Atmosphere of Titan#Composition

The presence of a significant atmosphere was first suspected by Spanish astronomer Josep Comas i Solà, who observed distinct limb darkening on Titan in 1903 from the Fabra Observatory in Barcelona, Catalonia.{{cite book|title=The Atlas of the Solar System|author=Moore, P.|year=1990|publisher=Mitchell Beazley|isbn=0-517-00192-6|url-access=registration|url=https://archive.org/details/atlasofsolarsyst0000unse}} This observation was confirmed by Dutch astronomer Gerard P. Kuiper in 1944 using a spectroscopic technique that yielded an estimate of an atmospheric partial pressure of methane of the order of 100 millibars (10 kPa).{{cite journal|author= Kuiper, G. P.|year= 1944|title = Titan: a Satellite with an Atmosphere|journal =Astrophysical Journal|volume = 100| doi =10.1086/144679|page= 378|bibcode=1944ApJ...100..378K}} Subsequent observations in the 1970s showed that Kuiper's figures had been significant underestimates; methane abundances in Titan's atmosphere were ten times higher, and the surface pressure was at least double what he had predicted. The high surface pressure meant that methane could only form a small fraction of Titan's atmosphere.Coustenis, pp. 13–15 In 1980, Voyager 1 made the first detailed observations of Titan's atmosphere, revealing that its surface pressure was higher than Earth's, at 1.5 bars (about 1.48 times that of Earth's).Coustenis, p. 22

The joint NASA/ESA Cassini-Huygens mission provided a wealth of information about Titan, and the Saturn system in general, since entering orbit on July 1, 2004. It was determined that Titan's atmospheric isotopic abundances were evidence that the abundant nitrogen in the atmosphere came from materials in the Oort cloud, associated with comets, and not from the materials that formed Saturn in earlier times.{{cite web |last1=Dyches |first1=Preston |last2=Clavin |first2=Clavin |title=Titan's Building Blocks Might Pre-date Saturn |url=http://www.jpl.nasa.gov/news/news.php?release=2014-200 |date=June 23, 2014 |work=NASA |access-date=June 24, 2014 }} It was determined that complex organic chemicals could arise on Titan,{{cite web|url=http://phys.org/news/2013-04-nasa-team-complex-chemistry-titan.html| title=NASA team investigates complex chemistry at Titan| author=Staff|date=April 3, 2013|work=Phys.Org|access-date=April 11, 2013}} including polycyclic aromatic hydrocarbons,{{cite news| url=http://www.iaa.es/content/pahs-titans-upper-atmosphere| title=PAH's in Titan's Upper Atmosphere| last=López-Puertas| first=Manuel| date=June 6, 2013| work=CSIC| access-date=June 6, 2013| archive-date=December 3, 2013| archive-url=https://web.archive.org/web/20131203030521/http://www.iaa.es/content/pahs-titans-upper-atmosphere| url-status=dead}} propylene,{{cite web |url=http://www.jpl.nasa.gov/news/news.php?release=2013-295| title=NASA's Cassini Spacecraft Finds Ingredient of Household Plastic in Space – NASA Jet Propulsion Laboratory| author=Jpl.Nasa.Gov|date=2013-09-30| publisher=Jpl.nasa.gov |access-date=2013-10-04}} and methane.{{cite web |url=http://www.jpl.nasa.gov/news/news.php?feature=4354| title=NASA Finds Methane Ice Cloud in Titan's Stratosphere| last1=Dyches|first1=Preston |last2=Zubritsky|first2=Elizabeth| date=October 24, 2014| work=NASA |access-date=October 31, 2014}}{{cite web| url=http://www.nasa.gov/content/goddard/nasa-identifies-ice-cloud-above-cruising-altitude-on-titan| title=NASA Identifies Ice Cloud Above Cruising Altitude on Titan| last1=Zubritsky| first1=Elizabeth| last2=Dyches|first2=Preston |date=October 24, 2014 |work=NASA|access-date=October 31, 2014}}

The Dragonfly mission by NASA is planning to land a large aerial vehicle on Titan in 2034.{{cite web |url=http://dragonfly.jhuapl.edu/News-and-Resources/news/20190109.php |title=Eyes on Titan: Dragonfly Team Shapes Science Instrument Payload |publisher=Johns Hopkins University Applied Physics Laboratory |date=9 January 2019 |access-date=15 March 2019}} The mission will study Titan's habitability and prebiotic chemistry at various locations.[https://www.hou.usra.edu/meetings/lpsc2017/eposter/1958.pdf Dragonfly: Exploring Titan's Prebiotic Organic Chemistry and Habitability] (PDF). E. P. Turtle, J. W. Barnes, M. G. Trainer, R. D. Lorenz, S. M. MacKenzie, K. E. Hibbard, D. Adams, P. Bedini, J. W. Langelaan, K. Zacny, and the Dragonfly Team. Lunar and Planetary Science Conference 2017. The drone-like aircraft will perform measurements of geologic processes, and surface and atmospheric composition.Langelaan J. W. et al. (2017) Proc. Aerospace Conf. IEEE

