Atmosphere of Mars#Vertical structure

{{See also|Atmospheric escape|Water on Mars}}The mass and composition of the Martian atmosphere are thought to have changed over the course of the planet's lifetime. A thicker, warmer and wetter atmosphere is required to explain several apparent features in the earlier history of Mars, such as the existence of liquid water bodies. Observations of the Martian upper atmosphere, measurements of isotopic composition and analyses of Martian meteorites, provide evidence of the long-term changes of the atmosphere and constraints for the relative importance of different processes.

In general, the gases found on modern Mars are depleted in lighter stable isotopes, indicating the Martian atmosphere has changed by some mass-selected processes over its history. Scientists often rely on these measurements of isotope composition to reconstruct conditions of the Martian atmosphere in the past.{{Cite web |url=http://www.jpl.nasa.gov/news/news.php?feature=4532 |title=Curiosity Sniffs Out History of Martian Atmosphere |website=NASA/JPL |access-date=2019-06-11 |archive-date=28 July 2020 |archive-url=https://web.archive.org/web/20200728150712/https://www.jpl.nasa.gov/news/news.php?feature=4532 |url-status=live }}{{Cite web |url=https://mars.nasa.gov/news/1976/nasas-maven-reveals-most-of-mars-atmosphere-was-lost-to-space |title=NASA's MAVEN reveals most of Mars' atmosphere was lost to space |last=mars.nasa.gov |website=NASA's Mars Exploration Program |access-date=2019-06-11 |archive-date=17 August 2020 |archive-url=https://web.archive.org/web/20200817130320/https://mars.nasa.gov/news/1976/nasas-maven-reveals-most-of-mars-atmosphere-was-lost-to-space/ |url-status=live }}{{cite magazine |first1=David C. |last1=Catling |first2=Kevin J. |last2=Zahnle |url=http://faculty.washington.edu/dcatling/Catling2009_SciAm.pdf |title=The planetary air leak |magazine=Scientific American |date=May 2009 |page=26 |access-date=10 June 2019 |archive-date=26 October 2020 |archive-url=https://web.archive.org/web/20201026095634/http://faculty.washington.edu/dcatling/Catling2009_SciAm.pdf |url-status=live }}

While Mars and Earth have similar ¹²C / ¹³C and ¹⁶O / ¹⁸O ratios, ¹⁴N is much more depleted in the Martian atmosphere. It is thought that the photochemical escape processes are responsible for the isotopic fractionation and has caused a significant loss of nitrogen on geological timescales. Estimates suggest that the initial partial pressure of N₂ may have been up to 30 hPa.{{cite journal |author1=McElroy, Michael B. |author2=Yung, Yuk Ling |author3=Nier, Alfred O. |s2cid=34066697 |date=1 Oct 1976 |title=Isotopic Composition of Nitrogen: Implications for the Past History of Mars' Atmosphere |journal=Science |volume=194 |issue=4260 |pages=70–72 |doi=10.1126/science.194.4260.70 |pmid=17793081 |bibcode=1976Sci...194...70M}}

Hydrodynamic escape in the early history of Mars may explain the isotopic fractionation of argon and xenon. On modern Mars, the atmosphere is not leaking these two noble gases to outer space owing to their heavier mass. However, the higher abundance of hydrogen in the Martian atmosphere and the high fluxes of extreme UV from the young Sun, together could have driven a hydrodynamic outflow and dragged away these heavy gases.{{cite journal |last1=Hunten |first1=Donald M. |last2=Pepin |first2=Robert O. |last3=Walker |first3=James C.G. |date=1987-03-01 |title=Mass fractionation in hydrodynamic escape |journal=Icarus |volume=69 |issue=3 |pages=532–549 |doi=10.1016/0019-1035(87)90022-4 |issn=0019-1035 |hdl=2027.42/26796 |bibcode=1987Icar...69..532H |hdl-access=free}}{{Cite journal |last1=Hans Keppler |last2=Shcheka |first2=Svyatoslav S. |date=October 2012 |title=The origin of the terrestrial noble-gas signature |journal=Nature |volume=490 |issue=7421 |pages=531–534 |doi=10.1038/nature11506 |pmid=23051754 |issn=1476-4687 |bibcode=2012Natur.490..531S |s2cid=205230813}} Hydrodynamic escape also contributed to the loss of carbon, and models suggest that it is possible to lose {{cvt|1|bar|hPa|disp=flip}} of CO₂ by hydrodynamic escape in one to ten million years under much stronger solar extreme UV on Mars.{{cite journal |last1=Tian |first1=Feng |last2=Kasting |first2=James F. |last3=Solomon |first3=Stanley C. |date=2009 |title=Thermal escape of carbon from the early Martian atmosphere |journal=Geophysical Research Letters |volume=36 |issue=2 |pages=n/a |doi=10.1029/2008GL036513 |bibcode=2009GeoRL..36.2205T |s2cid=129208608 |issn=1944-8007|doi-access=free }} Meanwhile, more recent observations made by the MAVEN orbiter suggested that sputtering escape is very important for the escape of heavy gases on the nightside of Mars and could have contributed to 65% loss of argon in the history of Mars.{{cite journal |last1=Jakosky |first1=B.M. |last2=Slipski |first2=M. |last3=Benna |first3=M. |last4=Mahaffy |first4=P. |last5=Elrod |first5=M. |last6=Yelle |first6=R. |last7=Stone |first7=S. |last8=Alsaeed |first8=N. |date=2017-03-31 |title=Mars' atmospheric history derived from upper-atmosphere measurements of {{sup|38}}Ar / {{sup|36}}Ar |journal=Science |volume=355 |issue=6332|pages=1408–1410 |doi=10.1126/science.aai7721 |pmid=28360326 |issn=0036-8075 |bibcode=2017Sci...355.1408J |doi-access=free}}

