Triton (moon)

Triton (moon)

Overview
Triton is the largest moon
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

 of the planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

 Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

, discovered on October 10, 1846, by English astronomer William Lassell
William Lassell
William Lassell FRS was an English merchant and astronomer.Born in Bolton and educated in Rochdale after the death of his father, he was apprenticed from 1814 to 1821 to a merchant in Liverpool. He then made his fortune as a beer brewer, which enabled him to indulge his interest in astronomy...

. It is the only large moon in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 with a retrograde orbit
Retrograde and direct motion
Apparent retrograde motion is the motion of a planetary body in a direction opposite to that of other bodies within its system as observed from a particular vantage point...

, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is the seventh-largest moon in the Solar System. Because of its retrograde orbit and composition similar to Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

's, Triton is thought to have been captured from the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

.
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Encyclopedia
Triton is the largest moon
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

 of the planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

 Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

, discovered on October 10, 1846, by English astronomer William Lassell
William Lassell
William Lassell FRS was an English merchant and astronomer.Born in Bolton and educated in Rochdale after the death of his father, he was apprenticed from 1814 to 1821 to a merchant in Liverpool. He then made his fortune as a beer brewer, which enabled him to indulge his interest in astronomy...

. It is the only large moon in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 with a retrograde orbit
Retrograde and direct motion
Apparent retrograde motion is the motion of a planetary body in a direction opposite to that of other bodies within its system as observed from a particular vantage point...

, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is the seventh-largest moon in the Solar System. Because of its retrograde orbit and composition similar to Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

's, Triton is thought to have been captured from the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

. Triton has a surface of mostly frozen nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

, a mostly water ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

 crust, an icy mantle and a substantial core of rock and metal. The core makes up two-thirds of its total mass. Triton has a mean density of 2.061 g/cm3 and is composed of approximately 15–35% water ice.

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
Cryovolcano
A cryovolcano is a volcano that erupts volatiles such as water, ammonia or methane, instead of molten rock. Collectively referred to as cryomagma or ice-volcanic melt, these substances are usually liquids and form plumes, but can also be in vapour form...

 and tectonic terrains. Part of its crust is dotted with geysers believed to erupt nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

. Triton has a tenuous nitrogen atmosphere less than 1/70 000 the pressure of Earth's atmosphere at sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

.

Discovery and naming


The moon was discovered by British
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...

 William Lassell
William Lassell
William Lassell FRS was an English merchant and astronomer.Born in Bolton and educated in Rochdale after the death of his father, he was apprenticed from 1814 to 1821 to a merchant in Liverpool. He then made his fortune as a beer brewer, which enabled him to indulge his interest in astronomy...

 on October 10, 1846, just 17 days after Neptune was discovered by German
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

 astronomers Johann Gottfried Galle
Johann Gottfried Galle
Johann Gottfried Galle was a German astronomer at the Berlin Observatory who, on 23 September 1846, with the assistance of student Heinrich Louis d'Arrest, was the first person to view the planet Neptune, and know what he was looking at...

 and Heinrich Louis d'Arrest
Heinrich Louis d'Arrest
Heinrich Louis d'Arrest was a German astronomer, born in Berlin. His name is sometimes given as Heinrich Ludwig d'Arrest....

, who were following coordinates given to 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
John Herschel
Sir John Frederick William Herschel, 1st Baronet KH, FRS ,was an English mathematician, astronomer, chemist, and experimental photographer/inventor, who in some years also did valuable botanical work...

 received news of Neptune's discovery, he wrote to Lassell suggesting he search for possible moons. Lassell did so and discovered Triton just eight days later. Lassell also claimed to have discovered rings. Although Neptune was later confirmed to have rings, they are so faint and dark that it is doubted he actually saw them.

Triton is named after the Greek sea god Triton
Triton (mythology)
Triton is a mythological Greek god, the messenger of the big sea. He is the son of Poseidon, god of the sea, and Amphitrite, goddess of the sea, whose herald he is...