File:AtmosphericComparison Titan Earth.svg

File:Titan atmosphere detail narrow.svg

Observations from the Voyager space probes have shown that the Titanean atmosphere is denser than Earth's, with a surface pressure about 1.48 times that of Earth's. Titan's atmosphere is about 1.19 times as massive as Earth's overall,{{cite book|title=Titan: Exploring an Earthlike World|author1=Coustenis, Athéna |author2=Taylor, F. W. |name-list-style=amp|publisher=World Scientific|year=2008|page=130|url=https://books.google.com/books?id=j3O47dxrDAQC&q=Titan|access-date=2010-03-25|isbn=978-981-270-501-3}} or about 7.3 times more massive on a per surface area basis. It supports opaque haze layers that block most visible light from the Sun and other sources and renders Titan's surface features obscure. The atmosphere is so thick and the gravity so low that humans could fly through it by flapping "wings" attached to their arms.{{cite book |author=Zubrin, Robert|title=Entering Space: Creating a Spacefaring Civilization |url=https://archive.org/details/enteringspacecre00zubr|url-access=registration|location=Section: Titan |pages=[https://archive.org/details/enteringspacecre00zubr/page/163 163–166] |publisher=Tarcher/Putnam |year=1999 |isbn=1-58542-036-0}} Titan's lower gravity means that its atmosphere is far more extended than Earth's; even at a distance of 975 km, the Cassini spacecraft had to make adjustments to maintain a stable trajectory against atmospheric drag.{{cite web|title=Exploring the Surface of Titan with Cassini–Huygens|author=Turtle, Elizabeth P.|year=2007|publisher=Smithsonian|url=https://www.youtube.com/watch?v=cfCTmv-9GkE| archive-url=https://web.archive.org/web/20130720123340/http://www.youtube.com/watch?v=cfCTmv-9GkE| archive-date=2013-07-20 | url-status=dead|access-date=2009-04-18}} The atmosphere of Titan is opaque at many wavelengths and a complete reflectance spectrum of the surface is impossible to acquire from the outside.{{cite journal |author=Schröder, S. E. |author2= Tomasko, M. G.|author3= Keller, H. U. |date=August 2005 |title= The reflectance spectrum of Titan's surface as determined by Huygens |page=726 |journal= American Astronomical Society, DPS Meeting #37, #46.15; Bulletin of the American Astronomical Society |volume=37 |issue=726 |bibcode=2005DPS....37.4615S}} It was not until the arrival of Cassini–Huygens in 2004 that the first direct images of Titan's surface were obtained. The Huygens probe was unable to detect the direction of the Sun during its descent, and although it was able to take images from the surface, the Huygens team likened the process to "taking pictures of an asphalt parking lot at dusk".{{cite news |url=http://www.space.com/missionlaunches/titan_update_050121.html |title=Huygens Probe Sheds New Light on Titan |author=de Selding, Petre|publisher=SPACE.com |date=January 21, 2005 |access-date=2005-03-28| archive-url= https://web.archive.org/web/20050404014313/http://www.space.com/missionlaunches/titan_update_050121.html| archive-date= 4 April 2005 | url-status= live}}

{{plain image with caption|File:Titan's atmosphere.svg|Diagram of Titan's atmosphere|300px|right|top|triangle|#ccc}}Titan's vertical atmospheric structure is similar to Earth. They both have a troposphere, stratosphere, mesosphere, and thermosphere. However, Titan's lower surface gravity creates a more extended atmosphere, with scale heights of {{convert|15|–|50|km|mi|abbr=on}} in comparison to {{convert|5|–|8|km|mi|abbr=on}} on Earth. Voyager data, combined with data from Huygens and radiative-convective models provide increased understanding of Titan's atmospheric structure.{{Cite journal|last1=Catling|first1=David C.|last2=Robinson|first2=Tyler D.|date=2012-09-09| title=An Analytic Radiative-Convective Model for Planetary Atmospheres|journal=The Astrophysical Journal|volume=757|issue=1|pages=104| language=en|doi=10.1088/0004-637X/757/1/104|arxiv=1209.1833|bibcode=2012ApJ...757..104R|s2cid=54997095}}

• Troposphere: This is the layer where a lot of the weather occurs on Titan. Since methane condenses out of Titan's atmosphere at high altitudes, its abundance increases below the tropopause at an altitude of {{convert|32|km|mi|abbr=on}}, leveling off at a value of 4.9% between {{convert|8|km|mi|abbr=on}} and the surface."Titan: Exploring an Earthlike World". By Athena Coustenis, F. W. Taylor. World Scientific, 2008. pp. 154–155. {{ISBN|9812705015}}, 9789812705013{{cite journal|author=Niemann, H. B.|display-authors=etal|year=2005 |title=The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe |url=https://deepblue.lib.umich.edu/bitstream/2027.42/62703/1/nature04122.pdf| journal=Nature|volume=438|issue=7069|pages=779–784|bibcode=2005Natur.438..779N|doi=10.1038/nature04122|pmid=16319830|hdl=2027.42/62703|s2cid=4344046|hdl-access=free}} Methane rain, haze rainout, and varying cloud layers are found in the troposphere.
• Stratosphere: The atmospheric composition in the stratosphere is 98.4% nitrogen—the only dense, nitrogen-rich atmosphere in the Solar System aside from Earth's—with the remaining 1.6% composed mostly of methane (1.4%) and hydrogen (0.1–0.2%). The main tholin haze layer lies in the stratosphere at about {{convert|100|–|210|km|mi|abbr=on}}. In this layer of the atmosphere there is a strong temperature inversion caused by the haze due to a high ratio of shortwave to infrared opacity.
• Mesosphere: A detached haze layer is found at about {{convert|450|–|500|km|mi|abbr=on}}, within the mesosphere. The temperature at this layer is similar to that of the thermosphere because of the cooling of hydrogen cyanide (HCN) lines.{{Cite journal|last=Yelle|first=Roger|date=1991-12-10|title=Non-LTE models of Titan's upper atmosphere| url=https://arizona.pure.elsevier.com/en/publications/non-lte-models-of-titans-upper-atmosphere|journal=Astrophysical Journal|volume=383|issue=1|pages=380–400|issn=0004-637X|doi=10.1086/170796|bibcode=1991ApJ...383..380Y}}
• Thermosphere: Particle production begins in the thermosphere This was concluded after finding and measuring heavy ions and particles.{{Cite journal|last1=Podolak|first1=M.|last2=Bar-Nun|first2=A.|date=1979-08-01|title=A constraint on the distribution of Titan's atmospheric aerosol|journal=Icarus|volume=39|issue=2|pages=272–276|doi=10.1016/0019-1035(79)90169-6|bibcode=1979Icar...39..272P|issn=0019-1035}} This was also Cassini's closest approach in Titan's atmosphere.
• Ionosphere: Titan's ionosphere is also more complex than Earth's, with the main ionosphere at an altitude of {{convert|1,200|km|mi|abbr=on}} but with an additional layer of charged particles at {{convert|63|km|mi|abbr=on}}. This splits Titan's atmosphere to some extent into two separate radio-resonating chambers. The source of natural extremely-low-frequency (ELF) waves on Titan, as detected by Cassini–Huygens, is unclear as there does not appear to be lightning activity. The main sources of Titan's ionosphere are solar irradiance, Saturn's magnetospheric electrons and ions ($H^+\_2$, $H^+$