The Martian atmosphere is particularly prone to impact erosion owing to the low escape velocity of Mars. An early computer model suggested that Mars could have lost 99% of its initial atmosphere by the end of late heavy bombardment period based on a hypothetical bombardment flux estimated from lunar crater density.{{Cite journal |first1=A.M. |last1=Vickery |last2=Melosh |first2=H.J. |date=April 1989 |title=Impact erosion of the primordial atmosphere of Mars |journal=Nature |volume=338 |issue=6215 |pages=487–489 |doi=10.1038/338487a0 |pmid=11536608 |issn=1476-4687 |bibcode=1989Natur.338..487M |s2cid=4285528}} In terms of relative abundance of carbon, the {{nowrap|C / {{sup|84}}Kr}} ratio on Mars is only 10% of that on Earth and Venus. Assuming the three rocky planets have the same initial volatile inventory, then this low {{nowrap|C / {{sup|84}}Kr}} ratio implies the mass of CO₂ in the early Martian atmosphere should have been ten times higher than the present value.{{Cite journal |last1=Owen |first1=Tobias |last2=Bar-Nun |first2=Akiva |date=1995-08-01 |title=Comets, impacts, and atmospheres |journal=Icarus |volume=116 |issue=2 |pages=215–226 |issn=0019-1035 |doi=10.1006/icar.1995.1122 |pmid=11539473 |bibcode=1995Icar..116..215O}} The huge enrichment of radiogenic ⁴⁰Ar over primordial ³⁶Ar is also consistent with the impact erosion theory.

One of the ways to estimate the amount of water lost by hydrogen escape in the upper atmosphere is to examine the enrichment of deuterium over hydrogen. Isotope-based studies estimate that 12 m to over 30 m global equivalent layer of water has been lost to space via hydrogen escape in Mars's history.{{Cite journal |last=Krasnopolsky |first=Vladimir A. |date=2002 |title=Mars' upper atmosphere and ionosphere at low, medium, and high solar activities: Implications for evolution of water |journal=Journal of Geophysical Research: Planets |volume=107 |issue=E12 |pages=11‑1–11‑11 |doi=10.1029/2001JE001809 |doi-access=free |issn=2156-2202 |bibcode=2002JGRE..107.5128K}} It is noted that atmospheric-escape-based approach only provides the lower limit for the estimated early water inventory.

To explain the coexistence of liquid water and faint young Sun during early Mars's history, a much stronger greenhouse effect must have occurred in the Martian atmosphere to warm the surface up above freezing point of water. Carl Sagan first proposed that a 1 bar H₂ atmosphere can produce enough warming for Mars.{{Cite journal |last=Sagan |first=Carl |date=September 1977 |title=Reducing greenhouses and the temperature history of Earth and Mars |journal=Nature |volume=269 |issue=5625 |pages=224–226 |doi=10.1038/269224a0 |issn=1476-4687 |bibcode=1977Natur.269..224S |s2cid=4216277}} The hydrogen can be produced by the vigorous outgassing from a highly reduced early Martian mantle and the presence of CO₂ and water vapor can lower the required abundance of H{{sub|2}} to generate such a greenhouse effect.{{Cite journal |last1=Kasting |first1=James F. |last2=Freedman |first2=Richard |first3=Tyler D. |last3=Robinson |last4=Zugger |first4=Michael E. |last5=Kopparapu |first5=Ravi |last6=Ramirez |first6=Ramses M. |date=January 2014 |title=Warming early Mars with CO{{sub|2}} and H{{sub|2}} |journal=Nature Geoscience |volume=7 |issue=1 |pages=59–63 |doi=10.1038/ngeo2000 |issn=1752-0908 |arxiv=1405.6701 |bibcode=2014NatGe...7...59R |s2cid=118520121}} Nevertheless, photochemical modeling showed that maintaining an atmosphere with this high level of H₂ is difficult.{{cite journal |last1=Batalha |first1=Natasha |last2=Domagal-Goldman |first2=Shawn D. |last3=Ramirez |first3=Ramses |last4=Kasting |first4=James F. |date=2015-09-15 |title=Testing the early Mars H{{sub|2}}–CO{{sub|2}} greenhouse hypothesis with a 1-D photochemical model |journal=Icarus |volume=258 |pages=337–349 |doi=10.1016/j.icarus.2015.06.016 |issn=0019-1035|bibcode=2015Icar..258..337B |arxiv=1507.02569 |s2cid=118359789}} SO₂ has also been one of the proposed effective greenhouse gases in the early history of Mars.{{cite journal |last1=Johnson |first1=Sarah Stewart |last2=Mischna |first2=Michael A. |last3=Grove |first3=Timothy L. |last4=Zuber |first4=Maria T. |s2cid=7525497 |date=2008-08-08 |title=Sulfur-induced greenhouse warming on early Mars |journal=Journal of Geophysical Research |volume=113 |issue=E8 |pages=E08005 |doi=10.1029/2007JE002962 |issn=0148-0227 |bibcode=2008JGRE..113.8005J|doi-access=free }}{{Cite journal |last1=Schrag |first1=Daniel P. |last2=Zuber |first2=Maria T. |last3=Halevy |first3=Itay |s2cid=7246517 |date=2007-12-21 |title=A sulfur dioxide climate feedback on early Mars |journal=Science |volume=318 |issue=5858 |pages=1903–1907 |doi=10.1126/science.1147039 |issn=0036-8075 |pmid=18096802 |bibcode=2007Sci...318.1903H}}{{Cite web |url=https://phys.org/news/2007-12-sulfur-dioxide-early-mars.html |title=Sulfur dioxide may have helped maintain a warm early Mars |website=phys.org |access-date=2019-06-08 |archive-date=8 June 2019 |archive-url=https://web.archive.org/web/20190608052030/https://phys.org/news/2007-12-sulfur-dioxide-early-mars.html |url-status=live }} However, other studies suggested that high solubility of SO₂, efficient formation of H₂SO₄ aerosol and surface deposition prohibit the long-term build-up of SO₂ in the Martian atmosphere, and hence reduce the potential warming effect of SO₂.

