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Polar cap, plains and ridges [ edit ] Triton's bright south polar cap above a region of cantaloupe terrain 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, [70] resulting from material swept up from orbit around Neptune. [54] Because it orbits with one side permanently facing the planet, astronomers expect that Triton should have fewer impacts on its trailing hemisphere, due to impacts on the leading hemisphere being more frequent and more violent. [70] Voyager 2 imaged only 40% of Triton's surface, so this remains uncertain. However, the observed cratering asymmetry exceeds what can be explained based on the impactor populations, and implies a younger surface age for the crater-free regions (≤ 6million years old) than for the cratered regions (≤ 50million years old). [53] Observation and exploration [ edit ] NASA illustration detailing the studies of the proposed Trident mission Neptune (top) and Triton (bottom) three days after flyby of Voyager 2 Main article: Atmosphere of Triton Artist's impression of Triton, showing its tenuous atmosphere just over the limb. 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 layer of frozen nitrogen overlying a darker substrate, which creates a kind of "solid greenhouse effect". Solar radiation passes through the thin surface ice sheet, slowly heating and vaporizing subsurface nitrogen until enough gas pressure accumulates for it to erupt through the crust. [7] [46] A temperature increase of just 4 K above the ambient surface temperature of 37K could drive eruptions to the heights observed. [59] 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 internal heat. CO 2 geysers on Mars are thought to erupt from its south polar cap each spring in the same way as Triton's geysers. [62]
Triton was discovered by British astronomer William Lassell on October 10, 1846, [17] just 17days after the discovery of Neptune. When John Herschel received news of Neptune's discovery, he wrote to Lassell suggesting he search for possible moons. Lassell discovered Triton eight days later. [17] [18] Lassell also claimed for a period [h] to have discovered rings. [19] Although Neptune was later confirmed to have rings, they are so faint and dark that it is not plausible he saw them. A brewer by trade, Lassell spotted Triton with his self-built 61cm (24in) aperture metal mirror reflecting telescope (also known as the "two-foot" reflector). [20] This telescope was donated to the Royal Observatory, Greenwich in the 1880s, but was eventually dismantled. [20]continuous energy and multigroup fixed-source Monte Carlo analysis with automated variance reduction General purpose point depletion and decay code to calculate isotopic concentrations, decay heat, radiation source terms, and curie levels Triton's revolution around Neptune has become a nearly perfect circle with an eccentricity of almost zero. Viscoelastic damping from tides alone is not thought 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. [4] [5] Tidal interactions also cause Triton's orbit, which is already closer to Neptune than the Moon is to Earth, to gradually decay further; predictions are that 3.6billion years from now, Triton will pass within Neptune's Roche limit. [26] This will result in either a collision with Neptune's atmosphere or the breakup of Triton, forming a new ring system similar to that found around Saturn. [26] Capture [ edit ] The Kuiper belt (green), in the Solar System's outskirts, is where Triton is thought to have originated. 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 giant planets. Triton's initially eccentric orbit would have intersected the orbits of irregular moons and disrupted those of smaller regular moons, dispersing them through gravitational interactions. [4] [5] The orbital properties of Triton were already determined 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 flew by in 1989. [7]
Stochastic uncertainty quantification in results based on uncertainties in nuclear data and input parameters Triton has a tenuous nitrogen atmosphere, with trace amounts of carbon monoxide and small amounts of methane near its surface. [11] [42] [43] Like Pluto's atmosphere, the atmosphere of Triton is thought to have resulted from the evaporation of nitrogen from its surface. [27] Its surface temperature is at least 35.6K (−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. [44] An upper limit in the low 40s (K) can be set from vapor pressure equilibrium with nitrogen gas in Triton's atmosphere. [45] This is colder than Pluto's average equilibrium temperature of 44K (−229.2°C). Triton's surface atmospheric pressure is only about 1.4–1.9 Pa (0.014–0.019 mbar). [7] Clouds observed above Triton's limb by Voyager 2. 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. Observations in late 1997 suggest that Triton is heating up and the atmosphere has become significantly denser since Voyager 2 flew past in 1989. [48] Simulated 2D and 3D analysis for light water reactor spent fuel assemblies (isotopic activation, depletion, and decay for light water reactor fuel assemblies)In 1997, observations from Earth were made of Triton's limb as it passed in front of stars. These observations indicated the presence of a denser atmosphere than was deduced from Voyager 2 data. [48] Other observations have shown an increase in temperature by 5% from 1989 to 1998. [49] These observations indicated Triton was approaching an unusually warm southern hemisphere summer season that happens only 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. [50] Another theory argues that temperature changes are a result of the deposition of dark, red material from geological processes. Because Triton's Bond albedo is among the highest in the Solar System, it is sensitive to small variations in spectral albedo. [51] Surface features [ edit ] Interpretative geomorphological map of Triton extended step characteristic transport with flexible geometry applied to neutronics analysis, especially within the TRITON sequences Eigenvalue Monte Carlo codes applied in many computational sequences for multigroup and continuous energy neutronics analysis Pre-generated burnup libraries for a variety of fuel assemblies for commercial and research reactors
Recent nuclear decay data, neutron reaction cross sections, energy-dependent neutron-induced fission product yields, delayed gamma ray emission data, neutron emission data, and photon yield data streamlined light water reactor lattice physics depletion calculations and generation of few-group cross section data for use in nodal core simulators Fixed source Monte Carlo code applied in the MAVRIC sequence for multigroup and continuous energy analysis Library used throughout SCALE that provides individual nuclides; elements with tabulated natural abundances; compounds, alloys, mixtures, and fissile solutions commonly encountered in engineering practice Temperature correction, resonance self-shielding, and flux weighting to provide problem-dependent microscopic and macroscopic multigroup cross section data integrated with computational sequences, but also available for stand-alone analysisSurface gravity derived from the mass m, the gravitational constant G and the radius r: G m r 2 {\displaystyle {\frac {Gm}{r Triton's western hemisphere consists of a strange series of fissures and depressions known as "cantaloupe terrain" because it resembles the skin of a cantaloupe melon. Although it has few craters, it is thought that this is the oldest terrain on Triton. [68] It probably covers much of Triton's western half. [7] Triton's orbit is associated with two tilts, the obliquity of Neptune's rotation to Neptune's orbit, 30°, and the inclination of Triton's orbit to Neptune's rotation, 157° (an inclination over 90° indicates retrograde motion). Triton's orbit precesses forward relative to Neptune's rotation with a period of about 678 Earth years (4.1 Neptunian years), [4] [5] 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's south polar region 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, but it is thought that Triton must also have a north polar ice cap. [44]Triton is the seventh-largest moon and sixteenth-largest object in the Solar System and is modestly larger than the dwarf planets Pluto and Eris. It is also the largest retrograde moon in the solar system. It comprises more than 99.5% of all the mass known to orbit Neptune, including the planet's rings and thirteen other known moons, [j] and is also more massive than all known moons in the Solar System smaller than itself combined. [k] Also, with a diameter 5.5% that of Neptune, it is the largest moon of a gas giant relative to its planet in terms of diameter, although Titan is bigger relative to Saturn in terms of mass (the ratio of Triton's mass to that of Neptune is approximately 1:4788). It has a radius, density (2.061 g/cm 3), temperature and chemical composition similar to that of Pluto. [33] 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's moon Miranda, which has only three percent of Triton's surface area. [70] The largest crater observed on Triton thought to have been created by an impact is a 27-kilometer-diameter (17mi) feature called Mazomba. [70] [71] Although larger craters have been observed, they are generally thought to be volcanic. [70]
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