, $O$

^+), drifting along the magnetic field lines, and galactic cosmic rays (see more in{{Cite journal |last1=Waite |first1=J. H. |last2=Lewis |first2=W. S. |last3=Kasprzak |first3=W. T. |last4=Anicich |first4=V. G. |last5=Block |first5=B. P. |last6=Cravens |first6=T. E. |last7=Fletcher |first7=G. G. |last8=Ip |first8=W.-H. |last9=Luhmann |first9=J. G. |last10=Mcnutt |first10=R. L. |last11=Niemann |first11=H. B. |date=2004-09-01 |title=The Cassini Ion and Neutral Mass Spectrometer (INMS) Investigation |url=https://doi.org/10.1007/s11214-004-1408-2 |journal=Space Science Reviews |language=en |volume=114 |issue=1 |pages=113–231 |doi=10.1007/s11214-004-1408-2 |bibcode=2004SSRv..114..113W |hdl=2027.42/43764 |s2cid=120116482 |issn=1572-9672|hdl-access=free }} ).

File:Formation of tholins in Titan's upper atmosphere.jpg

Titan's atmospheric chemistry is diverse and complex. Each layer of the atmosphere has unique chemical interactions occurring within that are then interacting with other sub layers in the atmosphere. For instance, 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.{{cite journal|author=Waite, J. H.|display-authors=etal|year=2007|title=The Process of Tholin Formation in Titan's Upper Atmosphere|journal=Science|volume=316|issue=5826|pages=870–5|bibcode=2007Sci...316..870W|doi=10.1126/science.1139727|pmid=17495166|s2cid=25984655}} The table below highlights the production and loss mechanisms of the most abundant photochemically produced molecules in Titan's atmosphere.

File:Titancloud.jpg

Titan's internal magnetic field is negligible, and perhaps even nonexistent, although studies in 2008 showed that Titan retains remnants of Saturn's magnetic field on the brief occasions when it passes outside Saturn's magnetosphere and is directly exposed to the solar wind.{{cite web|url=http://saturn.jpl.nasa.gov/news/features/feature20080911.cfm|title=Saturn's Magnetic Personality Rubs Off on Titan|year=2008|publisher=NASA/JPL|archive-url=https://web.archive.org/web/20090520090330/http://saturn.jpl.nasa.gov/news/features/feature20080911.cfm|archive-date=20 May 2009|url-status=dead|access-date=2009-04-20}}{{cite journal|author=H. Backes|s2cid=38778517|display-authors=etal|year=2005|title=Titan's magnetic field signature during the first Cassini encounter|journal=Science|volume=308|issue=5724|pages=992–995|bibcode=2005Sci...308..992B|doi=10.1126/science.1109763|pmid=15890875}} This may ionize and carry away some molecules from the top of the atmosphere. One interesting case was detected as an example of the coronal mass ejection impact onto Saturn's magnetosphere, causing Titan's orbit to be exposed to the shocked solar wind in the magnetosheath. This leads to the increased particle precipitation and the formation of extreme electron densities in Titan's ionosphere.{{Cite journal |last1=T. Edberg |first1=N. J. |last2=Andrews |first2=D. J. |last3=Shebanits |first3=O. |last4=Ågren |first4=K. |last5=Wahlund |first5=J.-E. |last6=Opgenoorth |first6=H. J. |last7=Roussos |first7=E. |last8=Garnier |first8=P. |last9=Cravens |first9=T. E. |last10=Badman |first10=S. V. |last11=Modolo |first11=R. |date=2013-06-17 |title=Extreme densities in Titan's ionosphere during the T85 magnetosheath encounter |journal=Geophysical Research Letters |volume=40 |issue=12 |pages=2879–2883 |doi=10.1002/grl.50579 |bibcode=2013GeoRL..40.2879E |s2cid=128369295 |issn=0094-8276|doi-access=free |hdl=1808/14414 |hdl-access=free }} Its orbital distance of 20.3 Saturn radii does place it within Saturn's magnetosphere occasionally. However, the difference between Saturn's rotational period (10.7 hours) and Titan's orbital period (15.95 days) causes a relative speed of about {{val|100|u=km/s}} between the Saturn's magnetized plasma and Titan. That can actually intensify reactions causing atmospheric loss, instead of guarding the atmosphere from the solar wind.{{cite journal|author=D.G. Mitchell|s2cid=6795525|display-authors=etal|year=2005|title=Energetic neutral atom emissions from Titan interaction with Saturn's magnetosphere|journal=Science|volume=308|issue=5724|pages=989–992|bibcode=2005Sci...308..989M|doi=10.1126/science.1109805|pmid=15890874}}

