Triton (moon)

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Triton on an image from the Voyager 2 spacecraft, 1989
Triton on an image from the Voyager 2 spacecraft , 1989
Central body Neptune
Properties of the orbit
Major semi-axis 354,759 km
Periapsis 354,753 km
Apoapsis 354,765 km
eccentricity 0.000016
Orbit inclination to the equator of the central body 156.885 °
Orbit inclination to the Laplace plain 129.608 °
Orbit inclination to the ecliptic 129.812 °
Orbital time 5.876854 d
Mean orbital velocity 4.39 km / s
Physical Properties
Albedo (0.756 ± 0.041)
Apparent brightness (13.472 ± 0.041) mag
Medium diameter (2,706.8 ± 1.8) km
Dimensions 2.147 x 10 22 kg
surface 23,000,000 km 2
Medium density 2.061 g / cm 3
Sidereal rotation 5.876854 days
Axis inclination 0.0 °
Acceleration of gravity on the surface 0.779 m / s 2
Escape speed 1455 m / s
Surface temperature −237.5 ° C / 35.6 K

William Lassell

Date of discovery October 10, 1846
Remarks Traces of one atmosphere (1.4-1.9 Pa) 99.9% N 2 ; 0.1% CH 4
Triton Earth Moon Comparison.png
Size comparison between Triton (bottom left), Earth's moon (top left) and Earth (true-to-scale photomontage).

Triton (also Neptune I ) is an ice moon and the eighth innermost moon of the planet Neptune . With a diameter of 2707 kilometers, it is by far the largest moon on the planet as well as the seventh largest moon and the sixteenth largest body in the solar system .

Discovery and naming

Triton was discovered on October 10, 1846 by the beer brewer and amateur astronomer William Lassell . Johann Gottfried Galle had only discovered the giant planet Neptune 17 days earlier . John Herschel had suggested in a letter to Lassell to keep an eye out for possible moons. Lassell did so and found Triton after eight days.

The moon was named after Triton , a sea god from Greek mythology , who is often referred to as the son of Poseidon . The name was only proposed by Camille Flammarion and other astronomers in 1880 , but the name was not officially used for a long time. Records from 1939 indicate that the moon had a name, but that name was not widely used. In the astronomical literature there was only ever talk of the moon of Neptune .

Track properties


Triton orbits Neptune in a retrograde , almost perfectly circular orbit at an average distance of 354,759 kilometers (approx. 14.326 Neptune radii) from its center, i.e. 330,000 kilometers above its cloud ceiling. The orbit eccentricity is 0.000016, the orbit is strongly inclined at 156.885 ° compared to the equator of Neptune .

The orbit of the next inner moon Proteus is 237,100 kilometers away from Triton's orbit, that of the next outer moon Nereid on average 5,159,000 kilometers, due to the high orbital eccentricity Nereid Triton approaches again and again up to about one million kilometers.

Triton needed 5 days and 21 hours to orbit Neptune. Unlike most of the moons in the solar system , it revolves around the planet in a retrograde manner , i.e. against its direction of rotation, which is extremely unusual for a moon of this size with a relatively small distance to the central body and unique in the solar system.

Triton orbits Neptune within a critical distance, whereby it is very exposed to the tidal forces of the gas planet. As Triton is continuously approaching Neptune, in 100 million years it will pass the Roche boundary inward and be torn apart, with its components forming a larger ring system similar to that of Saturn.

Triton is believed to be a larger object in the Kuiper Belt that was captured by Neptune through its gravitational effects . Its structure could be very similar to that of the dwarf planet Pluto , its moon Charon and other members of the Kuiper belt.


The rotation time is the same as the orbital time and Triton shows, like the earth's moon , a synchronous rotation , which takes place in the same retrograde direction of rotation within 5 days, 21 hours, 2 minutes and 40.2 seconds. Its axis of rotation is exactly perpendicular to its plane .

Physical Properties

Triton has an average diameter of 2706.8 kilometers, making it by far the largest Neptune moon . It accounts for 99.5% of the total mass that surrounds Neptune, the rest is distributed over the other 13 moons and Neptune's ring system . Triton also has more mass than all the moons in the solar system that are smaller than itself combined.

Triton compared to the Earth's moon
Surname Diameter
Orbit radius
Orbital period
Triton 2707 2.15 · 10 22 354,800 05.877
Earth moon 3476 7.35 · 10 22 384,400 27.320
Triton / Earth moon
rounded (exact value)
3: 4 (78%) 1: 3 (29%) 9:10 (92%) 1: 5 (22%)

Triton's mean density is 2.05 g / cm 3, the highest in the entire Neptune system. It has a high albedo of 0.76, which means that 76% of the incident sunlight is reflected. This is because a large part of its surface is covered by ice.

The lowest measured temperature on the surface is 35.6  K or −237.6 ° C, the lowest ever measured directly by a probe in the solar system.