Despite the lower gravity, Jeans escape is not efficient in the modern Martian atmosphere due to the relatively low temperature at the exobase (≈200 K at 200 km altitude). It can only explain the escape of hydrogen from Mars. Other non-thermal processes are needed to explain the observed escape of oxygen, carbon and nitrogen.

Molecular hydrogen (H₂) is produced from the dissociation of H₂O or other hydrogen-containing compounds in the lower atmosphere and diffuses to the exosphere. The exospheric H₂ then decomposes into hydrogen atoms, and the atoms that have sufficient thermal energy can escape from the gravitation of Mars (Jeans escape). The escape of atomic hydrogen is evident from the UV spectrometers on different orbiters.{{Cite journal|last=Anderson|first=Donald E.|date=1974|title=Mariner 6, 7, and 9 Ultraviolet Spectrometer Experiment: Analysis of hydrogen Lyman alpha data|journal=Journal of Geophysical Research|volume=79|issue=10|pages=1513–1518|doi=10.1029/JA079i010p01513|issn=2156-2202|bibcode=1974JGR....79.1513A}}{{Cite journal|last1=Chaufray|first1=J.Y.|last2=Bertaux|first2=J.L.|last3=Leblanc|first3=F.|last4=Quémerais|first4=E.|date=June 2008|title=Observation of the hydrogen corona with SPICAM on Mars Express|journal=Icarus|volume=195|issue=2|pages=598–613|doi=10.1016/j.icarus.2008.01.009|bibcode=2008Icar..195..598C}} While most studies suggested that the escape of hydrogen is close to diffusion-limited on Mars,{{Cite journal|last=Hunten|first=Donald M.|date=November 1973|title=The Escape of Light Gases from Planetary Atmospheres|journal=Journal of the Atmospheric Sciences|volume=30|issue=8|pages=1481–1494|doi=10.1175/1520-0469(1973)030<1481:TEOLGF>2.0.CO;2|issn=0022-4928|bibcode=1973JAtS...30.1481H|doi-access=free}}{{Cite journal|last1=Zahnle|first1=Kevin|last2=Haberle|first2=Robert M.|last3=Catling|first3=David C.|last4=Kasting|first4=James F.|s2cid=2199349|date=2008|title=Photochemical instability of the ancient Martian atmosphere|journal=Journal of Geophysical Research: Planets|volume=113|issue=E11|pages=E11004|doi=10.1029/2008JE003160|issn=2156-2202|bibcode=2008JGRE..11311004Z|doi-access=free}} more recent studies suggest that the escape rate is modulated by dust storms and has a large seasonality.{{Cite journal|last1=Bhattacharyya|first1=D.|last2=Clarke|first2=J. T.|last3=Chaufray|first3=J. Y.|last4=Mayyasi|first4=M.|last5=Bertaux|first5=J. L.|last6=Chaffin|first6=M. S.|last7=Schneider|first7=N. M.|last8=Villanueva|first8=G. L.|s2cid=119084288|date=2017|title=Seasonal Changes in Hydrogen Escape From Mars Through Analysis of HST Observations of the Martian Exosphere Near Perihelion|journal=Journal of Geophysical Research: Space Physics|volume=122|issue=11|pages=11,756–11,764|doi=10.1002/2017JA024572|issn=2169-9402|bibcode=2017JGRA..12211756B|url=https://hal-insu.archives-ouvertes.fr/insu-01636632/file/2017JA024572.pdf|access-date=6 January 2021|archive-date=5 November 2020|archive-url=https://web.archive.org/web/20201105034151/https://hal-insu.archives-ouvertes.fr/insu-01636632/file/2017JA024572.pdf|url-status=live}}{{Cite journal|last1=Schofield|first1=John T.|last2=Shirley|first2=James H.|last3=Piqueux|first3=Sylvain|last4=McCleese|first4=Daniel J.|last5=Paul O. Hayne|last6=Kass|first6=David M.|last7=Halekas|first7=Jasper S.|last8=Chaffin|first8=Michael S.|last9=Kleinböhl|first9=Armin|date=February 2018 |title=Hydrogen escape from Mars enhanced by deep convection in dust storms|journal=Nature Astronomy|volume=2|issue=2|pages=126–132|doi=10.1038/s41550-017-0353-4|issn=2397-3366|bibcode=2018NatAs...2..126H|s2cid=134961099}}{{Cite web|url=http://www.nasa.gov/feature/goddard/2019/martian-dust-could-help-explain-planet-s-water-loss-plus-other-learnings-from-recent-global|title=How Global Dust Storms Affect Martian Water, Winds, and Climate|last=Shekhtman|first=Svetlana|date=2019-04-29|website=NASA|access-date=2019-06-10|archive-date=17 June 2019|archive-url=https://web.archive.org/web/20190617062317/https://www.nasa.gov/feature/goddard/2019/martian-dust-could-help-explain-planet-s-water-loss-plus-other-learnings-from-recent-global/|url-status=live}} The estimated escape flux of hydrogen range from 10⁷ cm⁻² s⁻¹ to 10⁹ cm⁻² s⁻¹.