In November 2007, scientists uncovered evidence of negative ions with roughly 13 800 times the mass of hydrogen in Titan's ionosphere, which are thought to fall into the lower regions to form the orange haze which obscures Titan's surface.{{cite journal|author=Coates, A. J.|author2=F. J. Crary|author3=G. R. Lewis|author4=D. T. Young|author5=J. H. Waite|author6=E. C. Sittler|name-list-style=amp|year=2007|title=Discovery of heavy negative ions in Titan's ionosphere|journal=Geophys. Res. Lett.|volume=34|issue=22|pages=L22103|bibcode=2007GeoRL..3422103C|doi=10.1029/2007GL030978|s2cid=129931701 |url=http://discovery.ucl.ac.uk/168293/1/2007GL030978.pdf}} The smaller negative ions have been identified as linear carbon chain anions with larger molecules displaying evidence of more complex structures, possibly derived from benzene.{{cite journal|author=Desai, R. T.|author2=A. J. Coates|author3=A. Wellbrock|author4=V. Vuitton|author5=D. González-Caniulef|display-authors=et al|name-list-style=amp|year=2017|title=Carbon Chain Anions and the Growth of Complex Organic Molecules in Titan's Ionosphere|journal=Astrophys. J. Lett.|volume=844|issue=2|pages=L18|arxiv=1706.01610|bibcode=2017ApJ...844L..18D|doi=10.3847/2041-8213/aa7851|s2cid=32281365 |doi-access=free }} These negative ions appear to play a key role in the formation of more complex molecules, which are thought to be tholins, and may form the basis for polycyclic aromatic hydrocarbons, cyanopolyynes and their derivatives. Remarkably, negative ions such as these have previously been shown to enhance the production of larger organic molecules in molecular clouds beyond our Solar System,{{cite journal|author=Walsch, C.|author2=N. Harada|author3=E. Herbst|author4=T. J. Millar|name-list-style=amp|year=2017|title=The EFFECTS OF MOLECULAR ANIONS ON THE CHEMISTRY OF DARK CLOUDS|journal=Astrophys. J.|volume=700|issue=1|pages=752–761|arxiv=0905.0800|bibcode=2009ApJ...700..752W|doi=10.3847/2041-8213/aa7851|s2cid=32281365 |doi-access=free }} a similarity which highlights the possible wider relevance of Titan's negative ions.{{cite web|url=http://sci.esa.int/cassini-huygens/59350-has-cassini-found-a-universal-driver-for-prebiotic-chemistry-at-titan/|title=Has Cassini found a universal driver for prebiotic chemistry at Titan?|date=July 26, 2017|publisher=European Space Agency|access-date=2017-08-12}}File:PIA18431-SaturnMoon-Titan-SouthPoleVortex-Cloud-20121129.jpg ice cloud (November 29, 2012).]]

{{See also|Climate of Titan}}

There is a pattern of air circulation found flowing in the direction of Titan's rotation, from west to east. In addition, seasonal variation in the atmospheric circulation has also been detected. Observations by Cassini of the atmosphere made in 2004 also suggest that Titan is a "super rotator", like Venus, with an atmosphere that rotates much faster than its surface.{{cite web|url=http://www.astrobio.net/news/article1480.html|title=Wind or Rain or Cold of Titan's Night?|date=March 11, 2005|publisher=Astrobiology Magazine|archive-url=https://web.archive.org/web/20070927030216/http://www.astrobio.net/news/article1480.html|archive-date=27 September 2007 |url-status=live| access-date=2007-08-24}} The atmospheric circulation is explained by a big Hadley circulation that is occurring from pole to pole.

{{multiple image |header=Titan Clouds |align=right |direction=vertical |width=300 |image1=Titan clouds by NIRCam (2022-11-04 annotated).png |caption1=Clouds (Nov 4, 2022) |width1= |image2=Titan clouds by NIRCam and Keck NIRC-2 (2022-11-04,06 annotated).png |caption2=Clouds (Nov 6, 2022) |width2= |footer= }}

{{See also|Life on Titan}}Similar to the hydrological cycle on Earth, Titan features a methane cycle.{{Cite journal |last1=Lunine |first1=Jonathan I. |last2=Atreya |first2=Sushil K. |date=March 2008 |title=The methane cycle on Titan |url=http://www.nature.com/articles/ngeo125 |journal=Nature Geoscience |language=en |volume=1 |issue=3 |pages=159–164 |doi=10.1038/ngeo125 |bibcode=2008NatGe...1..159L |issn=1752-0894}}{{Cite journal |last1=MacKenzie |first1=Shannon M. |last2=Birch |first2=Samuel P. D. |last3=Hörst |first3=Sarah |last4=Sotin |first4=Christophe |last5=Barth |first5=Erika |last6=Lora |first6=Juan M. |last7=Trainer |first7=Melissa G. |last8=Corlies |first8=Paul |last9=Malaska |first9=Michael J. |last10=Sciamma-O’Brien |first10=Ella |last11=Thelen |first11=Alexander E. |date=2021-06-01 |title=Titan: Earth-like on the Outside, Ocean World on the Inside |journal=The Planetary Science Journal |volume=2 |issue=3 |pages=112 |doi=10.3847/PSJ/abf7c9 |arxiv=2102.08472 |bibcode=2021PSJ.....2..112M |s2cid=231942648 |issn=2632-3338 |doi-access=free }} This methane cycle results in surface formations that resemble formations we find on Earth. Lakes of methane and ethane are found across Titan's polar regions. Methane condenses into clouds in the atmosphere, and then precipitates onto the surface. This liquid methane then flows into the lakes. Some of the methane in the lakes will evaporate over time, and form clouds in the atmosphere again, starting the process over. However, since methane is lost in the thermosphere, there has to be a source of methane to replenish atmospheric methane. Energy from the Sun should have converted all traces of methane in Titan's atmosphere into more complex hydrocarbons within 50 million years — a short time compared to the age of the Solar System. This suggests that methane must be somehow replenished by a reservoir on or within Titan itself. Most of the methane on Titan is in the atmosphere. Methane is transported through the cold trap at the tropopause.{{Cite journal |title=Titan's Methane Weather |journal = Annual Review of Earth and Planetary Sciences|volume = 40|pages = 355–382|last=Roe |first=Henry G. |date=2012-05-02 |issue = 1|language=en |doi=10.1146/annurev-earth-040809-152548 |bibcode = 2012AREPS..40..355R}} Therefore, the circulation of methane in the atmosphere influences the radiation balance and chemistry of other layers in the atmosphere. If there is a reservoir of methane on Titan, the cycle would only be stable over geologic timescales.

