Triton

Triton is the largest moon of the planet Neptune, discovered on October 10, 1846 by William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation.[9] At 2700 km in diameter, it is the seventh-largest moon in the Solar System. Because of its retrograde orbit and composition similar to Pluto's, Triton is thought to have been captured from the Kuiper belt.[10] Triton consists of a crust of frozen nitrogen over an icy mantle believed to cover a substantial core of rock and metal.[4] The core makes up two-thirds of its total mass. Triton has a mean density of 2.061 g/cm3[3] and is composed of approximately 15–35% water ice.[4]

Triton is one of the few moons in the Solar System known to be geologically active. As a consequence, its surface is relatively young, with a complex geological history revealed in intricate and mysterious cryovolcanic and tectonic terrains.[4] Part of its crust is dotted with geysers believed to erupt nitrogen.[7] Triton has a tenuous nitrogen atmosphere less than 1/70 000th the pressure of Earth's atmosphere at sea level.[7]

Discovery and naming
William Lassell, the discoverer of Triton

The moon was discovered by British astronomer William Lassell on October 10, 1846,[11] just 17 days after Neptune itself was discovered by German astronomers Johann Gottfried Galle and Heinrich Louis d'Arrest, who were following coordinates given them by French astronomer and mathematician Urbain Le Verrier.

A brewer by trade, Lassell began making mirrors for his amateur telescope in 1820. When John Herschel received news of Neptune's discovery, he wrote to Lassell suggesting he search for possible moons. Lassell did so and discovered Triton after just eight more days.[12][11] Lassell also claimed to have discovered rings. Although Neptune was later confirmed to have rings, they are so faint and dark that his original claim is questioned.[13]

Triton is named after the Greek sea god Triton (Τρίτων), the son of Poseidon (the Greek god comparable to the Roman Neptune). The name was first proposed by Camille Flammarion in his 1880 book Astronomie Populaire,[14] although it was not officially adopted until many decades later.[15] Until the discovery of the second moon Nereid in 1949, Triton was commonly known as simply "the satellite of Neptune". Lassell did not name his own discovery, although he suggested names a few years later to his subsequent discovery of an eighth moon of Saturn (Hyperion). The third and fourth moons of Uranus (Ariel and Umbriel), which Lassell discovered in 1851, were named by John Herschel.[16]

Orbit and rotation

Triton is unique among all large moons in the Solar System for its retrograde orbit around its planet (i.e., it orbits in a direction opposite to the planet's rotation). Most of the outer irregular moons of Jupiter and Saturn also have retrograde orbits, as do some of Uranus' outer moons. However, these moons are all much more distant from their primaries, and are quite small in comparison; the largest of them (Phoebe)[h] has only 8% of the diameter (and 0.03% of the mass) of Triton.

Triton orbits in synchronous rotation about Neptune; it keeps one face oriented toward the planet at all times. As a result of its unusual orbital inclination, Triton's axis of rotation is tilted 157 degrees with respect to Neptune's axis, which is in turn inclined 30 degrees from the plane of Neptune's orbit.[2] The net result of these two axial tilts is that Triton's rotational axis lies close to the plane of Neptune's orbit, and hence during Neptune's year each pole points almost directly toward the Sun, much like Uranus'. As Neptune orbits the Sun, Triton's polar regions take turns facing the sun, probably resulting in radical seasonal changes as one pole, then the other, moves into the sunlight.

Triton's revolution around Neptune has become a nearly perfect circle with an eccentricity of almost zero. Viscoelastic damping from tides alone are not believed to be capable of circularizing Triton's orbit in the time since the origin of the system, and gas drag from a prograde debris disc is likely to have played a substantial role.[2] Tidal interactions have also meant that Triton's orbit, already close to Neptune, is slowly decaying further, and predictions are that some 3.6 billion years from now, Triton will pass within Neptune's Roche limit.[17] This will result in either a collision with Neptune's atmosphere or the breakup of Triton, forming a ring system similar to that found around Saturn.[17]

