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Triton is the planet Neptune's largest moon.

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Triton has a complex geological history and it is believed to have a relatively young surface. Triton was discovered by British Astronomer William Lassell on October 10, 1846, just 17 days after the planet itself was discovered by German astronomer Johann Gottfried Galle. Triton is also believed to be a captured Kuiper belt object.

Name of Triton.

Triton Neptune Moon.
Triton moon.
Discovered by William Lassell
Discovered on October 10, 1846
Orbital characteristics
Semimajor axis 354,800 km
eccentricity 0.0000
Orbital period -5.877 d
inclination 130.267º (to the ecliptic)
157.340º (to Neptune's equator)
130.063º (to Neptune's orbit)
Satellite of Neptune
Physical characteristics
Mean diameter 2706.81.8 km (0.2122 Earths)
Surface area 23,018,000 km
Volume 10,384,000,000 km
mass 2.1471022 kg (0.00359 Earths)
Mean density 2.05 g/cm
Surface gravity 0.782 m/s
escape velocity 1.455 km/s
rotation period 5 d, 21 h, 2 min, 28s
axial tilt zero
Albedo 0.76
Surface temp.
- min
- mean
- max

34.5 K
Atmospheric characteristics
pressure 0.001 kPa
Nitrogen 99.9%
methane 0.1%

Triton is named after the Greek sea god Triton, the son of Neptune. The name was proposed by Camille Flammarion in 1880. It is perhaps strange that Lassell, the discoverer, did not see it fit to name his own discovery, since he gave names a few years later to his subsequent discoveries of an eighth moon of Saturn (Hyperion), and of the third and fourth moons of Uranus (Ariel and Umbriel).

Orbit of Triton.

Triton is unique among all large moons in the solar system for its retrograde orbit around the planet (i.e., it orbits in a direction opposite to the planet's rotation). The small outer moons of Jupiter and Saturn also have retrograde orbits, as do three of Uranus' outer moons, but the largest of them (Phoebe) has only 8% of the diameter (and 0.03% of the mass) of Triton. Moons in retrograde orbits cannot form out of the same region of the Solar nebula as the planets they orbit, but must be captured from elsewhere or turn retrograde through collision. The latter scenario is the least likely with Triton and it is therefore suspected that it is a captured Kuiper belt object.

The capture of Triton may explain a number of features of the Neptunian system including the extremely eccentric orbit of Neptune's moon Nereid, the scarcity of moons as compared to the other gas giants (Triton's orbit could initially have crossed those of many other lighter moons, dispersing them through gravitational interaction), and the evidence of differentiation in Triton's interior (tidal heating resulting from an eccentric post-capture orbit being circularized could have kept Triton liquid for a billion years). Its similarity in size and composition to Pluto, as well as Pluto's eccentric Neptune-crossing orbit, provide further hints to Triton's possible origin as a Pluto-like planetary body.

Theoretical work suggests that prior to the capture Triton may have had a massive companion similar to Pluto's moon Charon. When the binary encountered Neptune, Triton's companion was expelled providing the required mechanism to capture Triton in an orbit around the planet. This theory has several notable advantages, including the fact that binaries are very common among the large Kuiper belt objects, the event was brief but gentle saving Triton from collisional disruption, and events like this may have been common during the formation of Neptune, or later when it migrated outward.

Due to its retrograde motion, the already-close Tritonian orbit is slowly decaying further from tidal interactions and it is predicted that some 3.6 billion years from now, Triton will pass within its Roche limit. The most likely outcome will be collision with Neptune's atmosphere, although ring formation due to tidal disruption is also possible.

Another unique feature of Triton's orbit, arising from tidal effects on such a large moon so close to its primary in a retrograde orbit, is that it is nearly a perfect circle with an eccentricity of zero to sixteen decimal places.

Physical characteristics of Triton.

A cloud over the limb of Triton.