File:Titan-SaturnMoon-Maps-TraceGases-20141022.jpg organic gases in Titan's atmosphere—HNC (left) and HC₃N (right).]]

Evidence that Titan's atmosphere contains over a thousand times more methane than carbon monoxide would appear to rule out significant contributions from cometary impacts, because comets are composed of more carbon monoxide than methane. That Titan might have accreted an atmosphere from the early Saturnian nebula at the time of formation also seems unlikely; in such a case, it ought to have atmospheric abundances similar to the solar nebula, including hydrogen and neon.{{cite journal|author=Coustenis, A.|year=2005|title=Formation and evolution of Titan's atmosphere|journal=Space Science Reviews|volume=116|issue=1–2|pages=171–184|bibcode=2005SSRv..116..171C|doi=10.1007/s11214-005-1954-2|s2cid=121298964}} Many astronomers have suggested that the ultimate origin for the methane in Titan's atmosphere is from within Titan itself, released via eruptions from cryovolcanoes.{{cite journal|author=Sushil K. Atreya|author2=Elena Y. Adams|author3=Hasso B. Niemann|display-authors=etal|date=October 2006|title=Titan's methane cycle|journal=Planetary and Space Science|volume=54|issue=12|page=1177|bibcode=2006P&SS...54.1177A|doi=10.1016/j.pss.2006.05.028}}{{cite journal|author=Stofan, E. R.|display-authors=etal|year=2007|title=The lakes of Titan.|journal=Nature|volume=445|issue=7123|pages=61–4|bibcode=2007Natur.445...61S|doi=10.1038/nature05438|pmid=17203056|s2cid=4370622}}{{cite journal|author1=Tobie, Gabriel|author2=Lunine, Jonathan|author3=Sotin, Cristophe|name-list-style=amp|year=2006|title=Episodic outgassing as the origin of atmospheric methane on Titan|journal=Nature|volume=440|issue=7080|pages=61–64|bibcode=2006Natur.440...61T|doi=10.1038/nature04497|pmid=16511489|s2cid=4335141}}

Another possible source for methane replenishment in Titan's atmosphere is methane clathrates.{{cite journal |author1=Tobie, Gabriel |author2=Lunine, Jonathan |author3=Sotin, Cristophe |name-list-style=amp |year=2006 |title=Episodic outgassing as the origin of atmospheric methane on Titan |journal=Nature |volume=440 |issue=7080 |pages=61–64 |bibcode=2006Natur.440...61T |doi=10.1038/nature04497 |pmid=16511489 |s2cid=4335141}} Clathrates are compounds in which an ice lattice surrounds a gas particle, much like a cage. In this case, methane gas is surrounded by a water crystal cage.{{Cite journal |last1=Choukroun |first1=Mathieu |last2=Grasset |first2=Olivier |last3=Tobie |first3=Gabriel |last4=Sotin |first4=Christophe |date=February 2010 |title=Stability of methane clathrate hydrates under pressure: Influence on outgassing processes of methane on Titan |url=https://linkinghub.elsevier.com/retrieve/pii/S0019103509003509 |journal=Icarus |language=en |volume=205 |issue=2 |pages=581–593 |doi=10.1016/j.icarus.2009.08.011|bibcode=2010Icar..205..581C }} These methane clathrates could be present underneath Titan's icy surface, having formed much earlier in Titan's history.{{Cite journal |last1=Maynard-Casely |first1=Helen E. |author-link=Helen Maynard-Casely |last2=Cable |first2=Morgan L. |last3=Malaska |first3=Michael J. |last4=Vu |first4=Tuan H. |last5=Choukroun |first5=Mathieu |last6=Hodyss |first6=Robert |date=2018-03-01 |title=Prospects for mineralogy on Titan |url=https://www.degruyter.com/document/doi/10.2138/am-2018-6259/html |journal=American Mineralogist |language=en |volume=103 |issue=3 |pages=343–349 |doi=10.2138/am-2018-6259 |bibcode=2018AmMin.103..343M |s2cid=104278344 |issn=0003-004X|doi-access=free }} Through the dissociation of methane clathrates, methane could be outgassed into the atmosphere, replenishing the supply.

On December 1, 2022, astronomers reported viewing clouds, likely made of methane, moving across Titan, using the James Webb Space Telescope.{{cite news |last=Bartels |first=Meghan |title=James Webb Space Telescope view of Saturn's weirdest moon Titan thrills scientists |url=https://www.space.com/james-webb-space-telescope-saturn-moon-titan |date=December 1, 2022 |work=Space.com |accessdate=December 2, 2022 }}{{cite news |last=Overbye |first=Dennis |authorlink=Dennis Overbye |title=Telescopes Team Up to Forecast an Alien Storm on Titan - Saturn's largest moon came under the gaze of NASA's powerful Webb space observatory, allowing it and another telescope to capture clouds drifting through Titan's methane-rich atmosphere. |url=https://www.nytimes.com/2022/12/05/science/titan-webb-telescope-pictures.html |date=December 5, 2022 |work=The New York Times |accessdate=December 6, 2022 }}

{{wide image|Titan-Earth-PolarClouds-20141024.jpg|600px|align-cap=center|Polar clouds, made of methane, on Titan (left) compared with polar clouds on Earth (right).}}

File:Visible and Infrared Sunsets on Titan .gif models{{Cite journal|last1=Barnes|first1=Jason W.|last2=MacKenzie|first2=Shannon M.|last3=Lorenz|first3=Ralph D.|last4=Turtle|first4=Elizabeth P.|date=2018-11-02|title=Titan's Twilight and Sunset Solar Illumination|journal=The Astronomical Journal|language=en|volume=156|issue=5|pages=247|doi=10.3847/1538-3881/aae519|bibcode=2018AJ....156..247B|s2cid=125886785 |issn=1538-3881|doi-access=free}} of a sunny day on Titan. The Sun is seen setting from noon until after dusk at 3 wavelengths: 5 μm, near-infrared (1-2 μm), and visible. Each image shows a "rolled-out" version of the sky as seen from the surface of Titan. The left side shows the Sun, while the right side points away from the Sun. The top and bottom of the image are the zenith and the horizon respectively. The solar zenith angle represents the angle between the Sun and zenith (0°), where 90° is when the Sun reaches the horizon.]]