Photochemistry of CO₂ and CO in ionosphere can produce CO₂⁺ and CO⁺ ions, respectively:

: {{chem2|CO2}} + {{mvar|hν}} ⟶ {{chem2|CO2+ + e-}}

: {{chem2|CO}} + {{mvar|hν}} ⟶ {{chem2|CO+ + e-}}

An ion and an electron can recombine and produce electronic-neutral products. The products gain extra kinetic energy due to the Coulomb attraction between ions and electrons. This process is called dissociative recombination. Dissociative recombination can produce carbon atoms that travel faster than the escape velocity of Mars, and those moving upward can then escape the Martian atmosphere:

: {{chem2|CO+ + e- ⟶ C + O}}

: {{chem2|CO2+ + e- ⟶ C + O2}}

UV photolysis of carbon monoxide is another crucial mechanism for the carbon escape on Mars:{{Cite journal|last1=Nagy|first1=Andrew F.|last2=Liemohn|first2=Michael W.|last3=Fox|first3=J. L.|author-link3=Jane Lee Fox|last4=Kim|first4=Jhoon|date=2001|title=Hot carbon densities in the exosphere of Mars|journal=Journal of Geophysical Research: Space Physics|volume=106|issue=A10|pages=21565–21568|doi=10.1029/2001JA000007|issn=2156-2202|bibcode=2001JGR...10621565N|url=https://corescholar.libraries.wright.edu/physics/18|doi-access=free|access-date=24 November 2019|archive-date=28 July 2020|archive-url=https://web.archive.org/web/20200728153549/https://corescholar.libraries.wright.edu/physics/18/|url-status=live}}

: {{chem2|CO}} + {{mvar|hν}} ({{mvar|λ}} < 116 nm) ⟶ {{chem2|C + O}}.

Other potentially important mechanisms include the sputtering escape of CO₂ and collision of carbon with fast oxygen atoms. The estimated overall escape flux is about 0.6 × 10⁷ cm⁻² s⁻¹ to 2.2 × 10⁷ cm⁻² s⁻¹ and depends heavily on solar activity.{{Cite journal|last1=Gröller|first1=H.|last2=Lichtenegger|first2=H.|last3=Lammer|first3=H.|last4=Shematovich|first4=V. I.|date=2014-08-01|title=Hot oxygen and carbon escape from the martian atmosphere|journal=Planetary and Space Science|series=Planetary evolution and life|volume=98|pages=93–105|doi=10.1016/j.pss.2014.01.007|issn=0032-0633|bibcode=2014P&SS...98...93G|arxiv=1911.01107|s2cid=122599784}}

Like carbon, dissociative recombination of N₂⁺ is important for the nitrogen escape on Mars.{{Cite journal|last=Fox|first=J. L.|author-link=Jane Lee Fox|date=1993|title=The production and escape of nitrogen atoms on Mars|journal=Journal of Geophysical Research: Planets|volume=98|issue=E2|pages=3297–3310|doi=10.1029/92JE02289|issn=2156-2202|url=https://corescholar.libraries.wright.edu/cgi/viewcontent.cgi?article=1378&context=physics|bibcode=1993JGR....98.3297F|access-date=24 June 2019|archive-date=21 July 2018|archive-url=https://web.archive.org/web/20180721133047/https://corescholar.libraries.wright.edu/cgi/viewcontent.cgi?article=1378&context=physics|url-status=live}}{{Cite journal|last1=Mandt|first1=Kathleen|last2=Mousis|first2=Olivier|last3=Chassefière|first3=Eric|date=July 2015|title=Comparative planetology of the history of nitrogen isotopes in the atmospheres of Titan and Mars|journal=Icarus|volume=254|pages=259–261|doi=10.1016/j.icarus.2015.03.025|pmc=6527424|pmid=31118538|bibcode=2015Icar..254..259M}} In addition, other photochemical escape mechanism also play an important role:{{Cite journal|last=Fox|first=J.L.|date=December 2007|title=Comment on the papers "Production of hot nitrogen atoms in the martian thermosphere" by F. Bakalian and "Monte Carlo computations of the escape of atomic nitrogen from Mars" by F. Bakalian and R.E. Hartle|journal=Icarus|volume=192|issue=1|pages=296–301|doi=10.1016/j.icarus.2007.05.022|bibcode=2007Icar..192..296F}}

: {{chem2|N2}} + {{mvar|hν}} ⟶ {{chem2|N+ + N + e-}}

: {{chem2|N2 + e- ⟶ N+ + N + 2e-}}

Nitrogen escape rate is very sensitive to the mass of the atom and solar activity. The overall estimated escape rate of ¹⁴N is 4.8 × 10⁵ cm⁻² s⁻¹.