Capture
The Kuiper belt, from which Triton is believed to have originated

Moons in retrograde orbits cannot have formed out of the same region of the solar nebula as the planets they orbit, but must have been captured from elsewhere. Triton is therefore suspected of being captured from the Kuiper belt.[10] The Kuiper belt is a ring of small icy objects extending outward from just inside the orbit of Neptune to about 55 AU from the Sun. Believed to be the point of origin for the majority of short-period comets observed from Earth, it is also home to several large, planet-like bodies including Pluto, which is now recognized as the largest in a population of Kuiper belt objects (the Plutinos) locked in orbital step with Neptune. Triton is only slightly larger than Pluto and nearly identical in composition, which has led to the hypothesis that the two share a common origin.[18]

The proposed capture of Triton may explain several features of the Neptunian system, including the extremely eccentric orbit of Neptune's moon Nereid and the scarcity of moons as compared to the other gas giants. Triton's initially eccentric orbit would have intersected irregular moons and disrupted those of smaller natural moons, dispersing them through gravitational interactions.[2]

Triton's eccentric post-capture orbit would have also resulted in tidal heating of the moon's interior. This would have kept Triton liquid for a billion years, which is supported by evidence of differentiation in the moon's interior.[7] This source of internal heat disappeared following circularization of the orbit.

There are two ways in which Triton's capture may have occurred. In order to be gravitationally captured by a planet, a passing body must lose sufficient energy to be slowed down to a speed less than that required to escape. An early theory of how Triton may have been slowed was by collision with another object, either one that happened to be passing by Neptune (which is unlikely), or a moon or proto-moon in orbit around Neptune (which is more likely).[4] Another hypothesis suggests that, before its capture, Triton may have had a massive companion similar to Pluto's moon Charon with which it formed a binary. When the binary encountered Neptune, it interacted in such a way that orbital energy was transferred from Triton to its companion; the latter was expelled, while Triton became bound to Neptune. This hypothesis is supported by several lines of evidence, including binaries being very common among the large Kuiper belt objects.[19] The event was brief but gentle, saving Triton from collisional disruption. Events like this may have been common during the formation of Neptune, or later when it migrated outward.[10]

Physical characteristics
Triton (blue), here shown as a fraction of the total mass, completely dominates the Neptunian moon system, with all other moons taken together comprising only one third of one percent. This imbalance may have come about when the capture of Triton destroyed much of the original Neptunian system.

Triton is the seventh largest moon and sixteenth largest object in the Solar System, and is larger than the dwarf planets Pluto and Eris. It comprises more than 99.5% of all the mass known to orbit Neptune, including the planet's rings and twelve other known moons,[f] and is also more massive than all known moons in the Solar System smaller than itself combined.[g] It has a radius, density (2.061 g/cm³), temperature and chemical composition similar to those of Pluto.[20]

As with Pluto, 55% of Triton's surface is covered with frozen nitrogen, with water ice comprising 15–35% and dry ice (frozen carbon dioxide) forming the remaining 10–20%. Trace ices include 0.1% methane and 0.05% carbon monoxide.[4] There could be ammonia on the surface that resulted from possible ammonia dihydrate in the lithosphere.[21] Triton's density implies it is probably about 30–45% water ice, with the remainder being rocky material.[4] Triton's surface area is 23 million km², which is 4.5% of Earth, or 15.5% of Earth's land area. Triton has a considerably high albedo, reflecting 60–95% of the sunlight that reaches it. By comparison, Earth's moon reflects only 11%.[22] Triton's reddish colour is believed to be the result of methane ice which reduces to carbon under bombardment from ultraviolet radiation.[4]

Because Triton's surface indicates a long history of melting, models of its interior posit that Triton is differentiated, like Earth, into a solid core, a mantle and a crust. Water, the most abundant volatile in the Solar System, comprises the moon's mantle, which lies over a core of rock and metal. There is enough rock in Triton's interior for solid-state convection to be occurring within its mantle, powered by radioactive decay. The heat may even be sufficient to maintain a "subterranean ocean" similar to that which is hypothesized to exist underneath the surface of Europa.[4] The possible presence of a layer of liquid water suggests the possibility, if unlikely, of life.[23]

Atmosphere
Main article: Atmosphere of Triton
Artist's impression of the atmosphere of Triton

