Atmosphere of Pluto
The atmosphere of Pluto is the tenuous layer of gases surrounding Pluto. It consists mainly of nitrogen (N2), with minor amounts of methane (CH4) and carbon monoxide (CO), all of which are vaporized from their ices on Pluto's surface. It contains layered haze, probably consisting of heavier compounds which form from these gases due to high-energy radiation. The atmosphere of Pluto is notable for its strong and not completely clear seasonal changes caused by peculiarities of the orbital and axial rotation of Pluto.
Surface pressure of the atmosphere of Pluto, measured by New Horizons in 2015, is about 1 Pa (10 μbar), roughly 100,000 times less than Earth's atmospheric pressure. Temperature on the surface is 40 to 60 K (−230 to −210 °C), but it quickly grows with altitude due to methane-generated greenhouse effect. Near the altitude 30 km it reaches 110 K (−163 °C), and then slowly decreases.
Pluto is the only trans-Neptunian object with a known atmosphere. Its closest analog is the atmosphere of Triton, although in some aspects it resembles even the atmosphere of Mars.
The atmosphere of Pluto has been studied since the 1980s by way of earth-based observation of occultations of stars by Pluto and spectroscopy. In 2015, it was studied from a close distance by the spacecraft New Horizons.
The main component of the atmosphere of Pluto is nitrogen. Content of methane, according to measurements by New Horizons, is 0.25%. For carbon monoxide, the Earth-based estimates are 0.025–0.15% (2010) and 0.05–0.075% (2015). Under influence of high-energy cosmic radiation, these gases react to form more complex compounds (not volatile at Pluto's surface temperatures), including ethane (C2H6), ethylene (C2H4), acetylene (C2H2), heavier hydrocarbons and nitriles and hydrogen cyanide (HCN) (amount of ethylene is about 0.0001%, and amount of acetylene is about 0.0003%). These compounds slowly precipitate on the surface. Probably, they also include tholins, which are responsible for the brown color of Pluto (like some other bodies in the outer solar system).
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