Moons of Neptune
The planet Neptune has 16 known moons, which are named for minor water deities and a water creature 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 century passed before the discovery of the second natural satellite, Nereid, in 1949, and another 40 years passed before Proteus, Neptune's second-largest moon, was discovered in 1989.
Triton is unique among moons of planetary mass in that its orbit is retrograde to Neptune's rotation and inclined relative to Neptune's equator, which suggests that it did not form in orbit around Neptune but was instead gravitationally captured by it. The next-largest satellite in the Solar System suspected to be captured, Saturn's moon Phoebe, has only 0.03% of Triton's mass. The capture of Triton, probably occurring some time after Neptune formed a satellite system, was a catastrophic event for Neptune's original satellites, disrupting their orbits so that they collided to form a rubble disc. Triton is massive enough to have achieved hydrostatic equilibrium and to retain a thin atmosphere capable of forming clouds and hazes.
Inward of Triton are seven small
History
Discovery
Triton was discovered by William Lassell in 1846, just seventeen days after the discovery of Neptune.[3] Nereid was discovered by Gerard P. Kuiper in 1949.[4] The third moon, later named Larissa, was first observed by Harold J. Reitsema, William B. Hubbard, Larry A. Lebofsky and David J. Tholen on May 24, 1981. The astronomers were observing a star's close approach to Neptune, looking for rings similar to those discovered around Uranus four years earlier.[5] If rings were present, the star's luminosity would decrease slightly just before the planet's closest approach. The star's luminosity dipped only for several seconds, which meant that it was due to a moon rather than a ring.
No further moons were found until
In 2013
In 2021,
Discovery of outer planet moons
Graphs are unavailable due to technical issues. There is more info on Phabricator and on MediaWiki.org. |
Names
Triton did not have an official name until the twentieth century. The name "Triton" was suggested by
For the "normal" irregular satellites, the general convention is to use names ending in "a" for prograde satellites, names ending in "e" for retrograde satellites, and names ending in "o" for exceptionally inclined satellites, exactly like the convention for the moons of Jupiter.[18] Two asteroids share the same names as moons of Neptune: 74 Galatea and 1162 Larissa.
Characteristics
The moons of Neptune can be divided into two groups:
Regular moons
In order of distance from Neptune, the regular moons are
The inner moons are closely associated with
Only the two largest regular moons have been imaged with a resolution sufficient to discern their shapes and surface features.
All of Neptune's inner moons are dark objects: their geometric albedo ranges from 7 to 10%.[25] Their spectra indicate that they are made from water ice contaminated by some very dark material, probably complex organic compounds. In this respect, the inner Neptunian moons are similar to the inner Uranian moons.[6]
Irregular moons
In order of their distance from the planet, the irregular moons are Triton, Nereid, Halimede, Sao, S/2002 N 5, Laomedeia, Psamathe, Neso, and S/2021 N 1, a group that includes both prograde and retrograde objects.[19] The seven outermost moons are similar to the irregular moons of other giant planets, and are thought to have been gravitationally captured by Neptune, unlike the regular satellites, which probably formed in situ.[8]
Triton and Nereid are unusual irregular satellites and are thus treated separately from the other seven irregular Neptunian moons, which are more like the outer irregular satellites of the other outer planets.[8] Firstly, they are the largest two known irregular moons in the Solar System, with Triton being almost an order of magnitude larger than all other known irregular moons. Secondly, they both have atypically small semi-major axes, with Triton's being over an order of magnitude smaller than those of all other known irregular moons. Thirdly, they both have unusual orbital eccentricities: Nereid has one of the most eccentric orbits of any known irregular satellite, and Triton's orbit is a nearly perfect circle. Finally, Nereid also has the lowest inclination of any known irregular satellite.[8]
Triton
Triton follows a retrograde and quasi-circular orbit, and is thought to be a gravitationally captured satellite. It was the second moon in the Solar System that was discovered to have a substantial
Nereid