The total area is about 23,018,000 square kilometers and thus roughly corresponds to the area of North America (excluding Greenland ).

the atmosphere

The surface temperature of Triton is on average low enough to hold an atmosphere despite the low gravity , which consists of 99% nitrogen, 1% methane and small traces of carbon monoxide . However, the pressure at 1.4-1.9 Pascal , approximately 1 / 70,000 of the atmospheric pressure on the earth corresponds to extremely low. However, convection currents caused by solar radiation in the thin atmosphere ensure that material lifted to the surface by Triton's geysers is distributed over large areas.


Geysers on Triton, recorded by Voyager 2, 1989. (post-colored photo)

When Voyager 2 passed Neptune and its moons on August 25, 1989, it sent images of Triton's surface. A network of faults emerged , where the ice crust was deformed and broken, with only a few impact craters . This suggests that the moon is geologically active, with the tracks of older craters being obliterated by geological or atmospheric processes. Large impact basins were evidently filled up several times with viscous material from the interior. So far unique in the solar system is the " honeydew melon terrain", a formation of hollows and ridges, presumably formed by diapirism , which covers large parts of the western hemisphere.

What was surprising was the evidence of a type of "cold" volcanism known as cryovolcanism (cold or ice volcanism). Active geysers have been found that emit a mixture of liquid nitrogen and entrained rock dust up to a height of 8 km. These are visible on the Voyager images as dark plumes of smoke. The reason for this is likely to be the seasonal warming due to solar radiation, which despite its low intensity is sufficient to evaporate frozen nitrogen. The ejected particles settle on the surface and form deposits of frozen methane and silicates. As a result of solar radiation, the methane is converted into other organic compounds , which are visible as dark streaks and stripes.

Like Pluto, Triton's surface is 55% covered with frozen nitrogen, 15–35% with water ice and 10–20% with dry ice. In addition, a 0.1% methane and a 0.05% carbon monoxide content could be determined.


Triton's axis of rotation is 157 ° inclined to the axis of rotation of Neptune, which in turn is inclined 30 ° to its orbit around the sun . As a result, Triton's poles are temporarily directly facing the sun, similar to those of the planet Uranus. During Neptune's 166 year orbit around the sun, between the times when he turns his equatorial region towards the sun, summer at the North Pole and once at the South Pole for over 40 years, while winter prevails on the opposite side. The associated temperature differences lead to strong seasonal effects.

At the time of the Voyager 2 spacecraft's flyby in 1989, the South Pole was facing the Sun, while the North Pole region had been in shadow for about 30 years, with temperatures down to −235 ° C (38 K). There, deposits of frozen nitrogen and methane could be seen , which are evidently heated over and over again with the seasons, evaporate and precipitate as ice on the pole in the shadow.

internal structure

Triton presumably consists of a differentiated structure, a core of silicate rock and a crust of water ice . Research from 2012 also suggests the possibility that a thin, ammonia-rich ocean exists beneath the surface. The energy to keep the ocean below the surface liquid at −90 ° C comes from the decay of radioactive materials in Triton's interior and the tidal friction that occurs when Neptune orbits.


Before the Voyager 2 probe's Neptune flyby in the summer of 1989, very little was known about Triton, although it had been known for 143 years. Previously it could only be observed through earth-based telescopes and its orbital elements and its brightness could be determined. Estimates of its size were based on the presumed retroreflective ability and varied between 3200 and 6000 km, since Triton was believed to be darker. Voyager 2 performed a swing-by on Neptune 4824 km away to pass Triton on August 25, 1989 at 39,790 km minimum distance. The geysers were discovered during later evaluations of the images on October 2, 1989.

Since the flyby, the Neptune system has been studied intensively by earth-based observations as well as the Hubble space telescope .

See also


Web links

Commons : Triton (moon)  - album with pictures, videos and audio files

Individual evidence

  1. See Holger Heuseler, Ralf Jaumann , Gerhard Neukum : Between the Sun and Pluto. The future of planetary research - the dawn of the third millennium. BLV, Munich / Vienna / Zurich 1999, ISBN 3-405-15726-9 , p. 165.
  2. William B. McKinnon, Randolph L. Kirk: Triton. In: Tilman Spohn, Doris Breuer, Torrence Johnson (Eds.): Encyclopedia of the Solar System. 3. Edition. Elsevier, Amsterdam / Boston, 2014, ISBN 978-0-12-416034-7 , pp. 861-882.
  3. Jodi Gaeman, Saswata Hier-Majumder, James H. Roberts, Sustainability of a Subsurface Ocean within Triton's Interior, Icarus, Available online May 18, 2012, ISSN  0019-1035 , doi : 10.1016 / j.icarus.2012.05.006 .
  4. Lars-C. Depka: A sea on Triton? ,, June 11, 2012, accessed: June 11, 2012