Dissociative recombination of CO₂⁺ and O₂⁺ (produced from CO₂⁺ reaction as well) can generate the oxygen atoms that travel fast enough to escape:

: {{chem2|CO2+ + e- ⟶ CO + O}}

: {{chem2|CO2+ + O ⟶ O2+ + CO}}

: {{chem2|O2+ + e- ⟶ O + O}}

However, the observations showed that there are not enough fast oxygen atoms the Martian exosphere as predicted by the dissociative recombination mechanism.{{Cite journal|last1=Feldman|first1=Paul D.|last2=Steffl|first2=Andrew J.|last3=Parker|first3=Joel Wm.|last4=A'Hearn|first4=Michael F.|last5=Bertaux|first5=Jean-Loup|last6=Alan Stern|first6=S.|last7=Weaver|first7=Harold A.|last8=Slater|first8=David C.|last9=Versteeg|first9=Maarten|date=2011-08-01|title=Rosetta-Alice observations of exospheric hydrogen and oxygen on Mars|journal=Icarus|volume=214|issue=2|pages=394–399|doi=10.1016/j.icarus.2011.06.013|issn=0019-1035|arxiv=1106.3926|bibcode=2011Icar..214..394F|s2cid=118646223}}{{Cite journal|last1=Leblanc|first1=F.|last2=Martinez|first2=A.|last3=Chaufray|first3=J. Y.|last4=Modolo|first4=R.|last5=Hara|first5=T.|last6=Luhmann|first6=J.|last7=Lillis|first7=R.|last8=Curry|first8=S.|last9=McFadden|first9=J.|s2cid=134561764|date=2018|title=On Mars's Atmospheric Sputtering After MAVEN's First Martian Year of Measurements|journal=Geophysical Research Letters|volume=45|issue=10|pages=4685–4691|doi=10.1002/2018GL077199|issn=1944-8007|bibcode=2018GeoRL..45.4685L|url=https://hal-insu.archives-ouvertes.fr/insu-03737439/file/Geophysical%20Research%20Letters%20-%202018%20-%20Leblanc%20-%20On%20Mars%20s%20Atmospheric%20Sputtering%20After%20MAVEN%20s%20First%20Martian%20Year%20of.pdf }} Model estimations of oxygen escape rate suggested it can be over 10 times lower than the hydrogen escape rate.{{Cite journal|last1=Lammer|first1=H.|last2=Lichtenegger|first2=H.I.M.|last3=Kolb|first3=C.|last4=Ribas|first4=I.|last5=Guinan|first5=E.F.|last6=Abart|first6=R.|last7=Bauer|first7=S.J.|date=September 2003|title=Loss of water from Mars|journal=Icarus|volume=165|issue=1|pages=9–25|doi=10.1016/S0019-1035(03)00170-2}} Ion pick and sputtering have been suggested as the alternative mechanisms for the oxygen escape, but this model suggests that they are less important than dissociative recombination at present.{{Cite journal|last1=Valeille|first1=Arnaud|last2=Bougher|first2=Stephen W.|last3=Tenishev|first3=Valeriy|last4=Combi|first4=Michael R.|last5=Nagy|first5=Andrew F.|date=2010-03-01|title=Water loss and evolution of the upper atmosphere and exosphere over martian history|journal=Icarus|series=Solar Wind Interactions with Mars|volume=206|issue=1|pages=28–39|doi=10.1016/j.icarus.2009.04.036|issn=0019-1035|bibcode=2010Icar..206...28V}} {{wide image|PIA18613-MarsMAVEN-Atmosphere-3UV-Views-20141014.jpg|600px|align-cap=center|Mars's escaping atmosphere—carbon, oxygen, hydrogen—measured by MAVEN's UV spectrograph).{{cite web |last1=Jones |first1=Nancy |last2=Steigerwald |first2=Bill |last3=Brown |first3=Dwayne |last4=Webster |first4=Guy |title=NASA Mission Provides Its First Look at Martian Upper Atmosphere |url=http://www.jpl.nasa.gov/news/news.php?release=2014-351 |date=14 October 2014 |work=NASA |access-date=15 October 2014 |archive-date=19 October 2014 |archive-url=https://web.archive.org/web/20141019184946/http://www.jpl.nasa.gov/news/news.php?release=2014-351 |url-status=live }}}}

The interaction of the solar wind and the interplanetary magnetic field with the Martian conductive ionosphere induces electrodynamic currents, that have been mapped and studied in detail, using MAVEN.{{Cite journal |last1=Ramstad |first1=Robin |last2=Brain |first2=David A. |last3=Dong |first3=Yaxue |last4=Espley |first4=Jared |last5=Halekas |first5=Jasper |last6=Jakosky |first6=Bruce |date=October 2020 |title=The global current systems of the Martian induced magnetosphere |url=https://www.nature.com/articles/s41550-020-1099-y |journal=Nature Astronomy |language=en |volume=4 |issue=10 |pages=979–985 |doi=10.1038/s41550-020-1099-y |bibcode=2020NatAs...4..979R |issn=2397-3366}} These currents can drive the ionospheric species to high altitudes, where the solar wind is able to sweep them away from the planet, resulting to global scale ion outflows. They are however not sufficient to explain the atmospheric and ionospheric losses of Mars over its lifetime.{{Cite journal |last1=Ramstad |first1=Robin |last2=Barabash |first2=Stas |last3=Futaana |first3=Yoshifumi |last4=Nilsson |first4=Hans |last5=Holmström |first5=Mats |date=November 2018 |title=Ion Escape From Mars Through Time: An Extrapolation of Atmospheric Loss Based on 10 Years of Mars Express Measurements |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018JE005727 |journal=Journal of Geophysical Research: Planets |language=en |volume=123 |issue=11 |pages=3051–3060 |doi=10.1029/2018JE005727 |bibcode=2018JGRE..123.3051R |issn=2169-9097}}

