Triton (moon)

**Triton**

Discovery
Discovered by	William Lassell
Discovered on	October 10, 1846
Orbital characteristics
Semimajor axis	354,800 km
Eccentricity	0.0000
Orbital period	-5.877 d (retrograde)
Inclination	130.267° (to the ecliptic) 157.340° (to Neptune's equator) 130.063° (to Neptune's orbit)
Satellite of	Neptune
Physical characteristics
Mean diameter	2706.8±1.8 km ^[1] (0.212 Earths)
Surface area	23,018,000 km²
Volume	10,384,000,000 km³
Mass	2.147×10²² kg (0.0036 Earths)
Mean density	2.05 g/cm³
Surface gravity	0.782 m/s²
Escape velocity	1.455 km/s
Rotation period	5 d, 21 h, 2 min, 28s
synchronous
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 (trye'-tən, IPA /ˈtraɪtn̩/, Greek Τρίτων), or Neptune I, is the planet Neptune's largest moon. Triton has a complex geological history and it is believed to have a relatively young surface. It was discovered by William Lassell on October 10, 1846, just 17 days after the planet itself was discovered.

Name

Triton is named after the Greek sea god Triton. 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).

After Flammarion's proposal, the name Triton was also independently proposed by others [2] [3] [4], but in 1909 it was reported as "not in general use". As late as 1939 it was noted that although Triton had a name, the name was "not generally used" [5]. In astronomical literature it was simply referred to as "the satellite of Neptune". Oddly enough, most of the references to Triton in the astronomical literature in the late 19th and early 20th centuries are to the name of a supposed Martian canal.

Perhaps it was the discovery of Nereid, the second moon of Neptune, in 1949 that finally prompted making Triton an official name.

Orbit

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; it is thought that Triton may be 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 outermost moon Nereid, the paucity 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 tantalizing hints to Triton's possible origin as a Pluto-like planetary body.

Due to its retrograde motion, the already-close Tritonian orbit is slowly decaying further from tidal interactions and it is predicted that between 1.4 and 3.6 billion years from now, Triton will pass within its Roche limit [6]. 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, is that it is nearly a perfect circle with an eccentricity of zero to sixteen decimal places.

Physical characteristics

Triton has a density of 2.05 g/cm³, 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 (-237 C) 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 (-229 C). 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 km² (4.5% of Earth's, or 15.5% of Earth's land area).

Planetary geology

Triton has a similar size, density, and chemical composition to that of Pluto, and upon verifying the eccentric orbit of Pluto that crosses the orbit of Neptune, we can track the origin of Triton as a similar planet captured by Neptune. Therefore, Triton could have been formed far from Neptune in the frozen dark 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 2/3 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/cm³ 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

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 grams per cubic centimetre. 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 prenumbra when Voyager 2 visited Triton. However, it is thought that it must have a polar cap.

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 and Cipagu and the Cipango plateau.

The plains zones of Sipagu Planitia and Abatus Planum in the southern hemisphere meet surrounded by black points, the maculae. Two groups of maculae, Acupara Maculae and Zin Maculae make up the eastern part of Abatus 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 Sipagu and Abatus Planum, there is a fresh crater that is 27 km in diameter called Mozamba. Following northwest, there are two smaller craters (Kurma and Llomba) following the Mozamba 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”

Tano Sulci is one of the long faults that cross the strange region of Bubeme on Triton. This region is also known as “cantaloupe terrain” because the appearance of a cantaloupe, one of the strangest regions in the entire solar system. 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 has current volcanic activity.

History of observation and exploration

In 1820, William Lassell started to make mirrors for his telescope and in 1844, made better mirrors that allowed him to discover the planet Neptune on September 23, 1846. When John Herschel received the news of the disovery, he wrote Lassell to search for moons of Neptune. Lassell started to search for the satellites and discovered Triton eight days after beginning to search, on 10 October, 1846, only 17 days after having discovered the planet Neptune. Even though Neptune has rings, they were so faint and dark that what Lassell saw was probably an illusion.

The first detailed observation of the satellites were only done after 100 years from when the satellite was discovered. The astronomers started to study the moon and discovered that it has an opposite orbit from that of Neptune and it is very inclined.

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

Prior to Voyager 2, it was suspected that Triton had liquid nitrogen seas and a nitrogen/methane atmosphere as much as 1/3 the density of Earth's. Like the famous overestimates of Mars' atmospheric density, these were found to be completely false.

The first attempt to measure the diameter of Triton correctly was done by Gerard Kuiper in 1954 and he obtained the value of 3800 km. After this, various attempts to measure the satellite were made and these dimensions varied from 2500 to 6000 km, or slightly smaller than our moon to nearly half of the diameter of Earth.

When Voyager 2 approached Neptune on August 25, 1989, it obtained data that allowed the correct measurement of the diameter (estimated at 2706 km) and it was decided to overfly the moon even if it affected the trajectory of Voyager 2.

In the 1990s, different observations from Earth were made of the limb of Triton using the occultation of stars by Triton. These observations indicated an atmosphere and an exotic surface. These observations indicated a denser atmosphere than that Voyager 2 measured.

NASA is planning a mission to Neptune and Triton that should begin between 2016 and 2018, but it will only arrive at Neptune in 2035. The mission will probably include two spacecraft that will land on the surface of Triton, study the atmosphere, and research geochemical information from the geysers.

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. But like Titan, Triton is extremely cold. 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.

However, 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[7]. This subsurface ocean could sustain primitive forms of life, very similar to what has been suggested on the moon Europa.