Atmosphere of Pluto

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Departure shot of Pluto by New Horizons, showing Pluto's atmosphere backlit by the Sun. The blue color is close to what a human eye would have seen, and is caused by layers of haze in the atmosphere.

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.[1][2][3][4] The surface pressure ranges from 6.5 to 24 μbar (0.65 to 2.4 Pa),[5] 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.[6] The lower atmosphere contains a higher concentration of methane than its upper atmosphere.[6]

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.[7]

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.[8]

Observational history[edit]

Atmospheric pressure of Pluto observed by New Horizons on 14 July 2015
More than a dozen haze layers in the atmosphere of Pluto are visible
Detection of Pluto's atmosphere by New Horizons during a solar occultation.[9]
Haze layers in the atmosphere of Pluto

Evidence of Pluto's atmosphere was first suggested by Noah Brosch and Haim Mendelson of the Wise Observatory in Israel in 1985,[10] and then confirmed by the Kuiper Airborne Observatory in 1988, from observations of occultations of stars by Pluto.[11] 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.[10] From the rate of dimming, the atmospheric pressure was determined to be 0.15 Pa, roughly 1/700,000 that of Earth.[12] In 2002, another occultation of a star by Pluto was observed and analysed by teams led by Bruno Sicardy of the Paris Observatory,[13] James L. Elliot of MIT,[14] and Jay Pasachoff of Williams College.[15] 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.[7] Spikes in the data from the same study revealed what may be the first evidence of wind in Pluto's atmosphere.[7] 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.[16]

In 2006, scientists using the Submillimeter Array calculated that Pluto's temperature is about 43 K (−230 °C), 10 K colder than would otherwise be expected.[17]

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.[18]

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.[19]

On 14 July 2015, New Horizons measured half the atmospheric surface pressure value of pressure values found earlier in Earth-based observations.[20]

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.[21][22]

Pluto - Norgay Montes (left-foreground); Hillary Montes (left-skyline); Sputnik Planum (right)
Near-sunset view includes several layers of atmospheric haze.


  1. ^ Sokol, Joshua (9 November 2015). "Pluto surprises with ice volcanoes". New Scientist. Retrieved 12 November 2015. 
  2. ^ Chang, Kenneth (24 July 2015). "Pluto's atmosphere is thinner than expected, but still looks hazy". New York Times. Retrieved 27 July 2015. 
  3. ^ "Conditions on Pluto: Incredibly hazy with flowing ice". New York Times. 24 July 2015. Retrieved 24 July 2015. 
  4. ^ Croswell, Ken (1992). "Nitrogen in Pluto's atmosphere". Retrieved 27 April 2007. 
  5. ^ Lellouch, Emmanuel; Sicardy, Bruno; de Bergh, Catherine; et al. (2009). "Pluto's lower atmosphere structure and methane abundance from high-resolution spectroscopy and stellar occultations". Astronomy and Astrophysics 495 (3): L17–L21. arXiv:0901.4882. Bibcode:2009A&A...495L..17L. doi:10.1051/0004-6361/200911633. 
  6. ^ a b Lellouch, Emmanuel; Sicardy, Bruno; de Bergh, Catherine (2009). "Pluto's lower atmosphere structure and methane abundance from high-resolution spectroscopy and stellar occultations". Astronomy & Astrophysics. 
  7. ^ a b c Britt, Robert Roy (2003). "Puzzling seasons and signs of wind found on Pluto". Archived from the original on 9 January 2009. Retrieved 26 March 2007. 
  8. ^
  9. ^ Staff (14 July 2015). "NASA – Video (00:17) – Alice occultation – Gladstone". NASA & YouTube. Retrieved 18 July 2015. 
  10. ^ a b Marsden, Brian G. (26 August 1985). "Occultation by Pluto on 1985 August 19". IAU Circular 4097. Retrieved 26 November 2011. 
  11. ^ Chester, Geoff. "NOFS contributes to SOFIA'S successful observation of challenging Pluto occultation" (PDF). US Naval Observatory. Retrieved 11 February 2012. 
  12. ^ Johnston, William Robert (2006). "The atmospheres of Pluto and other trans-Neptunian objects". Retrieved 26 March 2007. 
  13. ^ Sicardy, Bruno; Widemann, Thomas; Lellouch, Emmanuel; et al. (10 July 2003). "Large changes in Pluto's atmosphere as revealed by recent stellar occultations". Nature (Nature) 424 (6945): 168–70. Bibcode:2003Natur.424..168S. doi:10.1038/nature01766. PMID 12853950. 
  14. ^ "Pluto is undergoing global warming, researchers find". Massachusetts Institute of Technology. 9 October 2002. Retrieved 20 March 2007. 
  15. ^ "Williams scientists contribute to new finding about Pluto". Williams College. 9 July 2003. Retrieved 20 March 2007. 
  16. ^ Elliot, James L.; Person, Michael J.; Gulbis, Amanda A. S.; et al. (2006). "The size of Pluto's atmosphere as revealed by the 2006 June 12 occultation". Bulletin of the American Astronomical Society 38: 541. Bibcode:2006DPS....38.3102E. 
  17. ^ Than, Ker (2006). "Astronomers: Pluto colder than expected". (via Retrieved 30 November 2011. 
  18. ^ Stern, S. Alan (1 November 2006). "Making Old Horizons New". The PI's Perspective. Johns Hopkins University Applied Physics Laboratory. Retrieved 29 November 2011. 
  19. ^ Veronico, Nicholas A.; Squires, Kate K. (29 June 2015). "SOFIA in the right place at the right time for Pluto observations". NASA. Retrieved 1 July 2015. 
  20. ^ Gipson, Lillian (24 July 2015). "New Horizons Reveals Pluto’s Atmospheric Pressure Has Sharply Decreased". NASA. Retrieved 29 July 2015. 
  21. ^ Singer, Kelsi (10 August 2015). "Atmospheric Escape and Flowing N2 Ice Glaciers – What Resupplies Pluto's Nitrogen?". NASA. Retrieved 10 August 2015. 
  22. ^ Lewin, Sarah (17 August 2015). "If Pluto Keeps Spewing Nitrogen, Why Is It Still Full of It?". Retrieved 17 August 2015. 

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