 (Τρίτων), the son of Poseidon
Poseidon
Poseidon was the god of the sea, and, as "Earth-Shaker," of the earthquakes in Greek mythology. The name of the sea-god Nethuns in Etruscan was adopted in Latin for Neptune in Roman mythology: both were sea gods analogous to Poseidon...

 (the Greek god comparable to the Roman Neptune
Neptune (mythology)
Neptune was the god of water and the sea in Roman mythology and religion. He is analogous with, but not identical to, the Greek god Poseidon. In the Greek-influenced tradition, Neptune was the brother of Jupiter and Pluto, each of them presiding over one of the three realms of the universe,...

). The name was first proposed by Camille Flammarion
Camille Flammarion
Nicolas Camille Flammarion was a French astronomer and author. He was a prolific author of more than fifty titles, including popular science works about astronomy, several notable early science fiction novels, and several works about Spiritism and related topics. He also published the magazine...

 in his 1880 book Astronomie Populaire, although it was not officially adopted until many decades later. Until the discovery of the second moon Nereid
Nereid (moon)
Nereid , also known as Neptune II, is the third-largest moon of Neptune. It has a highly eccentric orbit. Nereid was discovered by Gerard Kuiper in 1949 and was the second moon of Neptune to be discovered.- Discovery and naming :...

 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 after his subsequent discovery of an eighth moon of Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

 (Hyperion
Hyperion (moon)
Hyperion , also known as Saturn VII, is a moon of Saturn discovered by William Cranch Bond, George Phillips Bond and William Lassell in 1848. It is distinguished by its irregular shape, its chaotic rotation, and its unexplained sponge-like appearance...

). The third and fourth moons of Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

 (Ariel
Ariel (moon)
Ariel is the brightest and fourth-largest of the 27 known moons of Uranus. Ariel orbits and rotates in the equatorial plane of Uranus, which is almost perpendicular to the orbit of Uranus, and so has an extreme seasonal cycle....

 and Umbriel
Umbriel (moon)
Umbriel is a moon of Uranus discovered on October 24, 1851, by William Lassell. It was discovered at the same time as Ariel and named after a character in Alexander Pope's poem The Rape of the Lock. Umbriel consists mainly of ice with a substantial fraction of rock, and may be differentiated into a...

), which Lassell discovered in 1851, were named by John Herschel
John Herschel
Sir John Frederick William Herschel, 1st Baronet KH, FRS ,was an English mathematician, astronomer, chemist, and experimental photographer/inventor, who in some years also did valuable botanical work...

.

Orbit and rotation


Triton is unique among all large moons in the Solar System for its retrograde orbit
Retrograde and direct motion
Apparent retrograde motion is the motion of a planetary body in a direction opposite to that of other bodies within its system as observed from a particular vantage point...

 around its planet (i.e., it orbits in a direction opposite to the planet's rotation). Most of the outer irregular moons of Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 and Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

 also have retrograde orbits, as do some of Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

' outer moons. However, these moons are all much more distant from their primaries, and are quite small in comparison; the largest of them (Phoebe
Phoebe (moon)
Phoebe is an irregular satellite of Saturn. It was discovered by William Henry Pickering on 17 March 1899 from photographic plates that had been taken starting on 16 August 1898 at the Boyden Observatory near Arequipa, Peru, by DeLisle Stewart...

) has only 8% of the diameter (and 0.03% of the mass) of Triton.

Triton's orbit is associated with two tilts, the inclination
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...

 of Neptune's spin to Neptune's orbit, 30°, and the inclination of Triton's orbit to Neptune's spin, 157° (an inclination over 90° indicates retrograde motion). Triton's orbit precesses forward relative to Neptune's spin with a period of about 678 Earth years (4.1 Neptunian years), making its Neptune-orbit-relative inclination vary between 127° and 173°. That inclination is currently 130°; Triton's orbit is now near its maximum departure from coplanarity with Neptune's.

