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Atmosphere

Triton has a tenuous but well-structured and global nitrogen atmosphere, with trace amounts of carbon monoxide and small amounts of methane near its surface. Like Pluto 's atmosphere, the atmosphere of Triton is thought to result from the evaporation of nitrogen from its surface. Its 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. An upper limit in the low 40s (K) can be set from vapor pressure equilibrium with nitrogen gas in Triton's atmosphere. This is colder than Pluto's average equilibrium temperature of 44 K (−229.2 °C). Triton's surface atmospheric pressure is only about 1.4–1.9 Pa (0.014–0.019 mbar).

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 over a micrometer in size. Unlike other atmospheres, Triton's lacks a stratosphere and instead has a thermosphere from altitudes of 8 to 950 km and an exosphere above that. The temperature of Triton's upper atmosphere, at 95 ± 5 K, is higher than that at its surface, due to heat absorbed from solar radiation and Neptune's magnetosphere. A haze permeates most of Triton's troposphere, thought to be composed largely of hydrocarbons and nitriles created by the action of sunlight on methane. Triton's atmosphere also has clouds of condensed nitrogen that lie between 1 and 3 km from its surface.

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. Other observations have shown an increase in temperature by 5% from 1989 to 1998. These observations indicated Triton was approaching an unusually warm southern hemisphere summer season that happens only once every few hundred years. Hypotheses 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. Another hypothesis 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. Based on the increase in atmospheric pressure between 1989 and 1997, it is estimated that by 2010 Triton's atmospheric pressure may have increased to as much as 4 Pa. By 2017, however, Triton's atmospheric surface pressure had nearly returned to Voyager 2 levels; the cause for the rapid spike in atmospheric pressure between 1989 and 2017 remains unexplained.