Editing Saturn (section)

=== Atmosphere ===
The outer atmosphere of Saturn contains 96.3% molecular hydrogen and 3.25% helium by volume. The proportion of helium is significantly deficient compared to the abundance of this element in the Sun.<ref name=guillot_et_al2009/> The quantity of elements heavier than helium ([[metallicity]]) is not known precisely, but the proportions are assumed to match the primordial abundances from the [[formation of the Solar System]]. The total mass of these heavier elements is estimated to be 19–31 times the mass of Earth, with a significant fraction located in Saturn's core region.<ref name="science286" />

Trace amounts of ammonia, [[acetylene]], [[ethane]], [[propane]], [[phosphine]], and [[methane]] have been detected in Saturn's atmosphere.<ref name="baas15_831" /><ref name="cain2009_24029" /><ref name="pfsaa2008" /> The upper clouds are composed of ammonia crystals, while the lower level clouds appear to consist of either [[ammonium hydrosulfide]] ({{chem2|NH4SH}}) or water.<ref name="martinez20050905" /> [[Ultraviolet radiation]] from the Sun causes methane [[photolysis]] in the upper atmosphere, leading to a series of [[hydrocarbon]] chemical reactions with the resulting products being carried downward by [[eddies]] and [[diffusion]]. This [[photochemical cycle]] is modulated by Saturn's annual seasonal cycle.<ref name=pfsaa2008/> ''Cassini'' observed a series of cloud features found in northern latitudes, nicknamed the "String of Pearls". These features are cloud clearings that reside in deeper cloud layers.<ref>{{cite press release |title=Cassini Image Shows Saturn Draped in a String of Pearls |url=http://www.nasa.gov/mission_pages/cassini/media/cassini-20061011a.html |publisher=Carolina Martinez, NASA |date=10 November 2006 |access-date=3 March 2013 |archive-date=1 May 2013 |archive-url=https://web.archive.org/web/20130501174037/http://www.nasa.gov/mission_pages/cassini/media/cassini-20061011a.html |url-status=live}}</ref>

==== Cloud layers ====
[[File:Saturn Storm.jpg|thumb|A global storm girdles the planet in 2011. The storm passes around the planet, such that the storm's head (bright area) passes its tail.|left]]

Saturn's atmosphere exhibits a banded pattern similar to Jupiter's, but Saturn's bands are much fainter and are much wider near the equator. The nomenclature used to describe these bands is the same as on Jupiter. Saturn's finer cloud patterns were not observed until the flybys of the ''[[Voyager Space Probe|Voyager]]'' spacecraft during the 1980s. Since then, Earth-based [[telescope|telescopy]] has improved to the point where regular observations can be made.<ref name=emp105_2_143/>

The composition of the clouds varies with depth and increasing pressure. In the upper cloud layers, with temperatures in the range of 100–160&nbsp;K and pressures extending between 0.5–2 [[Bar (unit)|bar]], the clouds consist of ammonia ice. Water [[ice cloud]]s begin at a level where the pressure is about 2.5 bar and extend down to 9.5 bar, where temperatures range from 185 to 270&nbsp;K. Intermixed in this layer is a band of ammonium hydrosulfide ice, lying in the pressure range 3–6 bar with temperatures of 190–235&nbsp;K. Finally, the lower layers, where pressures are between 10 and 20 bar and temperatures are 270–330&nbsp;K, contains a region of water droplets with ammonia in aqueous solution.<ref name=dougherty_esposito2009/>

Saturn's usually bland atmosphere occasionally exhibits long-lived ovals and other features common on Jupiter. In 1990, the [[Hubble Space Telescope]] imaged an enormous white cloud near Saturn's equator that was not present during the ''Voyager'' encounters, and in 1994 another smaller storm was observed. The 1990 storm was an example of a [[Great White Spot]], a short-lived phenomenon that occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere's [[summer solstice]].<ref name=icarus176_1_155/> Previous Great White Spots were observed in 1876, 1903, 1933, and 1960, with the 1933 storm being the best observed.<ref>{{cite book |editor1-link=Patrick Moore |editor1-last=Moore |editor1-first=Patrick |title=1993 Yearbook of Astronomy |location=London |publisher=W.W. Norton & Company |date=1992 |first1=Mark |last1=Kidger |chapter=The 1990 Great White Spot of Saturn |pages=176–215|bibcode=1992ybas.conf.....M }}</ref> The latest giant storm was observed in 2010. In 2015, researchers used [[Very Large Array]] telescope to study Saturnian atmosphere, and reported that they found "long-lasting signatures of all mid-latitude giant storms, a mixture of equatorial storms up to hundreds of years old, and potentially an unreported older storm at 70°N".<ref>{{cite journal |last1=Li |first1=Cheng |last2=de Pater |first2=Imke |last3=Moeckel |first3=Chris |last4=Sault |first4=R. J. |last5=Butler |first5=Bryan |last6=deBoer |first6=David |last7=Zhang |first7=Zhimeng |title=Long-lasting, deep effect of Saturn's giant storms |journal=Science Advances |date=11 August 2023 |volume=9 |issue=32 |pages=eadg9419 |doi=10.1126/sciadv.adg9419 |pmid=37566653 |pmc=10421028 |bibcode=2023SciA....9G9419L }}</ref>