Triton is in synchronous rotation
Synchronous rotation
In astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it is orbiting...

 with Neptune; it keeps one face oriented toward the planet at all times. Its equator is almost exactly aligned with its orbital plane. At the present time, Triton's rotational axis is about 40° from Neptune's orbital plane, and hence at some point during Neptune's year each pole points fairly close to the Sun, almost like the poles of Uranus. As Neptune orbits the Sun, Triton's polar regions take turns facing the Sun, resulting in seasonal changes as one pole, then the other, moves into the sunlight. Such changes have recently been observed.

Triton's revolution around Neptune has become a nearly perfect circle with an eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

 of almost zero. Viscoelastic
Viscoelasticity
Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like honey, resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain instantaneously when stretched and just...

 damping from tides alone is not believed to be capable of circularizing Triton's orbit in the time since the origin of the system, and gas drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

 from a prograde debris disc is likely to have played a substantial role. Tidal interactions also cause Triton's orbit, already closer to Neptune than the Moon's to Earth, to slowly decay further; predictions are that some 3.6 billion years from now, Triton will pass within Neptune's Roche limit
Roche limit
The Roche limit , sometimes referred to as the Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction...

. This will result in either a collision with Neptune's atmosphere or the breakup of Triton, forming a ring
Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...

 system similar to that found around Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

.

Capture


Because moons in retrograde orbits cannot have formed out of the same region of the solar nebula
Solar nebula
In cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. There is evidence that it was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation...

 as the planets they orbit, but must have been captured from elsewhere, it is suspected that Triton was captured from the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

, a ring of small icy objects extending outward from just inside the orbit of Neptune to about 50 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

 from the Sun. Believed to be the point of origin for the majority of short-period comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s observed from Earth, it is also home to several large, planet-like bodies including Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

, which is now recognized as the largest in a population of Kuiper belt objects (the plutino
Plutino
In astronomy, a plutino is a trans-Neptunian object in 2:3 mean motion resonance with Neptune. For every 2 orbits that a plutino makes, Neptune orbits 3 times. Plutinos are named after Pluto, which follows an orbit trapped in the same resonance, with the Italian diminutive suffix -ino...

s) 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.

The proposed capture of Triton may explain several features of the Neptunian system, including the extremely eccentric orbit
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

 of Neptune's moon Nereid
Nereid (moon)
Nereid , also known as Neptune II, is the third-largest moon of Neptune. It has a highly eccentric orbit. Nereid was discovered by Gerard Kuiper in 1949 and was the second moon of Neptune to be discovered.- Discovery and naming :...

 and the scarcity of moons as compared to the other gas giants. Triton's initially eccentric orbit would have intersected orbits of irregular moons and disrupted
Perturbation (astronomy)
Perturbation is a term used in astronomy in connection with descriptions of the complex motion of a massive body which is subject to appreciable gravitational effects from more than one other massive body....

 those of smaller regular moons, dispersing them through gravitation
Gravitation
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...

al interactions.

Triton's eccentric post-capture orbit would have also resulted in tidal heating
Tidal heating
Tidal heating occurs through the tidal friction processes: orbital and rotational energy are dissipated as heat in the crust of the moons and planets involved. Io, a moon of Jupiter, is the most volcanically active body in the solar system, with no impact craters surviving on its surface...

 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. This source of internal heat disappeared following circularization of the orbit.

Two types of mechanisms have been proposed for Triton's capture. 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). A more recent and now favored hypothesis suggests that, before its capture, Triton had a massive companion similar to Pluto's moon Charon
Charon (moon)
Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...

 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. 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
Planetary migration
Planetary migration occurs when a planet or other stellar satellite interacts with a disk of gas or planetesimals, resulting in the alteration of the satellite's orbital parameters, especially its semi-major axis...

.

Physical characteristics


Triton is the seventh largest moon and sixteenth largest object in the Solar System, and is modestly larger than the dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

s Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

 and Eris
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...

. It comprises more than 99.5% of all the mass known to orbit Neptune, including the planet's rings and twelve other known moons, and is also more massive than all known moons in the Solar System smaller than itself combined. It has a radius, density (2.061 g/cm3), temperature and chemical composition similar to those of Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

.