Triton has a tenuous nitrogen atmosphere, with trace amounts of carbon monoxide and small amounts of methane near the surface.[8][24][25] Like Pluto's atmosphere, the atmosphere of Triton is believed to have resulted from evaporation of nitrogen from the moon's surface.[18] The surface temperature is at least 35.6 K (−237.6 °C) because Triton's nitrogen ice is in the warmer, hexagonal crystalline state, and the phase transition between hexagonal and cubic nitrogen ice occurs at that temperature.[26] An upper limit in the low 40s (K) can be set from vapor pressure equilibrium with nitrogen gas in Triton's atmosphere.[27] This temperature range is colder than Pluto's average equilibrium temperature of 44 K (−229 °C). Triton's surface atmospheric pressure is only about 1.4–1.9 pascal (0.014–0.019 millibar).[4]
Limb cloud formation on Triton as seen by Voyager 2

Turbulence at Triton's surface creates a troposphere (a "weather region") rising to an altitude of 8 km. Streaks on Triton's surface left by geyser plumes suggest that the troposphere is driven by seasonal winds capable of moving material of over a micrometre in size.[28] Unlike other atmospheres, Triton's has no stratosphere, and instead consists of a thermosphere from 8 to 950 km above the surface, and an exosphere above that.[4] The temperature of Triton's upper atmosphere, at 95 ± 5 kelvins, is higher than the temperature at the surface due to heat deposited from space.[8] A haze permeates most of Triton's troposphere, believed to be composed largely of hydrocarbons and nitriles created by the action of sunlight on methane. Triton's atmosphere also possesses clouds of condensed nitrogen that lie between 1 and 3 km from the surface.[4]

In the 1990s, observations from Earth were made of Triton's limb as the moon passed in front of stars. These observations indicated the presence of a denser atmosphere than was thought from Voyager 2 data.[29] Other observations have shown an increase in temperature by 5% from 1989 to 1998.[30] These observations indicate Triton is approaching an unusually warm summer season that only happens once every few hundred years. Theories for this warming include a change of frost patterns on Triton's surface and a change in ice albedo, which would allow more heat to be absorbed.[31] Another theory argues the changes in temperature are a result of deposition of dark, red material from geological processes on the moon. Because Triton's Bond albedo is among the highest within the Solar System, it is sensitive to small variations in spectral albedo.[32]

Surface features

All detailed knowledge of the surface of Triton was acquired in a single encounter by the Voyager 2 spacecraft in 1989. The 40% of Triton's surface imaged by Voyager revealed rocky outcrops, canyons and icy melt, mainly frozen methane. Triton is relatively flat; its observed topography never varies beyond a kilometer.[4] There are relatively few impact craters on Triton. Recent analysis of crater density and distribution has suggested that in geological terms, Triton's surface is extremely young, with regions varying from 50 million years old to just 6 million years old.[33]

Cryovolcanism
Main article: Cryovolcano
Streaks across Triton's surface believed to have been left by eruptions from nitrogen geysers

Triton is geologically active; its surface is young and has relatively few impact craters. Although Triton is made of various ices, its subsurface processes are similar to those that produce volcanoes and rift valleys on Earth, but with water and ammonia lavas as opposed to liquid rock.[4] Triton's entire surface is cut by complex valleys and ridges, probably the result of tectonics and icy volcanism. The vast majority of surface features on Triton are endogenic—the result of internal geological processes rather than external processes such as impacts. Most are volcanic and extrusive in nature, rather than tectonic.[4] 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.)[34]

When the Voyager 2 probe studied Triton, it observed numerous geyser-like eruptions of invisible nitrogen gas and entrained dust from beneath the surface in plumes up to 8 km high.[20] Triton thus joins the Earth, Io, and Enceladus as one of the few worlds of the Solar System on which active eruptions of some sort have been observed.[35] (Venus, Mars, Europa, Titan, and Dione may also be volcanically active.)

