Two natural satellites discovered in 2002 and 2003, Psamathe and Neso, have the largest orbits of any natural satellites discovered in the Solar system to date. They take 25 years to orbit Neptune at an average of 125 times the distance between Earth and the Moon. Neptune has the largest Hill sphere in the solar system, owing primarily to its large distance from the Sun; this allows it to retain control of such distant moons.
Neptune's moons are named for aquatic personages in Greek and Roman mythology, many of them after Nereids, in keeping with Neptune's position as god of the sea.
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[edit] Theories of origin
Neptune has the most lopsided of all moon systems. One moon, Triton, makes up nearly all of the mass of the system, with all other moons together only one third of one percent (see diagram). This may be because the capture of Triton destroyed much of the original Neptunian system.[citation needed]
It is likely that Neptune's inner satellites are not the original bodies that formed with Neptune but accreted rubble from the havoc that was wreaked after Triton's capture. Triton's original captured orbit would have been highly eccentric, and caused chaotic perturbations in the orbits of the original inner Neptunian satellites, causing them to collide and become reduced to a rubble disc. Only after Triton's orbit became circularised did some of the rubble disc re-accrete into the present-day satellites [1].
The mechanism of the Triton’s capture have been the subject of a few theories over the years. The most recent postulates that Triton was captured in a three body encounter. In this scenario, Triton is the surviving member of a binary object1 disrupted by the encounter with Neptune.[2].
Numerical simulations show that another moon discovered in 2002, Halimede has had a high probability of collision with Nereid during the lifespan of the system.[3] As both moons appear to have similar (grey) colours, the satellite could be a fragment of Nereid.[4]
1Binary objects, gravitational association of two objects, are quite common among Trans-Neptunian Objects (>10%; the most known is Pluto-Charon) and less so among the asteroids (e.g. Ida and Dactyl).
[edit] The moons
The Neptunian moons are listed here by orbital period, from shortest to longest. Triton, which is not only massive enough for its surface to have collapsed into a spheroid, but is comparable in size to our own moon, is highlighted in purple. Irregular (captured) moons are shown in grey; prograde in light grey and retrograde in dark grey. (Triton is also thought to be captured.)
| Order | Name (spheroidal moon in bold) |
Image | Diameter (km) | Mass (1016 kg) |
Semi-major axis (km) | Orbital period‡ (d) | Inclination (°) to Neptune's Equator | Discovery Date | ||
|---|---|---|---|---|---|---|---|---|---|---|
| 1 | Neptune III | Naiad | ˈneɪəd | 67 (96×60×52) | ~19 | 48 227 | 0.294 | 4.7° | 1989 | |
| 2 | Neptune IV | Thalassa | θəˈlæsə | 83 (108×100×52) | ~35 | 50 075 | 0.311 | 0.2° | 1989 | |
| 3 | Neptune V | Despina | dɨsˈpiːnə |
 |
152 (180×150×130) | ~210 | 52 526 | 0.335 | 0.1° | 1989 |
| 4 | Neptune VI | Galatea | ˌgæləˈtiːə | 175 (204×184×144) | 212 | 61 953 | 0.429 | 0.1° | 1989 | |
| 5 | Neptune VII | Larissa | ləˈrɪsə |
 |
195 (216×204×164) | ~420 | 73 548 | 0.555 | 0.2° | 1981 |
| 6 | Neptune VIII | Proteus | ˈproʊtiəs |
.jpg) |
418 (436 × 416 × 402) | ~4 400 | 117 647 | 1.122 | 0.6° | 1989 |
| 7 | Neptune I | Triton | ˈtraɪtən |
.jpg) |
2707 | 2 140 000 | 354 800 | −5.877 | 156.8° | 1846 |
| 8 | Neptune II | Nereid | ˈnɪəriɪd |
 |
340 | ~2 200 | 5 513 400 | 360.14 | 27.6° | 1949 |