As with Pluto, 55% of Triton's surface is covered with frozen nitrogen, with water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 ice comprising 15–35% and dry ice
Dry ice
Dry ice, sometimes referred to as "Cardice" or as "card ice" , is the solid form of carbon dioxide. It is used primarily as a cooling agent. Its advantages include lower temperature than that of water ice and not leaving any residue...

 (frozen carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

) forming the remaining 10–20%. Trace ices include 0.1% methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 and 0.05% carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

. There could be also be ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 on the surface if ammonia dihydrate
Hydrate
Hydrate is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains water. The chemical state of the water varies widely between hydrates, some of which were so labeled before their chemical structure was understood....

 is present as suspected in the lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

. Triton's density implies it is probably about 30–45% water ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

, with the remainder being rocky material. Triton's surface area is 23 million km2, which is 4.5% of Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, or 15.5% of Earth's land area. Triton has a considerably high albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

, reflecting 60–95% of the sunlight that reaches it. By comparison, Earth's moon reflects only 11%. Triton's reddish colour is believed to be the result of methane ice, which is converted to tholin
Tholin
Tholin [after the ancient Greek word meaning "not clear"] is a heteropolymer molecule formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane. Tholins do not form naturally on modern-day Earth, but are found in great abundance on the surface of icy bodies in...

s under bombardment from ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 radiation.

Because Triton's surface indicates a long history of melting, models of its interior posit that Triton is differentiated, like Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, into a solid core, a mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....

 and a crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...

. Water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, the most abundant volatile
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...

 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 radioactive decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

 to power convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

 in the mantle. The heat may even be sufficient to maintain a "subterranean ocean" similar to that which is hypothesized to exist underneath the surface of Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...

. If present, a layer of liquid water would suggest the possibility, however unlikely, of life.

Atmosphere


Triton has a tenuous nitrogen atmosphere, with trace amounts of carbon monoxide and small amounts of methane near the surface. Like Pluto's atmosphere, the atmosphere of Triton is believed to have resulted from evaporation of nitrogen from the moon's surface. The surface temperature is at least 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. An upper limit in the low 40s (K) can be set from vapor pressure equilibrium with nitrogen gas in Triton's atmosphere. This temperature range is colder than Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

's average equilibrium temperature of . Triton's surface atmospheric pressure is only about (.

Turbulence at Triton's surface creates a troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

 (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. Unlike other atmospheres, Triton's lacks a stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

, and instead has a thermosphere
Thermosphere
The thermosphere is the biggest of all the layers of the Earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. The International Space Station has a stable orbit within the middle of the thermosphere, between...

 from altitudes of 8 to 950 km, and an exosphere above that. The temperature of Triton's upper atmosphere, at , is higher than that at the surface, due to heat absorbed from solar radiation and Neptune's magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...

. A haze permeates most of Triton's troposphere, believed to be composed largely of hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....

s and nitrile
Nitrile
A nitrile is any organic compound that has a -C≡N functional group. The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, one example being super glue .Inorganic compounds containing the -C≡N group are not called...

s 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.

In the 1990s, observations from Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 were made of Triton's limb as the moon passed in front of stars
Occultation
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...

. These observations indicated the presence of a denser atmosphere than was deduced from Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

 data. Other observations have shown an increase in temperature by 5% from 1989 to 1998. 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
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

, which would allow more heat to be absorbed. 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
Bond albedo
The Bond albedo, named after the American astronomer George Phillips Bond , 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...

 is among the highest within the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

, it is sensitive to small variations in spectral albedo.

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 blocky outcrops, ridges, troughs, furrows, hollows, plateaus, icy plains and few craters. Triton is relatively flat; its observed topography never varies beyond a kilometer. There are relatively few impact crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

s 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.