All the geysers observed were located between 50° and 57°S, the part of Triton's surface close to the subsolar point. This indicates that solar heating, although very weak at Triton's great distance from the Sun, plays a crucial role. It is thought that the surface of Triton probably consists of a semi-transparent layer of frozen nitrogen overlying a darker substrate, which creates a kind of "solid greenhouse effect". Solar radiation passes through the surface ice, slowly heating and vaporizing subsurface nitrogen until enough gas pressure accumulates for it to erupt through the crust.[4][28] A temperature increase of just 4 K above the ambient surface temperature of 37 K could drive eruptions to the heights observed. Although commonly termed "cryovolcanic", this nitrogen plume activity is distinct from Triton's larger scale cryovolcanic eruptions, as well as volcanic processes on other worlds, which are powered by the internal heat of the body in question. Analogous plumes of gaseous CO2 are believed to erupt from the south polar cap of Mars each spring.[36]

Each eruption of a Triton geyser may last up to a year, and during this time about 100 million cubic metres (3.5×10^9 cu ft) of material may be deposited up to 150 km downwind.[citation needed] Voyager's images of Triton's southern hemisphere show many streaks of dark material laid down by geyser activity.[37] Between 1977 and the Voyager flyby in 1989, Triton shifted from a reddish colour, similar to Pluto, to a far paler hue, suggesting that in the intervening decade lighter nitrogen frosts had covered older reddish material.[4] The eruption of volatiles from Triton's equator and its deposition at the poles may redistribute enough mass over the course of 10 000 years to cause polar wander.[38]

Polar cap, plains and ridges
Triton's bright red south polar cap

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 because it was on the night side during the Voyager 2 encounter. However, it is thought that Triton must also have a north polar cap.[26]

The high plains found on Triton's eastern hemisphere cover over and blot out older features, and are therefore almost certainly the result of icy lava washing over the previous landscape. The plains are dotted with pits, such as Leviathan Patera, which are probably the vents from which this lava emerged. The composition of the lava is unknown, although a mixture of ammonia and water is suspected.[4]

Four roughly circular "walled plains" have been identified on Triton. They are the flattest regions so far discovered, with a variance in altitude of less than 200 m. They are believed to have formed from eruption of icy lava.[4] The plains near Triton's eastern limb are dotted with black points, the maculae. Each of the maculae comprises a dark central patch surrounded by a white halo of material. They all have similar diameters of between 20 and 30 km. Some speculate the maculae are outliers of the southern polar cap, which is in retreat in summer.[4]

There are extensive ridges and valleys in complex patterns across Triton's surface, probably the result of freeze–thaw cycles.[39] Many also appear to be tectonic in nature and may result from extension or strike-slip faulting.[40] Some bear a strong resemblance to ridges on Europa, and may have a similar origin.[4] In the equatorial region, long faults with parallel mountain ranges of ice expelled from the interior cross complex terrain with valleys. These ridges, or sulci, such as Yasu Sulci, Ho Sulci, and Lo Sulci,[41] are believed to be of intermediate age in Triton's geological history, and in many cases to have formed concurrently. They tend to be clustered in groups or "packets".[40]

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

Triton's western hemisphere consists of a strange series of fissures and depressions known as "cantaloupe terrain" because of its resemblance to the skin of a cantaloupe melon. Although it has few craters, it is believed that this is the oldest terrain on Triton.[42] It probably covers much of the western half of the moon.[4]

Cantaloupe terrain, which is mostly dirty water ice, is known to exist only on Triton. It contains depressions 30–40 km in diameter.[42] The depressions (cavi) are probably not impact craters because they are all of similar size and have smooth curves. The leading hypothesis for their formation is diapirism, the rising of "lumps" of less dense material through a stratum of denser material.[43][4] Alternate hypotheses include formation by collapses, or by flooding caused by cryovolcanism.[42]

Impact craters
The paucity of craters on Triton is evidence of intense geologic activity.

Due to constant erasure and modification by ongoing geological activity, impact craters on Triton's surface are relatively rare. A census of Triton's craters imaged by Voyager 2 found only 179 that were incontestably of impact origin, compared with 835 observed for Uranus' moon Miranda, which has only three percent of Triton's surface area.[44] The largest crater observed on Triton believed to have been created by an impact is a 27 km-diameter feature called Mazomba.[44][45] Although larger craters have been observed, they are generally believed to be volcanic in nature.[44]

The few impact craters on Triton are almost all concentrated in the leading hemisphere–that facing the direction of the orbital motion—with the majority concentrated around the equator between 30° and 70° longitude,[44] resulting from material swept up from orbit around Neptune.[33] Because it orbits with one side permanently facing the planet, astronomers expect that Triton should have fewer impacts on its trailing hemisphere, as impacts on the leading hemisphere would be more frequent and more violent.[44] However, as Voyager only imaged 40% of Triton's surface, this remains uncertain.
[edit] Observation and exploration
Neptune (top) and Triton (bottom) three days after the flyby of Voyager 2