| 9 | Neptune IX | Halimede | ˌhælɨˈmiːdi | 60 | ~9 | 15 728 000 | −1 879.71 | 2002 | ||
| 10 | Neptune XI | Sao | ˈseɪ.oʊ | 38 | ~9 | 22 422 000 | 2 914.07 | 2002 | ||
| 11 | Neptune XII | Laomedeia | ˌleɪəmɨˈdiːə | 38 | ~9 | 23 571 000 | 3 167.85 | 2002 | ||
| 12 | Neptune X | Psamathe | ˈsæməθi | 28 | ~1.5 | 46 695 000 | −9 115.91 | 2003 | ||
| 13 | Neptune XIII | Neso | ˈniːsoʊ | 60 | ~9 | 48 387 000 (0.32 AU) |
−9 373.99 | 2002 | ||
‡ Negative orbital periods indicate a retrograde orbit around Neptune (opposite to the planet's rotation)
[edit] Irregular moons
The diagram illustrates the orbits of Neptune’s irregular moons discovered so far. The eccentricity of the orbits is represented by the yellow segments (extending from the pericentre to the apocentre) with the inclination represented on Y axis. The satellites above the X axis are prograde, the satellites beneath are retrograde. The X axis is labelled in Gm (million km) and the fraction of the Hill sphere's (gravitational influence) radius (~116 Gm for Neptune).
Given the similarity of their orbits, it was suggested that Neso and Psamathe could have a common origin in the break-up of a larger moon.[5]
Triton, the biggest moon following a retrograde but a quasi-circular orbit, also conjectured to be a captured satellite, is not shown. Nereid, which has a prograde but very eccentric orbit, is believed to have been scattered during Triton's capture.[6]
[edit] Discovery
Triton was discovered by William Lassell in 1846, seventeen days after Neptune was discovered. Nereid was discovered by Gerard P. Kuiper in 1949. In 1981 Larissa was first observed by Harold J. Reitsema, William B. Hubbard, Larry A. Lebofsky and David J. Thole.
No further moons were found until Voyager 2 flew by Neptune in 1989. Voyager 2 recovered Larissa and discovered five new inner moons, bringing the total of known moons to eight.
From 2002 onwards, telescopic surveys found the remaining five outer moons.
[edit] Naming notes
Some asteroids share the same names as moons of Neptune: 74 Galatea, 1162 Larissa. See also Name conflicts of solar system objects.
Note that Triton did not have an official name until the twentieth century. Although the name was suggested in 1880 by Camille Flammarion, it did not come into common use until at least the 1930s. Usually, it was simply known as "the satellite of Neptune" (the second satellite, Nereid, was not discovered until 1949).
[edit] References
- ^ D. Banfield and N. Murray (1992). "A dynamical history of the inner Neptunian satellites". Icarus 99: 390. doi:.
- ^ C.B. Agnor & D.P. Hamilton Neptune's capture of its moon Triton in a binary-planet gravitational encounter, Nature, 441 (2006), pp. 192. (pdf)
- ^ M.Holman, JJ Kavelaars, B.Gladman, T.Grav, W.Fraser, D.Milisavljevic, P.Nicholson, J.Burns, V.Carruba, J-M.Petit, P.Rousselot, O.Mousis, B.Marsden, R.Jacobson Discovery of five irregular moons of Neptune, Nature, 430 (2004), pp. 865-867. Final preprint(pdf)
- ^ T.Grav, M.Holman and W.Fraser, Photometry of Irregular Satellites of Uranus and Neptune, The Astrophysical Journal, 613 (2004), pp.L77–L80 (preprint)
- ^ Scott S. Sheppard, David C. Jewitt, Jan Kleyna, A Survey for "Normal" Irregular Satellites Around Neptune: Limits to Completeness (preprint)
- ^ Goldreich, P.; Murray, N.; Longaretti, P. Y.; Banfield, D. Neptune's story, Science, 245, (1989), p. 500-504.
[edit] See also
- Natural satellites of Mars · Jupiter · Saturn · Uranus · Pluto
- Timeline of discovery of Solar System planets and their natural satellites
- Naming of natural satellites
[edit] External links
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