Cryovolcanism


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 volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

es and rift valley
Rift valley
A rift valley is a linear-shaped lowland between highlands or mountain ranges created by the action of a geologic rift or fault. This action is manifest as crustal extension, a spreading apart of the surface which is subsequently further deepened by the forces of erosion...

s on Earth, but with water and ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 lavas as opposed to liquid rock. Triton's entire surface is cut by complex valleys and ridges, probably the result of tectonics and icy volcanism
Volcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....

. 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
Tectonics
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...

.

The Voyager 2 probe observed a handful of geyser
Geyser
A geyser is a spring characterized by intermittent discharge of water ejected turbulently and accompanied by a vapour phase . The word geyser comes from Geysir, the name of an erupting spring at Haukadalur, Iceland; that name, in turn, comes from the Icelandic verb geysa, "to gush", the verb...

-like eruptions of invisible nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 gas and entrained
Entrainment (physical geography)
In physical geography, entrainment is the process by which surface sediment is incorporated into a fluid flow as part of the operation of erosion....

 dust from beneath the surface of Triton in plumes up to 8 km high. Triton thus joins the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....

, and Enceladus
Enceladus (moon)
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...

 as one of the few worlds of the Solar System on which active eruptions of some sort have been observed. (Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

, Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

, Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...

, Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

, and Dione
Dione (moon)
Dione is a moon of Saturn discovered by Cassini in 1684. It is named after the titan Dione of Greek mythology. It is also designated Saturn IV.- Name :...

 may also be volcanically active.) The best observed examples were named Hili and Mahilani (after a Zulu
Zulu mythology
Zulu mythology contains numerous deities, commonly associated with animals or general classes of natural phenomena.Unkulunkulu is the highest God and is the creator of humanity. Unkulunkulu was created in Uhlanga , a huge swamp of reeds, before he came to Earth...

 water sprite
Tikoloshe
Tokoloshe or Tikoloshe. From the Xhosa word uthikoloshe.The tokoloshe is a short, hairy, dwarf-like creature from Bantu folklore. It is a mischievous and evil spirit that can become invisible by swallowing a pebble. Tokoloshes are called upon by malevolent people to cause trouble for others...

 and a Tonga
Tonga
Tonga, officially the Kingdom of Tonga , is a state and an archipelago in the South Pacific Ocean, comprising 176 islands scattered over of ocean in the South Pacific...

n sea spirit, respectively).

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 translucent
Transparency and translucency
In the field of optics, transparency is the physical property of allowing light to pass through a material; translucency only allows light to pass through diffusely. The opposite property is opacity...

 layer of frozen nitrogen overlying a darker substrate, which creates a kind of "solid greenhouse effect
Greenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...

". 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. A temperature increase of just 4 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

 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
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 are believed to erupt from the south polar cap of Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 each spring.

Each eruption of a Triton geyser may last up to a year, driven by the sublimation of about 100 million m3s (3,531,466,621.3 cu ft) of nitrogen ice over this interval; dust entrained may be deposited up to 150 km downwind in visible streaks, and perhaps much farther in more diffuse deposits. Voyager's images of Triton's southern hemisphere show many such streaks of dark material. 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. The eruption of volatiles from Triton's equator and their deposition at the poles may redistribute enough mass over the course of 10 000 years to cause polar wander
Polar wander
Polar wander is the movement of the North and South Poles of the Earth with respect to the continents. This motion can be divided into two components: that due to continental drift , and true polar wander, in which the mantle and the crust rotate together into new orientations....

.

Polar cap, plains and ridges


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.

The high plains found on Triton's eastern hemisphere, such as Cipango Planum, 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.

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
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...

. They are believed to have formed from eruption of icy lava. The plains near Triton's eastern limb are dotted with black spots, the maculae
Macula (planetary geology)
Macula is the Latin word for 'spot'. It is used in planetary geology to refer to unusually dark areas on the surface of a planet or moon. They are seen on the icy surfaces of Jupiter's moon Europa and Neptune's moon Triton. The term was adopted for planetary geology when high resolution pictures...

. 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.