Triton has a density of 2.05 g/cm, and is probably about 25% water ice, with the remainder being rocky material. It has a tenuous nitrogen atmosphere with small amounts of methane. Tritonian atmospheric pressure is only about 0.01 millibar. The surface temperature is at least 35.6 K (-237ºC) because Triton's nitrogen ice is in the warmer, hexagonal beta crystalline state, and the phase transition between beta and cubic alpha nitrogen ice is that temperature. An upper limit in the low 40s can be set from vapor pressure equilibrium with nitrogen gas in Triton's atmosphere. This temperature range is colder than Pluto's average equilibrium temperature of 44 K (-229ºC). Surprisingly, however, Triton is geologically active; its surface is fresh and sparsely cratered, and the Voyager 2 probe observed numerous icy volcanoes or geysers erupting liquid nitrogen, dust, or methane compounds from beneath the surface in plumes up to 8 km high. This volcanic activity is thought to be driven by seasonal heating from the Sun, unlike the tidal heating responsible for the volcanoes of Io. There are extensive ridges and valleys in complex patterns all over Triton's surface, probably the result of freezing/thawing cycles. Triton's surface area is 23 million km (4.5% of Earth, or 15.5% of Earth's land area).

Planetary geology of Triton.

Triton is a geologically active moon with a complex and young surface.

Triton has a similar size, density, and chemical composition to that of Pluto. Noting the eccentric orbit of Pluto, which crosses the orbit of Neptune, we can postulate the origin of Triton as a similar Planetoid captured by Neptune. Therefore, Triton may well have been formed far from Neptune, in the far reaches of the solar system.

Even though there are various differences between Triton and other frozen moons of the solar system, the terrain is similar to Ariel (moon of Uranus), Enceladus (moon of Saturn), and three moons of Jupiter: Io, Europa, and Ganymede. It is also similar to Mars with its polar caps.

The gravitational effect of Triton on the trajectory of Voyager 2 suggests that the icy mantle covers a substantial core of rock (probably containing metal). The core makes up ? of the total mass of Triton, which is more than any other moon in the solar system, with the exception of Io and Europa. Triton has a mean density of 2.05 g/cm and is composed of approximately 25% of water ice, especially in the mantle.

The surface is mainly composed of frozen nitrogen, but it also has dry ice (carbon dioxide), water ice, Carbon monoxide ice, and methane. It is thought that there could be large amounts of ammonia on the surface. Triton is very bright, reflecting 60%-95% of the sunlight that reaches it while Earth's moon reflects only 11%.

General topography of Triton.

The total surface area is about 15.5% of the land area of Earth, or 4.5% of the total area. The dimensions of Triton suggest that there are regions with different densities, varying from 2.07 to 2.3 g/cm. There are areas with rocky outcrops, and there are areas of canyons. Icy substances, mainly frozen methane covers part of the surface.

The southern polar region of Triton is covered by a highly reflective cap of frozen nitrogen and methane sprinkled by impact craters and openings of geysers. Little is known about the north pole of Triton because it was already in the penumbra when Voyager 2 visited Triton. However, it is thought that it must have a polar cap.

Triton geologic activity.
The few craters that exist on Triton reveal intense geologic activity.

In the equatorial region, long faults with parallel mountain ranges of ice expelled from the interior cross complex terrain with valleys. Yasu Sulci, Ho Sulci, and Lo Sulci are some of these systems known as sulci, a term that means 'ridges'. East of these ridges are the plains of Ryugu Planitia and Sipapu Planitia and the plateau of Cipango Planum.

The plains zones of Sipagu Planitia and Abatos Planum in the southern hemisphere meet surrounded by black points, the maculae. Two groups of maculae, Akupara Maculae and Zin Maculae make up the eastern part of Abatos Planum. These marks appear to be deposits on the surface left by ice that evaporated, but neither the composition nor the origin of the ice is known.

Next to Sipapu and Abatos Planum, there is a fresh crater that is 27 km in diameter called Mazomba. Following northwest, there are two smaller craters (Kurma and Ilomba) following the Mazomba crater almost in a straight line. The majority of the holes and wasteland was caused by slipping and collapse of ice, opposite of what happened on other moons, where impact craters dominate the surface. However, Voyager photographed a 500 km impact crater, that was changed extensively by repeated flooding, slipping of ice, and collapses.