The orbital properties of Triton had been defined with high accuracy in the 19th century. It was found to have a retrograde orbit, at a very high angle of inclination to the plane of Neptune's orbit. The first detailed observations of Triton were not made until 1930. Little was known about the satellite until Voyager 2 arrived at the end of the 20th century.[4]

Before the arrival of Voyager 2, astronomers suspected that Triton might have liquid nitrogen seas and a nitrogen/methane atmosphere with a density as much as 30% that of the Earth. Like the famous overestimates of the atmospheric density of Mars, this was completely false. As with Mars, a denser atmosphere is postulated for the body's early history.[46]

The first attempt to measure the diameter of Triton was made by Gerard Kuiper in 1954. He obtained a value of 3800 km. Subsequent measurement attempts arrived at values ranging from 2500 to 6000 km, or from slightly smaller than our Moon to nearly half the diameter of Earth.[47] Data from the approach of Voyager 2 to Neptune on August 25, 1989 led to a more accurate estimate of Triton's diameter (2706 km).[48]

In the 1990s, various observations from Earth were made of the limb of Triton using the occultation of nearby stars, which indicated the presence of an atmosphere and an exotic surface. The observations suggest that the atmosphere is denser than the Voyager 2 measurements had indicated.[29]

New concepts for missions to the Neptune system to be conducted in the 2010s have been brought forward by NASA scientists on numerous occasions over the last decades. All of them identified Triton as being a prime target and a separate Triton lander comparable to the Huygens probe for Titan was frequently included in those plans. To date, however, no efforts aimed at Neptune and Triton went beyond the proposal-phase and NASA's funding on missions to the outer solar system is currently focused on the Jupiter- and Saturn-systems.[49]

See also

* List of geological features on Triton
* Neptune's moons in fiction
* Triton's sky

Notes

1. ^ In US dictionary transcription, us dict: trī′·tən, or as Greek Τρίτων, according to "Theogonia Heseodou". Neoteron Enkuklopaidikon Lexikon. http://www.astro.gr/arxaia-sofia/Hesiod.htm. Retrieved 2008-02-19.

1. ^ Surface area derived from the radius r: 4*pi*r2.
2. ^ Volume v derived from the radius r: 4/3*pi*r3.
3. ^ Mass m derived from the density d and the volume v: m=d*v.
4. ^ Surface gravity derived from the mass m, the gravitational constant g and the radius r: g*m/r2 .
5. ^ Escape velocity derived from the mass m, the gravitational constant g and the radius r: sqrt((2*g*m)/r).
6. ^ Mass of Triton: 2.14 × 1022 kg. Combined mass of 12 other known moons of Neptune: 7.53 × 1019 kg, or 0.35 percent. The mass of the rings is negligible.
7. ^ The masses of other spherical moons are: Titania—3.5 × 1021, Oberon—3.0 × 1021, Rhea—2.3 × 1021, Iapetus—1.8 × 1021, Charon—1.5 × 1021, Ariel—1.3 × 1021, Umbriel—1.2 × 1021, Dione—1.0 × 1021, Tethys—0.6 × 1021, Enceladus—0.12 × 1021, Miranda—0.06 × 1021, Proteus—0.05 × 1021, Mimas—0.04 × 1021. The total mass of remaining moons is about 0.09 × 1021. So, the total mass of all moons smaller than Triton is about 1.65 × 1022. (See List of moons by diameter)
8. ^ Largest irregular moons: Saturn's Phoebe (210 km), Uranus' Sycorax (150 km), and Jupiter's Himalia (85 km)

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External links

* Triton Profile at NASA's Solar System Exploration site
* Triton page at The Nine8 Planets
* Triton page at Views of the Solar System
* Movie of Triton's rotation from the National Oceanic and Atmospheric Administration site
* Triton images from Planetary Photojournal
* Gazetteer of Triton Nomenclature from USGS Neptune system page
* Paul Schenk's 3D images and flyover videos of Triton and other outer solar system satellites

Moons of Neptune