File:Saturnset on Titan.gif setting behind Titan.]]

Sky brightness and viewing conditions are expected to be quite different from Earth and Mars due to Titan's farther distance from the Sun (~10 AU) and complex haze layers in its atmosphere. The sky brightness model videos show what a typical sunny day may look like standing on the surface of Titan based on radiative transfer models.

For astronauts who see with visible light, the daytime sky has a distinctly dark orange color and appears uniform in all directions due to significant Mie scattering from the many high-altitude haze layers. The daytime sky is calculated to be ~100–1000 times dimmer than an afternoon on Earth, which is similar to the viewing conditions of a thick smog or dense fire smoke. The sunsets on Titan are expected to be "underwhelming events", where the Sun disappears about half-way up in the sky (~50° above the horizon) with no distinct change in color. After that, the sky will slowly darken until it reaches night. However, the surface is expected to remain as bright as the full Moon up to 1 Earth day after sunset.

In near-infrared light, the sunsets resemble a Martian sunset or dusty desert sunset. Mie scattering has a weaker influence at longer infrared wavelengths, allowing for more colorful and variable sky conditions. During the daytime, the Sun has a noticeable solar corona that transitions color from white to "red" over the afternoon. The afternoon sky brightness is ~100 times dimmer than Earth. As evening time approaches, the Sun is expected to disappear fairly close to the horizon. Titan's atmospheric optical depth is the lowest at 5 microns.{{Cite journal|last1=Sotin|first1=C.|last2=Lawrence|first2=K. J.|last3=Reinhardt|first3=B.|last4=Barnes|first4=J. W.|last5=Brown|first5=R. H.|last6=Hayes|first6=A. G.|last7=Le Mouélic|first7=S.|last8=Rodriguez|first8=S.|last9=Soderblom|first9=J. M.|last10=Soderblom|first10=L. A.|last11=Baines|first11=K. H.|date=2012-11-01|title=Observations of Titan's Northern lakes at 5μm: Implications for the organic cycle and geology|url=http://www.sciencedirect.com/science/article/pii/S0019103512003363|journal=Icarus|language=en|volume=221|issue=2|pages=768–786|doi=10.1016/j.icarus.2012.08.017|bibcode=2012Icar..221..768S|issn=0019-1035}} So, the Sun at 5 microns may even be visible when it is below the horizon due to atmospheric refraction. Similar to images of Martian sunsets from Mars rovers, a fan-like corona is seen to develop above the Sun due to scattering from haze or dust at high-altitudes.

In regards to Saturn, the planet is nearly fixed in its position in the sky because Titan's orbit is tidally locked around Saturn. However, there is a small 3° east-to-west motion over a Titan year due to the orbital eccentricity,{{Cite book |last=Lorenz |first=Ralph |url=https://books.google.com/books?id=xyNRygEACAAJ |title=Saturn's Moon Titan: From 4. 5 Billion Years Ago to the Present – an Insight Into the Workings and Exploration of the Most Earth-Like World in the Outer Solar System |date= |publisher=Haynes Publishing Group P.L.C. |year=2020 |isbn=978-1-78521-643-5 |pages=130–131 |language=en |access-date=30 November 2020}} similar to the analemma on Earth. Sunlight reflected off of Saturn, Saturnshine, is about 1000 times weaker than solar insolation on the surface of Titan. Even though Saturn appears several times bigger in the sky than the Moon in Earth's sky, the outline of Saturn is masked out by the brighter Sun during the daytime. Saturn may become discernible at night, but only at a wavelength of 5 microns. This is due to two factors: the small optical depth of Titan's atmosphere at 5 microns{{Cite journal|last1=Barnes|first1=Jason W.|last2=Clark|first2=Roger N.|last3=Sotin|first3=Christophe|last4=Ádámkovics|first4=Máté|last5=Appéré|first5=Thomas|last6=Rodriguez|first6=Sebastien|last7=Soderblom|first7=Jason M.|last8=Brown|first8=Robert H.|last9=Buratti|first9=Bonnie J.|last10=Baines|first10=Kevin H.|last11=Le Mouélic|first11=Stéphane|title=A Transmission Spectrum of Titan's North Polar Atmosphere from a Specular Reflection of the Sun|date=2013-10-24|url=https://iopscience.iop.org/article/10.1088/0004-637X/777/2/161|journal=The Astrophysical Journal|volume=777|issue=2|pages=161|doi=10.1088/0004-637X/777/2/161|bibcode=2013ApJ...777..161B|issn=0004-637X|hdl=1721.1/94552|s2cid=16929531 |hdl-access=free}} and the strong 5 μm emissions from Saturn's night side.{{Cite journal|last1=BAINES|first1=K. H.|last2=DROSSART|first2=P.|last3=MOMARY|first3=T. W.|last4=FORMISANO|first4=V.|last5=GRIFFITH|first5=C.|last6=BELLUCCI|first6=G.|last7=BIBRING|first7=J. P.|last8=BROWN|first8=R. H.|last9=BURATTI|first9=B. J.|last10=CAPACCIONI|first10=F.|last11=CERRONI|first11=P.|title=The Atmospheres of Saturn and Titan in the Near-Infrared: First Results of Cassini/Vims|date=2005-06-01|url=https://doi.org/10.1007/s11038-005-9058-2|journal=Earth, Moon, and Planets|language=en|volume=96|issue=3|pages=119–147|doi=10.1007/s11038-005-9058-2|bibcode=2005EM&P...96..119B|s2cid=53480412|issn=1573-0794}} In visible light, Saturn will make the sky on Titan's Saturn-facing side appear slightly brighter, similar to an overcast night with a full moon on Earth. Saturn's rings are hidden from view owing to the alignment of Titan's orbital plane and the plane of the rings. Saturn is expected to show phases, akin to the phases of Venus on Earth, that partially illuminate the surface of Titan at night, except for eclipses.