The winds on Saturn are the second fastest among the Solar System's planets, after Neptune's. ''Voyager'' data indicate peak easterly winds of {{convert|500|m/s|km/h|abbr=on}}.<ref name="Voyager Summary 1">{{cite web |title=Voyager Saturn Science Summary |url=http://www.solarviews.com/eng/vgrsat.htm |first=Calvin J. |last=Hamilton |access-date=5 July 2007 |date=1997 |publisher=Solarviews |archive-url=https://web.archive.org/web/20110926211656/http://www.solarviews.com/eng/vgrsat.htm |archive-date=26 September 2011 |url-status=dead }}</ref> In images from the ''[[Cassini–Huygens|Cassini]]'' spacecraft during 2007, Saturn's northern hemisphere displayed a bright blue hue, similar to Uranus. The color was most likely caused by [[Rayleigh scattering]].<ref>{{cite web |url=http://www.nasa.gov/mission_pages/cassini/multimedia/pia09188.html |title=Saturn's Strange Hexagon |access-date=6 July 2007 |date=27 March 2007 |last=Watanabe |first=Susan |publisher=NASA |archive-url=https://web.archive.org/web/20100116155833/http://www.nasa.gov/mission_pages/cassini/multimedia/pia09188.html |archive-date=16 January 2010 |url-status=live }}</ref> [[Thermography]] has shown that Saturn's south pole has a warm [[polar vortex]], the only known example of such a phenomenon in the Solar System.<ref name=MCP>{{cite web |url=http://www.mcpstars.org/node/353 |title=Warm Polar Vortex on Saturn |date=2007 |publisher=Merrillville Community Planetarium |access-date=25 July 2007 |archive-url=https://web.archive.org/web/20110921212018/http://www.mcpstars.org/node/353 |archive-date=21 September 2011 |url-status=dead }}</ref> Whereas temperatures on Saturn are normally −185&nbsp;°C, temperatures on the vortex often reach as high as −122&nbsp;°C, suspected to be the warmest spot on Saturn.<ref name=MCP/>

==== Hexagonal cloud patterns ====
{{main|Saturn's hexagon}}
{{multiple image
| align             = right
| direction         = horizontal
| total_width       = 400
| image1            = Rotatingsaturnhexagon.gif
| image2            = Looking saturn in the eye.jpg
| image3            = |thumb
| caption3          = Hexagonal storm pattern around the north pole of Saturn
| footer            = Saturn's north and south pole in [[infrared]]
}}