Nereid is the third-largest moon of Neptune. It has a prograde but very eccentric orbit and is believed to be a former regular satellite that was scattered to its current orbit through gravitational interactions during Triton's capture.[30] Water ice has been spectroscopically detected on its surface. Early measurements of Nereid showed large, irregular variations in its visible magnitude, which were speculated to be caused by forced precession or chaotic rotation combined with an elongated shape and bright or dark spots on the surface.[31] This was disproved in 2016, when observations from the Kepler space telescope showed only minor variations. Thermal modeling based on infrared observations from the Spitzer and Herschel space telescopes suggest that Nereid is only moderately elongated which disfavours forced precession of the rotation.[32] The thermal model also indicates that the surface roughness of Nereid is very high, likely similar to the Saturnian moon Hyperion.[32]
Nereid dominates the normal irregular satellites of Neptune, having about 98% of the mass of Neptune's entire irregular satellite system altogether (if Triton is not counted). This is similar to the situation of Phoebe at Saturn. If it is counted as a normal irregular satellite (but not Triton), then Nereid is also by far the largest normal irregular satellite known, having about two-thirds the mass of all normal irregular moons combined.[33]
Normal irregular moons
Among the remaining irregular moons, Sao, S/2002 N 5, and Laomedeia follow prograde orbits, whereas Halimede, Psamathe, Neso and S/2021 N 1 follow retrograde orbits. There are at least two groups of moons that share similar orbits, with the prograde moons Sao, S/2002 N 5, and Laomedeia belonging to the Sao group and the retrograde moons Psamathe, Neso, and S/2021 N 1 belonging to the Neso group.[12] The moons of the Neso group have the largest orbits of any natural satellites discovered in the Solar System to date, with average orbital distances over 125 times the distance between Earth and the Moon and orbital periods over 25 years.[34] 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.[19] Nevertheless, the Jovian moons in the Carme and Pasiphae groups orbit at a greater percentage of their primary's Hill radius than the Neso group moons.[19]
Formation
The mass distribution of the Neptunian moons is the most lopsided of the satellite systems of the giant planets in the Solar System. One moon, Triton, makes up nearly all of the mass of the system, with all other moons together comprising only one third of one percent. This is similar to the moon system of Saturn, where Titan makes up more than 95% of the total mass, but is different from the more balanced systems of Jupiter and Uranus. The reason for the lopsidedness of the present Neptunian system is that Triton was captured well after the formation of Neptune's original satellite system, and experts conjecture much of the system was destroyed in the process of capture.[30][35]
Triton's orbit upon capture would have been highly eccentric, and would have caused chaotic perturbations in the orbits of the original inner Neptunian satellites, causing them to collide and reduce to a disc of rubble.[30] This means it is likely that Neptune's present inner satellites are not the original bodies that formed with Neptune. Only after Triton's orbit became circularised could some of the rubble re-accrete into the present-day regular moons.[24]
The mechanism of Triton's capture has been the subject of several theories over the years. One of them postulates that Triton was captured in a three-body encounter. In this scenario, Triton is the surviving member of a binary Kuiper belt object[note 2] disrupted by its encounter with Neptune.[36]
Numerical simulations show that there is a 0.41 probability that the moon Halimede collided with Nereid at some time in the past.[7] Although it is not known whether any collision has taken place, both moons appear to have similar ("grey") colors, implying that Halimede could be a fragment of Nereid.[37]
List
The Neptunian moons are listed here by orbital period, from shortest to longest. Irregular (captured) moons are marked by color. The orbits and mean distances of the irregular moons are variable over short timescales due to frequent planetary and solar perturbations, therefore the listed orbital elements of all irregular moons are averaged over a 30,000-year period: these may differ from osculating orbital elements provided by other sources.[38] Their orbital elements are all based on the epoch of 1 January 2020.[1] Triton, the only Neptunian moon massive enough for its surface to have collapsed into a spheroid, is emboldened.
Key | |||||
---|---|---|---|---|---|
Inner moons |
♠ Triton |
† Nereid |
‡ Halimede |
♦ Sao group |
♥ Neso group |
Label [note 3] |
Name | Pronunciation ( key )
|
Image | Abs. magn. |
Diameter (km)[note 4] |
Mass (×1016 kg) [note 5] |
Semi-major axis
(km)[17] |
Orbital period (d)[1] |
Orbital inclination (°)[1] |
Eccentricity [17][note 6] |
Discovery year[16] |
Year announced | Discoverer [16] |
Group |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