{{See also|Martian polar ice caps|}}

CO₂ is the main component of the Martian atmosphere. It has a mean volume (molar) ratio of 94.9%. In winter polar regions, the surface temperature can be lower than the frost point of CO_2. CO₂ gas in the atmosphere can condense on the surface to form 1–2 m thick solid dry ice. In summer, the polar dry ice cap can undergo sublimation and release the CO₂ back to the atmosphere. As a result, significant annual variability in atmospheric pressure (≈25%) and atmospheric composition can be observed on Mars.{{Cite web|url=https://www.msss.com/http/ps/seasons/seasons.html|title=Seasons on Mars|website=www.msss.com|access-date=2019-06-07|archive-date=3 November 2020|archive-url=https://web.archive.org/web/20201103161520/https://www.msss.com/http/ps/seasons/seasons.html|url-status=live}} The condensation process can be approximated by the Clausius–Clapeyron relation for CO₂.{{Cite journal |last1=Soto |first1=Alejandro |last2=Mischna |first2=Michael |last3=Schneider |first3=Tapio |last4=Lee |first4=Christopher |last5=Richardson |first5=Mark |date=2015-04-01 |title=Martian atmospheric collapse: Idealized GCM studies |journal=Icarus |volume=250 |pages=553–569 |doi=10.1016/j.icarus.2014.11.028 |issn=0019-1035 |bibcode=2015Icar..250..553S |url=https://authors.library.caltech.edu/53068/1/1-s2.0-S0019103514006575-main.pdf |access-date=30 August 2020 |archive-date=15 August 2017 |archive-url=https://web.archive.org/web/20170815231941/http://authors.library.caltech.edu/53068/1/1-s2.0-S0019103514006575-main.pdf |url-status=live }}

There also exists the potential for adsorption of CO₂ into and out of the regolith to contribute to the annual atmospheric variability. Although the sublimation and deposition of CO₂ ice in the polar caps is the driving force behind seasonal cycles, other processes such as dust storms, atmospheric tides, and transient eddies also play a role.{{Cite journal |last1=Hess |first1=Seymour L. |last2=Henry |first2=Robert M. |last3=Tillman |first3=James E. |date=1979 |title=The seasonal variation of atmospheric pressure on Mars as affected by the south polar cap |url=http://doi.wiley.com/10.1029/JB084iB06p02923 |journal=Journal of Geophysical Research |language=en |volume=84 |issue=B6 |pages=2923 |doi=10.1029/JB084iB06p02923 |bibcode=1979JGR....84.2923H |issn=0148-0227}}{{Cite journal |last1=Hess |first1=S. L. |last2=Ryan |first2=J. A. |last3=Tillman |first3=J. E. |last4=Henry |first4=R. M. |last5=Leovy |first5=C. B. |date=March 1980 |title=The annual cycle of pressure on Mars measured by Viking Landers 1 and 2 |url=http://doi.wiley.com/10.1029/GL007i003p00197 |journal=Geophysical Research Letters |language=en |volume=7 |issue=3 |pages=197–200 |doi=10.1029/GL007i003p00197|bibcode=1980GeoRL...7..197H }}{{Cite journal |last1=Ordonez-Etxeberria |first1=Iñaki |last2=Hueso |first2=Ricardo |last3=Sánchez-Lavega |first3=Agustín |last4=Millour |first4=Ehouarn |last5=Forget |first5=Francois |date=January 2019 |title=Meteorological pressure at Gale crater from a comparison of REMS/MSL data and MCD modelling: Effect of dust storms |journal=Icarus |language=en |volume=317 |pages=591–609 |doi=10.1016/j.icarus.2018.09.003|bibcode=2019Icar..317..591O |s2cid=125851495 |doi-access=free }}{{Cite journal |last1=Guzewich |first1=Scott D. |last2=Newman |first2=C.E. |last3=de la Torre Juárez |first3=M. |last4=Wilson |first4=R.J. |last5=Lemmon |first5=M. |last6=Smith |first6=M.D. |last7=Kahanpää |first7=H. |last8=Harri |first8=A.-M. |date=April 2016 |title=Atmospheric tides in Gale Crater, Mars |url=https://linkinghub.elsevier.com/retrieve/pii/S0019103515005850 |journal=Icarus |language=en |volume=268 |pages=37–49 |doi=10.1016/j.icarus.2015.12.028|bibcode=2016Icar..268...37G }}{{Cite journal |last1=Haberle |first1=Robert M. |last2=Juárez |first2=Manuel de la Torre |last3=Kahre |first3=Melinda A. |last4=Kass |first4=David M. |last5=Barnes |first5=Jeffrey R. |last6=Hollingsworth |first6=Jeffery L. |last7=Harri |first7=Ari-Matti |last8=Kahanpää |first8=Henrik |date=June 2018 |title=Detection of Northern Hemisphere transient eddies at Gale Crater Mars |url=https://linkinghub.elsevier.com/retrieve/pii/S0019103517304311 |journal=Icarus |language=en |volume=307 |pages=150–160 |doi=10.1016/j.icarus.2018.02.013|bibcode=2018Icar..307..150H |s2cid=92991001 }} Understanding each of these more minor processes and how they contribute to the overall atmospheric cycle will give a clearer picture as to how the Martian atmosphere works as a whole. It has been suggested that the regolith on Mars has high internal surface area, implying that it might have a relatively high capacity for the storage of adsorbed gas.{{Cite journal |last1=Fanale |first1=F. P. |last2=Cannon |first2=W. A. |date=April 1971 |title=Adsorption on the Martian Regolith |url=https://www.nature.com/articles/230502a0 |journal=Nature |language=en |volume=230 |issue=5295 |pages=502–504 |doi=10.1038/230502a0 |bibcode=1971Natur.230..502F |s2cid=4263086 |issn=0028-0836}} Since adsorption works through the adhesion of a film of molecules onto a surface, the amount of surface area for any given volume of material is the main contributor for how much adsorption can occur. A solid block of material, for example, would have no internal surface area, but a porous material, like a sponge, would have high internal surface area. Given the loose, finely grained nature of the Martian regolith, there is the possibility of significant levels of CO₂ adsorption into it from the atmosphere.{{Cite journal |last1=Zent |first1=Aaron P. |last2=Quinn |first2=Richard C. |date=1995 |title=Simultaneous adsorption of CO 2 and H 2 O under Mars-like conditions and application to the evolution of the Martian climate |url=http://doi.wiley.com/10.1029/94JE01899 |journal=Journal of Geophysical Research |language=en |volume=100 |issue=E3 |pages=5341 |doi=10.1029/94JE01899 |bibcode=1995JGR...100.5341Z |hdl=2060/19940030969 |s2cid=129616949 |issn=0148-0227|hdl-access=free }} Adsorption from the atmosphere into the regolith has previously been proposed as an explanation for the observed cycles in the methane and water mixing ratios.{{Cite journal |last1=Moores |first1=John E. |last2=Gough |first2=Raina V. |last3=Martinez |first3=German M. |last4=Meslin |first4=Pierre-Yves |last5=Smith |first5=Christina L. |last6=Atreya |first6=Sushil K. |last7=Mahaffy |first7=Paul R. |last8=Newman |first8=Claire E. |last9=Webster |first9=Christopher R. |date=May 2019 |title=Methane seasonal cycle at Gale Crater on Mars consistent with regolith adsorption and diffusion |url=http://www.nature.com/articles/s41561-019-0313-y |journal=Nature Geoscience |language=en |volume=12 |issue=5 |pages=321–325 |doi=10.1038/s41561-019-0313-y |bibcode=2019NatGe..12..321M |s2cid=135136911 |issn=1752-0894}}{{Cite journal |last1=Meslin |first1=P.-Y. |last2=Gough |first2=R. |last3=Lefèvre |first3=F. |last4=Forget |first4=F. |date=February 2011 |title=Little variability of methane on Mars induced by adsorption in the regolith |url=https://linkinghub.elsevier.com/retrieve/pii/S0032063310002941 |journal=Planetary and Space Science |language=en |volume=59 |issue=2–3 |pages=247–258 |doi=10.1016/j.pss.2010.09.022|bibcode=2011P&SS...59..247M }} More research is needed to help determine if CO₂ adsorption is occurring, and if so, the extent of its impact on the overall atmospheric cycle.