From outer space, Cassini images from near-infrared to UV wavelengths have shown that the twilight periods (phase angles > 150°) are brighter than the daytime on Titan.{{Cite journal|last1=García Muñoz|first1=A.|last2=Lavvas|first2=P.|last3=West|first3=R. A.|date=2017-04-24|title=Titan brighter at twilight than in daylight|url=https://www.nature.com/articles/s41550-017-0114|journal=Nature Astronomy|language=en|volume=1|issue=5|page=0114|doi=10.1038/s41550-017-0114|arxiv=1704.07460|bibcode=2017NatAs...1E.114G|s2cid=119491241|issn=2397-3366}} This observation has not been observed on any other planetary body with a thick atmosphere. The Titanean twilight outshining the dayside is due to a combination of Titan's atmosphere extending hundreds of kilometers above the surface and intense [http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c3 forward Mie scattering] from the haze. Radiative transfer models have not reproduced this effect.

The temperature of Titan is increased over the blackbody temperature by a strong greenhouse effect caused by infrared absorption by pressure-induced opacity of Titan's atmosphere, but the greenhouse warming is somewhat reduced by an effect tagged by Pollack the anti-greenhouse effect,{{cite journal | url=https://www.science.org/doi/10.1126/science.11538492 | doi=10.1126/science.11538492 | title=The Greenhouse and Antigreenhouse Effects on Titan | date=1991 | last1=McKay | first1=Christopher P. | last2=Pollack | first2=James B. | last3=Courtin | first3=Régis | journal=Science | volume=253 | issue=5024 | pages=1118–1121 | bibcode=1991Sci...253.1118M }}"[https://www.spacedaily.com/news/saturn-titan-05zi.html Titan: Greenhouse And Anti-Greenhouse]," ''Space Daily, Nov 4, 2005. Retrieved 13 August 2024. absorbing some incoming solar energy before it can reach the surface, leading to cooler surface temperatures than if methane were less abundant. The greenhouse effect increases surface temperature by 21 K, while the anti-greenhouse takes away half this effect, reducing this to an increase of 12 K.

The persistence of a dense atmosphere on Titan has been enigmatic as the atmospheres of the structurally similar satellites of Jupiter, Ganymede and Callisto, are negligible. Although the disparity is still poorly understood, data from recent missions have provided basic constraints on the evolution of Titan's atmosphere.

File:Titan's Many Layers.jpg spacecraft]]

Roughly speaking, at the distance of Saturn, solar insolation and solar wind flux are sufficiently low that elements and compounds that are volatile on the terrestrial planets tend to accumulate in all three phases.{{cite journal

|author=P.A. Bland

|year=2005

|title=Trace element carrier phases in primitive chondrite matrix: implications for volatile element fractionation in the inner solar system

|url=http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1841.pdf

|journal=Lunar and Planetary Science

|volume=XXXVI |page=1841

|bibcode=2005LPI....36.1841B

|display-authors=etal}} Titan's surface temperature is also quite low, about 94 K (–179 C/–290 F).{{cite journal

|author=F.M. Flasar

|s2cid=31833954

|year=2005

|title=Titan's atmospheric temperatures, winds, and composition

|journal=Science

|volume=308 |issue=5724 |pages=975–978

|doi=10.1126/science.1111150

|pmid=15894528

|bibcode = 2005Sci...308..975F |display-authors=etal}}{{cite journal

|author=G. Lindal

|year=1983

|title=The atmosphere of Titan: An analysis of the Voyager 1 radio occultation measurements

|journal=Icarus

|volume=53

|issue=2 |pages=348–363

|doi=10.1016/0019-1035(83)90155-0

|bibcode=1983Icar...53..348L

|display-authors=etal}} Consequently, the mass fractions of substances that can become atmospheric constituents are much larger on Titan than on Earth. In fact, current interpretations suggest that only about 50% of Titan's mass is silicates,{{cite journal

|year=2006

|author1=G. Tobie |author2=J.I. Lunine |author3=C. Sotin |title=Episodic outgassing as the origin of atmospheric methane on Titan

|journal=Nature

|volume=440 |issue=7080 |pages=61–64

|doi=10.1038/nature04497

|pmid=16511489

|bibcode = 2006Natur.440...61T |s2cid=4335141 }} with the rest consisting primarily of various H₂O (water) ices and NH₃·H₂O (ammonia hydrates). NH₃, which may be the original source of Titan's atmospheric N₂ (dinitrogen), may constitute as much as 8% of the NH₃·H₂O mass. Titan is most likely differentiated into layers, where the liquid water layer beneath ice I_h may be rich in NH₃.{{technical inline|date=January 2014}}

File:Titan-Complex 'Anti-greenhouse'.jpg

{{multiple image

| align = left

| image1 = PIA 08211 Titan backlit.jpg

| width1 = 160

| alt1 =

| caption1 = Titan's atmosphere backlit by the Sun, with Saturn's rings behind. An outer haze layer merges at top with the northern polar hood.