"Cantaloupe terrain" of Triton.

Voyager 2.
The cantaloupe-skin terrain as seen from 130,000 km by Voyager 2.

Tano Sulci is one of the long faults that cross the strange region of Bubembe on Triton. This region is also known as "cantaloupe terrain" because of its resemblance to the skin of a cantaloupe. The origin of this region is unknown, but it could have been caused by diapirism (the rising and falling of frozen nitrogen or other ices), by collapses, and by flooding caused by cryovolcanism. Even though the terrain has few craters, it is believed that this is the oldest terrain on Triton. This terrain probably covers much of the northern hemisphere.

This cantaloupe terrain is known to exist only on Triton. It contains depressions that are 30-50 km in diameter. The depressions ("cavi") are probably not impact craters by meteorites because they are regular and have smooth curves. These depressions might have been caused by viscous ice eruptions. Triton is geologically active; its surface is young and has relatively few impact craters. There are valleys and ridges that are very complex on the entire surface, probably the result of tectonism and icy volcanism. Volcanic activity could be related to tidal heating from when Triton was captured by Neptune, somewhat like the way volcanoes on Io are powered today. Hili and Mahilani are two candidate cryovolcanoes that have been observed on Triton. They are named after a Zulu water sprite and a Tongan sea spirit, respectively. Triton is then like the Earth, Io, Enceladus, and perhaps Venus and Titan, one of the few worlds of the solar system that have current volcanic activity.

History of observation and exploration of Triton.

William Lassell started making mirrors for his telescope in 1820, and produced better mirrors in 1844. When John Herschel received news of Neptune's discovery, he wrote to Lassell suggesting he search for moons. Lassell did so and discovered Triton just eight days later, on 10 October 1846, only 17 days after the discovery of the planet itself. Lassell also claimed to have discovered rings. However, although Neptune does have rings, they are so faint and dark that what Lassell saw was probably an illusion.

The first detailed observations of the satellite were not made until 100 years after its discovery, when it was found to have a retrograde orbit around Neptune (i.e., it orbits in a direction opposite to the planet's rotation), which is at a very high angle of inclination to the plane of Neptune's orbit.

Even though the orbital properties of Triton had been defined almost correctly in the 19th century, little was known about Triton itself until Voyager 2 arrived at the end of the 20th century. The satellite appeared pink-yellowish in the first photograph taken.

Before the arrival of Voyager 2, it was suspected that Triton might have liquid nitrogen seas and a nitrogen/methane atmosphere with a density as much as ? that of the Earth. Like the famous overestimates of the atmospheric density of Mars, this was found to be completely false, but like on Mars a denser early atmosphere is postulated.

The first attempt to measure the diameter of Triton was made by Gerard Kuiper in 1954. He obtained a value of 3800 km. Various subsequent attempts to measure the diameter of the satellite arrived at values ranging from 2500 to 6000 km, or slightly smaller than our Moon to nearly half the diameter of Earth.

Neptune and Triton.
Neptune and Triton three days after the flyby of Voyager 2.

Data from Voyager 2's approach to Neptune on August 25, 1989 enabled a more accurate estimate of Triton's diameter to be made (estimated at 2706 km).

In the 1990s, different observations from Earth were made of the limb of Triton using the occultation of stars by Triton. These observations indicated the presence of an atmosphere and an exotic surface. These observations suggest that the atmosphere is denser than was thought on the basis of the measurements made by Voyager 2.

Potential for life

Like the atmosphere of Titan, Triton's atmosphere is composed of nitrogen and methane. Nitrogen is also the principal constituent of Earth’s atmosphere, and the methane on Earth is normally associated with life, being a by-product of life. However, like Titan, Triton is extremely cold (coldest measured object in the solar system). In addition, Triton's atmosphere is almost non-existent (corresponding to a medium vacuum on Earth) and therefore not suitable to any known life forms.

Due to the geological activity and the possible internal heating, it is possible that below the surface of Triton there is a layer of liquid water, supported by antifreezers like ammonia. This subsurface ocean could sustain primitive forms of life, very similar to what has been suggested on the moon Europa.

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