III | Naiad | /ˈneɪəd, ˈnaɪæd/[42] | 9.6 | 60.4 (96 × 60 × 52) |
≈ 13 | 48224 | +0.2944 | 4.691 | 0.0047 | 1989 | 1989 | Voyager Science Team | inner | |
IV | Thalassa | /θəˈlæsə/ | 8.7 | 81.4 (108 × 100 × 52) |
≈ 35 | 50074 | +0.3115 | 0.135 | 0.0018 | 1989 | 1989 | Voyager Science Team | inner | |
V | Despina | /dəˈspaɪnə/ | 7.3 | 156 (180 × 148 × 128) |
≈ 170 | 52526 | +0.3346 | 0.068 | 0.0004 | 1989 | 1989 | Voyager Science Team | inner | |
VI | Galatea | /ˌɡæləˈtiːə/ | 7.2 | 174.8 (204 × 184 × 144) |
≈ 280 | 61953 | +0.4287 | 0.034 | 0.0001 | 1989 | 1989 | Voyager Science Team | inner | |
VII | Larissa | /ləˈrɪsə/ | 6.8 | 194 (216 × 204 × 168) |
≈ 380 | 73548 | +0.5555 | 0.205 | 0.0012 | 1981 | 1981 | Reitsema et al. | inner | |
XIV | Hippocamp | /ˈhɪpəkæmp/ | 10.5 | 34.8±4.0 | ≈ 2.2 | 105283 | +0.9500 | 0.064 | 0.0005 | 2013 | 2013 | Showalter et al. | inner | |
VIII | Proteus | /ˈproʊtiəs/ | 5.0 | 420 (436 × 416 × 402) |
≈ 3900 | 117646 | +1.1223 | 0.075 | 0.0005 | 1989 | 1989 | Voyager Science Team | inner | |
I | Triton♠ | /ˈtraɪtən/ | –1.2 | 2705.2±4.8 (2709 × 2706 × 2705) |
2139000 | 354759 | −5.8769 | 156.865 | 0.0000 | 1846 | 1846 | Lassell | ||
II | Nereid† | /ˈnɪəriəd/ | 4.4 | 357 ± 13 | ≈ 2400 | 5504000 | +360.14 | 5.8 | 0.749 | 1949 | 1949 | Kuiper | ||
IX | Halimede‡ | /ˌhæləˈmiːdiː/ | 10.0 | ≈ 62 | ≈ 12 | 16590500 | −1879.78 | 119.6 | 0.521 | 2002 | 2003 | Holman et al. | ||
XI | Sao♦ | /ˈseɪoʊ/ | 11.1 | ≈ 44 | ≈ 3.4 | 22239900 | +2919.43 | 50.2 | 0.296 | 2002 | 2003 | Holman et al. | Sao | |
S/2002 N 5♦ | 11.2 | ≈ 38 | ≈ 3 | 23414700 | +3156.56 | 46.3 | 0.433 | 2002 | 2024 | Holman et al. | Sao | |||
XII | Laomedeia♦ | /ˌleɪəməˈdiːə/ | 10.8 | ≈ 42 | ≈ 3.4 | 23499900 | +3176.13 | 36.9 | 0.419 | 2002 | 2003 | Holman et al. | Sao | |
X | Psamathe♥ | /ˈsæməθiː/ | 11.0 | ≈ 40 | ≈ 2.9 | 47615100 | −9149.51 | 127.8 | 0.414 | 2003 | 2003 | Sheppard et al. | Neso | |
XIII | Neso♥ | /ˈniːsoʊ/ | 10.7 | ≈ 60 | ≈ 11 | 49895300 | −9794.99 | 128.4 | 0.455 | 2002 | 2003 | Holman et al. | Neso | |
S/2021 N 1♥ | 12.1 | ≈ 25 | ≈ 0.8 | 50700200 | −10036.65 | 135.2 | 0.503 | 2021 | 2024 | Sheppard et al. | Neso |
See also
Notes
- ^ The geometric albedo of an astronomical body is the ratio of its actual brightness at zero phase angle (i.e. as seen from the light source) to that of an idealized flat, fully reflecting, diffusively scattering (Lambertian) disk with the same cross-section. The Bond albedo, named after the American astronomer George Phillips Bond (1825–1865), who originally proposed it, is the fraction of power in the total electromagnetic radiation incident on an astronomical body that is scattered back out into space. The Bond albedo is a value strictly between 0 and 1, as it includes all possible scattered light (but not radiation from the body itself). This is in contrast to other definitions of albedo such as the geometric albedo, which can be above 1. In general, though, the Bond albedo may be greater or smaller than the geometric albedo, depending on surface and atmospheric properties of the body in question.
- ^ Binary objects, objects with moons such as the Pluto–Charon system, are quite common among the larger trans-Neptunian objects (TNOs). Around 11% of all TNOs may be binaries.[36]
- ^ Label refers to the Roman numeral attributed to each moon in order of their discovery.[16]
- ^ Diameters with multiple entries such as "60×40×34" reflect that the body is not spherical and that each of its dimensions has been measured well enough to provide a 3-axis estimate. The dimensions of the five inner moons were taken from Karkoschka, 2003.[25] Dimensions of Proteus are from Stooke, 1994.[23] Dimensions of Triton are from Thomas, 2000,[39] whereas its diameter is taken from Davies et al., 1991.[40] The size of Nereid is from Kiss et al., 2016,[32] and the sizes of the other outer moons are from Sheppard, with the diameters of S/2002 N 5 and S/2021 N 1 calculated assuming an albedo of 0.04.[34]
- ^ Of all known moons of Neptune, only Triton has a reliably measured mass.[41] The masses of all regular satellites were estimated by JPL,[41] while all other irregular moons of Neptune were calculated assuming a density of 1 g/cm3.
- ^ Since the reference Showalter et al. (2019) does not cover irregular moons (with colored background), their eccentricities are taken from Planetary Satellite Mean Elements of JPL.[1]
References
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- ^ a b "MPEC 2024-D112 : S/2021 N 1". Minor Planet Electronic Circular. Minor Planet Center. 23 February 2024. Retrieved 23 February 2024.
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- ^
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- ^ Hussmann, Hauke; Sohl, Frank; Spohn, Tilman (November 2006). "Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects". .
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