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), hydrogen cyanide (HCN) 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 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, 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. 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.
Evidence of Pluto's atmosphere was first suggested by Noah Brosch and Haim Mendelson of the Wise Observatory in Israel in 1985, and then confirmed by the Kuiper Airborne Observatory in 1988, from observations of occultations of stars by Pluto. When an object with no atmosphere moves in front of a star, the star abruptly disappears; in the case of Pluto, the star dimmed out gradually. From the rate of dimming, the atmospheric pressure was determined to be 0.15 Pa, roughly 1/700,000 that of Earth. In 2002, another occultation of a star by Pluto was observed and analysed by teams led by Bruno Sicardy of the Paris Observatory, James L. Elliot of MIT, and Jay Pasachoff of Williams College. The atmospheric pressure was estimated to be 0.3 Pa, even though Pluto was farther from the Sun than in 1988 and thus should have a colder and more-rarefied atmosphere. One explanation for the discrepancy is that in 1987, the north pole of Pluto came out of shadow for the first time in 120 years, causing extra nitrogen to sublimate from the polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere as it freezes onto the south pole's now continuously dark ice cap. Spikes in the data from the same study revealed what may be the first evidence of wind in Pluto's atmosphere. Another stellar occultation was observed by the MIT-Williams College team of James L. Elliot, Jay Pasachoff, and a Southwest Research Institute team led by Leslie A. Young on 12 June 2006, from sites in Australia.
In October 2006, Dale Cruikshank of NASA/Ames Research Center (a New Horizons co-investigator) and his colleagues announced the spectroscopic discovery of ethane (C2H6) on Pluto's surface. This ethane is produced from the photolysis or radiolysis (i.e. the chemical conversion driven by sunlight and charged particles) of frozen methane on Pluto's surface and suspended in its atmosphere.
On 29 June 2015, Pluto passed between a distant star and Earth, producing a shadow on Earth near New Zealand that allowed SOFIA, an airborne observatory, to study the atmosphere of Pluto. Results will be made public in July 2016.
On 14 July 2015, New Horizons measured half the atmospheric surface pressure value of pressure values found earlier in Earth-based observations.
On 10 August 2015, NASA scientists stated that simulations suggested that the nitrogen of Pluto's atmosphere is unlikely to be chiefly replenished by impact events, and therefore that Pluto's nitrogen is most likely supplied by geological processes.
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