Atmosphere of Pluto
The atmosphere of Pluto is the thin layer of gases surrounding Pluto. It consists mainly of nitrogen (N2), with minor components of methane (CH4) and carbon monoxide (CO), all of which are in equilibrium with their ices on Pluto's surface. The surface pressure ranges from 6.5 to 24 μbar (0.65 to 2.4 Pa), roughly one million to 100,000 times less than Earth's atmospheric pressure. Pluto's elliptical orbit is predicted to have a major effect on its atmosphere: as Pluto moves away from the Sun, its atmosphere should gradually freeze out. When Pluto is closer to the Sun, the temperature of Pluto's solid surface increases, causing the ices to sublimate. Just like sweat cools the body as it evaporates from the skin, this sublimation cools the surface of Pluto, a kind of anti-greenhouse effect.The presence of methane, a powerful greenhouse gas, in Pluto's atmosphere creates a temperature inversion, with average temperatures 36 K warmer 10 km above the surface. The lower atmosphere contains a higher concentration of methane than its upper atmosphere.Even though Pluto is receding from the Sun, in 2002, the atmospheric pressure (0.3 Pa) was higher than in 1988, because in 1987, the north pole of Pluto came out of the shadow for the first time in 120 years, causing extra nitrogen to start sublimating from the polar cap, which will take decades to condense out of the atmosphere as it freezes onto Pluto's now continuously dark south pole's ice cap.Some of the molecules that form the atmosphere have enough energy to overcome Pluto’s weak gravity and escape into space, where they are ionized by solar ultraviolet radiation. As the solar wind encounters the obstacle formed by the ions, it is slowed and diverted (depicted in the red region), possibly forming a shock wave upstream of Pluto. The ions are ""picked up"" by the solar wind and carried in its flow past the dwarf planet to form an ion or plasma tail (blue region). The Solar Wind around Pluto (SWAP) instrument on the New Horizons spacecraft made the first measurements of this region of low-energy atmospheric ions shortly after its closest approach on 14 July 2015. Such measurements will enable the SWAP team to determine the rate at which Pluto loses its atmosphere and, in turn, will yield insight into the evolution of the Pluto’s atmosphere and surface.