File:Composition-comparison Mars-Venus-Earth.png

Despite the high concentration of CO₂ in the Martian atmosphere, the greenhouse effect is relatively weak on Mars (about 5 °C) because of the low concentration of water vapor and low atmospheric pressure. While water vapor in Earth's atmosphere has the largest contribution to greenhouse effect on modern Earth, it is present in only very low concentration in the Martian atmosphere. Moreover, under low atmospheric pressure, greenhouse gases cannot absorb infrared radiation effectively because the pressure-broadening effect is weak.{{Cite web |url=https://www.esa.int/Our_Activities/Space_Science/Venus_Express/Greenhouse_effects_also_on_other_planets |title=Greenhouse effects ... also on other planets |publisher=European Space Agency |access-date=2019-06-07 |archive-date=29 September 2019 |archive-url=https://web.archive.org/web/20190929025950/http://www.esa.int/Our_Activities/Space_Science/Venus_Express/Greenhouse_effects_also_on_other_planets |url-status=live }}{{Cite journal |last1=Yung |first1=Yuk L. |last2=Kirschvink |first2=Joseph L. |last3=Pahlevan |first3=Kaveh |last4=Li |first4=King-Fai |date=2009-06-16 |title=Atmospheric pressure as a natural climate regulator for a terrestrial planet with a biosphere |journal=Proceedings of the National Academy of Sciences |volume=106 |issue=24 |pages=9576–9579 |doi=10.1073/pnas.0809436106 |issn=0027-8424 |pmc=2701016 |pmid=19487662 |bibcode=2009PNAS..106.9576L|doi-access=free }}

In the presence of solar UV radiation (hν, photons with wavelength shorter than 225 nm), CO₂ in the Martian atmosphere can be photolyzed via the following reaction:

: {{chem2|CO2}} + {{mvar|hν}} ({{mvar|λ}} < 225 nm) ⟶ {{chem2|CO + O}}.