{{Anchor|North pole hexagonal cloud pattern|South pole vortex}}A persisting [[hexagon]]al wave pattern around the north polar vortex in the atmosphere at about 78°N was first noted in the ''Voyager'' images.<ref>{{cite journal |bibcode=1988Icar...76..335G |doi=10.1016/0019-1035(88)90075-9 |title=A hexagonal feature around Saturn's North Pole |date=1988 |page=335 |author=Godfrey, D. A. |volume=76 |journal=Icarus |issue=2}}</ref><ref>{{cite journal |title=Ground-based observations of Saturn's north polar SPOT and hexagon |first4=P. |last4=Laques |first3=F. |last3=Colas |first2=J. |last2=Lecacheux |journal=Science |display-authors=1 |first1=A. |last1=Sanchez-Lavega |volume=260 |issue=5106 |date=1993 |pmid=17838249 |doi=10.1126/science.260.5106.329 |bibcode=1993Sci...260..329S |pages=329–32|s2cid=45574015 }}</ref><ref name="NYT-20140806">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Storm Chasing on Saturn |url=https://www.nytimes.com/2014/08/06/science/space/storm-chasing-on-saturn.html |date=6 August 2014 |work=[[New York Times]] |access-date=6 August 2014 |archive-date=12 July 2018 |archive-url=https://web.archive.org/web/20180712004804/https://www.nytimes.com/2014/08/06/science/space/storm-chasing-on-saturn.html |url-status=live }}</ref> The sides of the hexagon are each about {{Convert|14500|km|mi|-2|abbr=on}} long, which is longer than the diameter of the Earth.<ref name="Hexagon ref">{{cite news |url=https://www.nbcnews.com/id/wbna34352533 |title=New images show Saturn's weird hexagon cloud |publisher=NBC News |date=12 December 2009 |access-date=29 September 2011 |archive-date=21 October 2020 |archive-url=https://web.archive.org/web/20201021075355/http://www.nbcnews.com/id/34352533 |url-status=live }}</ref> The entire structure rotates with a period of {{RA|10|39|24}} (the same period as that of the planet's radio emissions) which is assumed to be equal to the period of rotation of Saturn's interior.<ref name=science247_4947_1206/> The hexagonal feature does not shift in longitude like the other clouds in the visible atmosphere.<ref name=pss57_14_1671/> The pattern's origin is a matter of much speculation. Most scientists think it is a [[standing wave]] pattern in the atmosphere. Polygonal shapes have been replicated in the laboratory through differential rotation of fluids.<ref>{{cite journal |doi=10.1038/news060515-17 |last1=Ball |first1=Philip |title=Geometric whirlpools revealed |journal=[[Nature (journal)|Nature]] |date=19 May 2006|s2cid=129016856 |doi-access=free }} Bizarre geometric shapes that appear at the center of swirling vortices in planetary atmospheres might be explained by a simple experiment with a bucket of water but correlating this to Saturn's pattern is by no means certain.</ref><ref name=labmodel>{{cite journal |doi=10.1016/j.icarus.2009.10.022 |display-authors=1 |last1=Aguiar |first1=Ana C. Barbosa |last2=Read |first2=Peter L. |last3=Wordsworth |first3=Robin D |title=A laboratory model of Saturn's North Polar Hexagon |volume=206 |issue=2 |date=April 2010 |pages=755–763 |last4=Salter |first4=Tara |last5=Hiro Yamazaki |first5=Y. |journal=Icarus |bibcode=2010Icar..206..755B}} Laboratory experiment of spinning disks in a liquid solution forms vortices around a stable hexagonal pattern similar to that of Saturn's.</ref>

[[Hubble Space Telescope|HST]] imaging of the south polar region indicates the presence of a [[jet stream]], but no strong polar vortex nor any hexagonal standing wave.<ref>{{Cite journal |display-authors=1 |author=Sánchez-Lavega, A. |author2=Pérez-Hoyos, S. |author3=French, R. G. |url=http://aas.org/archives/BAAS/v34n3/dps2002/10.htm |title=Hubble Space Telescope Observations of the Atmospheric Dynamics in Saturn's South Pole from 1997 to 2002 |journal=Bulletin of the American Astronomical Society |volume=34 |page=857 |access-date=6 July 2007 |date=8 October 2002 |bibcode=2002DPS....34.1307S |archive-date=30 June 2010 |archive-url=https://web.archive.org/web/20100630162647/http://aas.org/archives/BAAS/v34n3/dps2002/10.htm |url-status=live }}</ref> [[NASA]] reported in November 2006 that ''Cassini'' had observed a "[[hurricane]]-like" storm locked to the south pole that had a clearly defined [[eyewall]].<ref>{{cite web |url=http://photojournal.jpl.nasa.gov/catalog/PIA09187 |title=NASA catalog page for image PIA09187 |access-date=23 May 2007 |publisher=NASA Planetary Photojournal |archive-url=https://web.archive.org/web/20111109044235/http://photojournal.jpl.nasa.gov/catalog/PIA09187 |archive-date=9 November 2011 |url-status=live }}</ref><ref>{{cite news |url=http://news.bbc.co.uk/2/hi/6135450.stm?lsm |title=Huge 'hurricane' rages on Saturn |work=BBC News |date=10 November 2006 |access-date=29 September 2011 |archive-url=https://web.archive.org/web/20120803115422/http://news.bbc.co.uk/2/hi/6135450.stm?lsm |archive-date=3 August 2012 |url-status=live }}</ref> Eyewall clouds had not previously been seen on any planet other than Earth. For example, images from the ''[[Galileo (spacecraft)|Galileo]]'' spacecraft did not show an eyewall in the [[Great Red Spot]] of Jupiter.<ref>{{cite web |url=http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=703 |title=NASA Sees into the Eye of a Monster Storm on Saturn |publisher=NASA |date=9 November 2006 |access-date=20 November 2006 |archive-url=https://web.archive.org/web/20080507154317/http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=703 |archive-date=7 May 2008 |url-status=dead }}</ref>

The south pole storm may have been present for billions of years.<ref name="spole"/> This vortex is comparable to the size of Earth, and it has winds of 550&nbsp;km/h.<ref name="spole">{{Cite APOD | title=A Hurricane Over the South Pole of Saturn | date=13 November 2006 | access-date=1 May 2013}}</ref>