| image2 = Two Halves of Titan.png

| width2 = 120

| alt2 =

| caption2 = Titan's winter hemisphere (top) is slightly darker in visible light due to a high-altitude haze

| footer =

}}

Tentative constraints are available, with the current loss mostly due to low gravity{{cite journal

|author=J.H. Waite (Jr)

|s2cid=20551849

|year=2005

|title=Ion neutral mass spectrometer results from the first flyby of Titan

|journal=Science

|volume=308 |issue=5724 |pages=982–986

|doi=10.1126/science.1110652

|pmid=15890873

|bibcode = 2005Sci...308..982W |display-authors=etal}} and solar wind{{cite journal

|author1=T. Penz |author2=H. Lammer |author3=Yu.N. Kulikov |author4=H.K. Biernat |year=2005

|title=The influence of the solar particle and radiation environment on Titan's atmosphere evolution

|journal=Advances in Space Research

|volume=36

|issue=2 |pages=241–250

|doi=10.1016/j.asr.2005.03.043

|bibcode=2005AdSpR..36..241P

}} aided by photolysis. The loss of Titan's early atmosphere can be estimated with the ¹⁴N–¹⁵N isotopic ratio, because the lighter ¹⁴N is preferentially lost from the upper atmosphere under photolysis and heating. Because Titan's original ¹⁴N–¹⁵N ratio is poorly constrained, the early atmosphere may have had more N₂ by factors ranging from 1.5 to 100 with certainty only in the lower factor. Because N₂ is the primary component (98%) of Titan's atmosphere,{{cite journal

|author=A. Coustenis

|year=2005

|title=Formation and Evolution of Titan's Atmosphere

|journal=Space Science Reviews

|volume=116

|issue=1–2 |pages=171–184

|doi=10.1007/s11214-005-1954-2

|bibcode = 2005SSRv..116..171C |s2cid=121298964

}} the isotopic ratio suggests that much of the atmosphere has been lost over geologic time. Nevertheless, atmospheric pressure on its surface remains nearly 1.5 times that of Earth as it began with a proportionally greater volatile budget than Earth or Mars. It is possible that most of the atmospheric loss was within 50 million years of accretion, from a highly energetic escape of light atoms carrying away a large portion of the atmosphere (hydrodynamic escape). Such an event could be driven by heating and photolysis effects of the early Sun's higher output of X-ray and ultraviolet (XUV) photons.

Because Callisto and Ganymede are structurally similar to Titan, it is unclear why their atmospheres are insignificant relative to Titan's. Nevertheless, the origin of Titan's N₂ via geologically ancient photolysis of accreted and degassed NH₃, as opposed to degassing of N₂ from accretionary clathrates, may be the key to a correct inference. Had N₂ been released from clathrates, ³⁶Ar and ³⁸Ar that are inert primordial isotopes of the Solar System should also be present in the atmosphere, but neither has been detected in significant quantities.{{cite journal

|author=H.B. Niemann

|year=2005

|title=The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe

|journal=Nature

|volume=438 |issue=7069 |pages=779–784

|doi=10.1038/nature04122

|pmid=16319830

|bibcode = 2005Natur.438..779N |display-authors=etal|hdl=2027.42/62703

|s2cid=4344046

|url=https://deepblue.lib.umich.edu/bitstream/2027.42/62703/1/nature04122.pdf

|hdl-access=free

}} The insignificant concentration of ³⁶Ar and ³⁸Ar also indicates that the ~40 K temperature required to trap them and N₂ in clathrates did not exist in the Saturnian sub-nebula. Instead, the temperature may have been higher than 75 K, limiting even the accumulation of NH₃ as hydrates.{{cite journal

|author1=T.C. Owen |author2=H. Niemann |author3=S. Atreya |author4=M.Y. Zolotov |title=Between heaven and Earth: the exploration of Titan

|year=2006

|journal=Faraday Discussions

|volume=133

|pmid=17191458 |pages=387–391

|doi=10.1039/b517174a

|bibcode = 2006FaDi..133..387O |citeseerx=10.1.1.610.9932}} Temperatures would have been even higher in the Jovian sub-nebula due to the greater gravitational potential energy release, mass, and proximity to the Sun, greatly reducing the NH₃ inventory accreted by Callisto and Ganymede. The resulting N₂ atmospheres may have been too thin to survive the atmospheric erosion effects that Titan has withstood.

An alternative explanation is that cometary impacts release more energy on Callisto and Ganymede than they do at Titan due to the higher gravitational field of Jupiter. That could erode the atmospheres of Callisto and Ganymede, whereas the cometary material would actually build Titan's atmosphere. However, the ²H–¹H (i.e. D–H) ratio of Titan's atmosphere is {{val|2.3|0.5|e=-4}}, nearly 1.5 times lower than that of comets. The difference suggests that cometary material is unlikely to be the major contributor to Titan's atmosphere.{{Cite journal|last1=Bockelée-Morvan|first1=Dominique|author1-link=Dominique Bockelée-Morvan|last2=Calmonte|first2=Ursina|last3=Charnley|first3=Steven|last4=Duprat|first4=Jean|last5=Engrand|first5=Cécile|last6=Gicquel|first6=Adeline|last7=Hässig|first7=Myrtha|last8=Jehin|first8=Emmanuël|last9=Kawakita|first9=Hideyo|date=2015-12-01|title=Cometary Isotopic Measurements|journal=Space Science Reviews|language=en|volume=197|issue=1|pages=47–83|doi=10.1007/s11214-015-0156-9|bibcode=2015SSRv..197...47B|s2cid=53457957|issn=1572-9672|url=https://research.chalmers.se/en/publication/223477}} Titan's atmosphere also contains over a thousand times more methane than carbon monoxide which supports the idea that cometary material is not a likely contributor since comets are composed of more carbon monoxide than methane.File:PIA22484-SaturnMoon-Titan-3DustStorms-20180924.jpg |access-date=24 September 2018 }}]]

{{Clear}}

• Climate of Titan
• Atmosphere of Venus
• Atmosphere of Earth
• Atmosphere of Mars

{{Reflist|2}}

• {{Cite journal | last1 = Roe | first1 = H. G. | doi = 10.1146/annurev-earth-040809-152548 | title = Titan's Methane Weather | journal = Annual Review of Earth and Planetary Sciences | volume = 40 | issue = 1 | pages = 355–382 | year = 2012 |bibcode = 2012AREPS..40..355R }}

• {{Commons category-inline|Atmosphere of Titan}}

{{Titan}}

{{Atmospheres}}

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Category:Titan (moon)

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