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Titan is the only moon with an atmosphere denser than Earth's, and it is one of only two moons whose atmospheres are able to support clouds, hazes, and weather—the other being Neptune's moon Triton.[1][2]: 872 Titan's atmosphere is significantly denser than the Earth's, with a surface pressure of 1.448 atm.[2]: 834 The presence of a significant atmosphere was first suspected by Catalan astronomer Josep Comas i Solà, who observed distinct limb darkening on Titan in 1903.[3] Due to the extensive, hazy atmosphere, Titan was once thought to be the largest moon in the Solar System until the Voyager missions revealed that Ganymede is slightly larger.[2]: 831 The haze also shrouded Titan's surface from view—it was not until the arrival of the Cassini–Huygens spacecraft in 2004 that the first direct images of Titan's surface were obtained.[4]
The primary constituents of Titan's atmosphere are nitrogen, methane, and hydrogen.[5]: 243 The precise atmospheric composition varies depending on altitude and latitude due to the methane cycle in Titan's lower atmosphere.[6][7] Nitrogen is the most abundant gas, with a concentration of around 98.6% in the stratosphere and decreasing to 95.1% in the troposphere. Direct observations by the Huygens probe determined that methane concentrations are highest near the surface, with a concentration of 4.92% that remains relatively constant up to 8 kilometres (5.0 mi) above the surface. Methane concentrations then gradually decrease with increasing altitude, down to a concentration of 1.41% in the stratosphere.[5]: 243–244 Methane also increases in concentration near Titan's winter pole, probably due to evaporation from the surface in high-latitude regions.[7]: 385 Hydrogen is the third-most abundant gas, with a concentration of around 0.1%.[5]: 243 There are trace amounts of other hydrocarbons, such as ethane, diacetylene, methylacetylene, acetylene, and propane; and of other gases, such as cyanoacetylene, hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen, argon, and helium.[8] The hydrocarbons are thought to form in Titan's upper atmosphere in reactions resulting from the breakup of methane by the Sun's ultraviolet light, producing a thick orange smog.[9]
Titan's atmosphere is divided into distinct layers, much like those of Earth's atmosphere. The layers of Titan's atmosphere are the troposphere, stratosphere, mesosphere, and thermosphere.[2]: 834 Titan's lower surface gravity and high surface pressure means that its atmosphere is very extended. The exobase—the top of its thermosphere—is roughly 1,450 kilometres (900 mi) above its surface, which is over half of Titan's radius. [10]: 19
Origin
edit- ^ Forget, F.; Bertrand, T.; Vangvichith, M.; Leconte, J.; Millour, E.; Lellouch, E. (May 2017). "A post-New Horizons Global climate model of Pluto including the N 2, CH 4 and CO cycles" (PDF). Icarus. 287: 54–71. Bibcode:2017Icar..287...54F. doi:10.1016/j.icarus.2016.11.038.
- ^ a b c d Tilman, Spohn; Breuer, Doris; Johnson, Torrence V., eds. (2014). Encyclopedia of the Solar System (3rd ed.). doi:10.1016/C2010-0-67309-3. ISBN 978-0-12-415845-0.
- ^ Moore, P. (1990). The Atlas of the Solar System. Mitchell Beazley. ISBN 0-517-00192-6.
- ^ de Selding, Petre (January 21, 2005). "Huygens Probe Sheds New Light on Titan". Space.com. Archived from the original on October 19, 2012. Retrieved March 28, 2005.
- ^ a b c Brown, Robert H.; Lebreton, Jean-Pierre; Waite, J. Hunter, eds. (2010). Titan from Cassini-Huygens (1st ed.). Springer Dordrecht. doi:10.1007/978-1-4020-9215-2. ISBN 978-94-017-8107-7.
- ^ Penteado, Paulo F.; Griffith, Caitlin A. (2010). "Ground-based measurements of the methane distribution on Titan". Icarus. 210 (1): 345–351. Bibcode:2010Icar..206..345P. doi:10.1016/j.icarus.2009.08.022.
- ^ a b Ádámkovics, Máté; et al. (2016). "Meridional variation in tropospheric methane on Titan observed with AO spectroscopy at Keck and VLT". Icarus. 270: 376–388. arXiv:1509.08835. Bibcode:2016Icar..270..376A. doi:10.1016/j.icarus.2015.05.023.
- ^ Niemann, H. B.; et al. (2005). "The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe" (PDF). Nature. 438 (7069): 779–784. Bibcode:2005Natur.438..779N. doi:10.1038/nature04122. hdl:2027.42/62703. PMID 16319830. S2CID 4344046. Archived from the original on April 14, 2020. Retrieved April 17, 2018.
- ^ Waite, J. H.; Cravens, T. E.; Coates, A. J.; Crary, F. J.; Magee, B.; Westlake, J. (2007). "The Process of Tholin Formation in Titan's Upper Atmosphere". Science. 316 (5826): 870–5. Bibcode:2007Sci...316..870W. doi:10.1126/science.1139727. PMID 17495166. S2CID 25984655.
- ^ Müller-Wodarg, Ingo; Griffith, Caitlin; Lellouch, Emmanuel; Cravens, Thomas E., eds. (February 2014). Titan: Interior, surface, atmosphere, and space environment (1st ed.). Cambridge University Press. ISBN 9780521199926.