There are extensive ridges and valleys in complex patterns across Triton's surface, probably the result of freeze–thaw cycles. Many also appear to be tectonic in nature and may result from extension or strike-slip faulting. There are long double ridges of ice with central troughs bearing a strong resemblance to Europan lineae (although they have a larger scale), and which may have a similar origin, possibly shear heating from strike-slip motion along faults caused by diurnal tidal stresses experienced before Triton's orbit was fully circularized. These faults with parallel ridges expelled from the interior cross complex terrain with valleys in the equatorial region. The ridges and furrows, or sulci
Sulcus (geology)
Sulcus is, in astrogeology, a long parallel groove on a planet or a moon.For example, Uruk Sulcus is a bright region of grooved terrain adjacent to Galileo Regio on Jupiter's moon Ganymede....

, such as Yasu Sulci, Ho Sulci, and Lo Sulci, 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".

Cantaloupe terrain


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
Cantaloupe
"Rockmelon" redirects here, for the band see Rockmelons. See also Cantaloupe .Cantaloupe refers to a variety of Cucumis melo, a species in the family Cucurbitaceae which includes nearly all melons and squashes. Cantaloupes range in size from...

 melon. Although it has few craters, it is believed that this is the oldest terrain on Triton. It probably covers much of the western half of the moon.

Cantaloupe terrain, which is mostly dirty water ice, is known to exist only on Triton. It contains depressions in diameter. 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 diapir
Diapir
A diapir is a type of intrusion in which a more mobile and ductily-deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh-Taylor instability-type structures in regions with low tectonic stress...

ism, the rising of "lumps" of less dense material through a stratum of denser material. Alternate hypotheses include formation by collapses, or by flooding caused by cryovolcanism.

Impact craters


Due to constant erasure and modification by ongoing geological activity, impact crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

s 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'
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

 moon Miranda
Miranda (moon)
-External links:* at * at The Nine8 Planets* at Views of the Solar System* * from the...

, which has only three percent of Triton's surface area
Surface area
Surface area is the measure of how much exposed area a solid object has, expressed in square units. Mathematical description of the surface area is considerably more involved than the definition of arc length of a curve. For polyhedra the surface area is the sum of the areas of its faces...

. The largest crater observed on Triton believed to have been created by an impact is a 27 km-diameter feature called Mazomba. Although larger craters have been observed, they are generally believed to be volcanic in nature.

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, resulting from material swept up from orbit around Neptune. 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. However, as Voyager only imaged 40% of Triton's surface, this remains uncertain.

Observation and exploration


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
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

 arrived at the end of the 20th century.

Before the arrival of Voyager 2, astronomers suspected that Triton might have liquid nitrogen
Liquid nitrogen
Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...

 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.

The first attempt to measure the diameter of Triton was made by Gerard Kuiper
Gerard Kuiper
Gerard Peter Kuiper , Netherlands – December 24, 1973, Mexico City) was a Dutch-American astronomer after whom the Kuiper belt was named.-Early life:...

 in 1954. He obtained a value of 3,800 km. Subsequent measurement attempts arrived at values ranging from 2,500 to 6,000 km, or from slightly smaller than our Moon to nearly half the diameter of Earth. Data from the approach of Voyager 2 to Neptune on August 25, 1989, led to a more accurate estimate of Triton's diameter (2,706 km).

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.

New concepts for missions to the Neptune system
Neptune Orbiter
Neptune Orbiter was a proposed NASA unmanned planetary spacecraft to explore the planet Neptune. It was envisioned that it would be launched sometime around 2016 and take 8 to 12 years to reach the planet; however, no longer lists any possible launch date. The Neptune Orbiter was designed to...

 to be conducted in the 2010s have been brought forward by NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 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
Huygens probe
The Huygens probe was an atmospheric entry probe carried to Saturn's moon Titan as part of the Cassini–Huygens mission. The probe was supplied by the European Space Agency and named after the Dutch 17th century astronomer Christiaan Huygens....

 for Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

 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.

See also




External links