If there is no chemical production of CO₂, all the CO₂ in the current Martian atmosphere would be removed by photolysis in about 3,500 years. The hydroxyl radicals (OH) produced from the photolysis of water vapor, together with the other odd hydrogen species (e.g. H, HO₂), can convert carbon monoxide (CO) back to CO₂. The reaction cycle can be described as:{{Cite journal|last1=McElroy |first1=M.B. |last2=Donahue |first2=T.M. |s2cid=30958948 |date=1972-09-15 |title=Stability of the Martian atmosphere |journal=Science |volume=177 |issue=4053 |pages=986–988 |doi=10.1126/science.177.4053.986 |pmid=17788809 |issn=0036-8075 |hdl=2060/19730010098 |hdl-access=free |bibcode=1972Sci...177..986M}}{{Cite journal |last1=Parkinson |first1=T.D. |last2=Hunten|first2=D.M. |date=October 1972 |title=Spectroscopy and acronomy of O{{sub|2}} on Mars |journal=Journal of the Atmospheric Sciences |volume=29 |issue=7 |pages=1380–1390 |doi=10.1175/1520-0469(1972)029<1380:SAAOOO>2.0.CO;2 |issn=0022-4928 |bibcode=1972JAtS...29.1380P |doi-access=free}}

: {{chem2|CO + OH ⟶ CO2 + H}}

: {{chem2|H + O2 + M ⟶ HO2 + M}}

: {{chem2|HO2 + O ⟶ OH + O2}}

: {{chem2|Net: CO + O ⟶ CO2}}

Mixing also plays a role in regenerating CO₂ by bringing the O, CO, and O₂ in the upper atmosphere downward. The balance between photolysis and redox production keeps the average concentration of CO₂ stable in the modern Martian atmosphere.

CO₂ ice clouds can form in winter polar regions and at very high altitudes (>50 km) in tropical regions, where the air temperature is lower than the frost point of CO₂.{{Cite journal|last1=Stevens |first1=M.H. |last2=Siskind|first2=D.E. |last3=Evans |first3=J.S. |last4=Jain |first4=S.K. |last5=Schneider |first5=N.M. |last6=Deighan |first6=J. |last7=Stewart |first7=A.I.F. |last8=Crismani |first8=M. |last9=Stiepen |first9=A. |date=2017-05-28 |title=Martian mesospheric cloud observations by IUVS on MAVEN: Thermal tides coupled to the upper atmosphere: IUVS Martian Mesospheric Clouds |journal=Geophysical Research Letters |volume=44 |issue=10 |pages=4709–4715 |doi=10.1002/2017GL072717 |hdl=10150/624978 |s2cid=13748950 |hdl-access=free}}{{Cite journal |last1=González-Galindo |first1=Francisco |last2=Määttänen |first2=Anni |last3=Forget |first3=François |last4=Spiga |first4=Aymeric |date=2011-11-01 |title=The martian mesosphere as revealed by CO{{sub|2}} cloud observations and general circulation modeling |journal=Icarus |volume=216 |issue=1 |pages=10–22 |doi=10.1016/j.icarus.2011.08.006 |issn=0019-1035 |bibcode=2011Icar..216...10G}}

N₂ is the second most abundant gas in the Martian atmosphere. It has a mean volume ratio of 2.6%. Various measurements showed that the Martian atmosphere is enriched in ¹⁵N.{{cite journal |last1=Stevens |first1=M.H. |last2=Evans |first2=J.S. |last3=Schneider |first3= N.M. |last4=Stewart |first4=A.I.F. |last5=Deighan |first5=J. |last6=Jain |first6=S.K. |last7=Crismani |first7=M. |last8=Stiepen |first8=A. |last9=Chaffin |first9=M.S. |last10=McClintock |first10=W.E. |last11=Holsclaw |first11=G.M. |last12=Lefèvre |first12=F. |last13=Lo |first13=D.Y. |last14=Clarke |first14=J.T. |last15=Montmessin |first15=F. |last16=Bougher |first16=S.W. |last17=Jakosky |first17=B.M. |year=2015 |title=New observations of molecular nitrogen in the Martian upper atmosphere by IUVS on MAVEN |journal=Geophysical Research Letters |volume=42 |issue=21 |pages=9050–9056 |doi=10.1002/2015GL065319 |doi-access=free |bibcode=2015GeoRL..42.9050S}}{{cite journal |last1=Avice |first1=G. |last2=Bekaert |first2=D.V. |last3=Chennaoui Aoudjehane |first3=H. |last4=Marty |first4=B. |date=9 February 2018 |title=Noble gases and nitrogen in Tissint reveal the composition of the Mars atmosphere |journal=Geochemical Perspectives Letters |volume=6 |pages=11–16 |doi=10.7185/geochemlet.1802 |doi-access=free}} The enrichment of heavy isotopes of nitrogen is possibly caused by mass-selective escape processes.{{cite journal |author1=Mandt, Kathleen |author2=Mousis, Olivier |author3=Chassefière, Eric |date=1 July 2015 |title=Comparative planetology of the history of nitrogen isotopes in the atmospheres of Titan and Mars |journal=Icarus |volume=254 |pages=259–261 |doi=10.1016/j.icarus.2015.03.025 |pmc=6527424 |pmid=31118538 |bibcode=2015Icar..254..259M}}

File:PIA16818-MarsCuriosityRover-Argon-AtmosphericLoss.png are a signature of atmospheric loss on Mars.{{cite press release |url=https://mars.nasa.gov/news/1461/remaining-martian-atmosphere-still-dynamic/ |title=Remaining Martian atmosphere still dynamic |last=Webster |first=Guy |date=8 April 2013 |publisher=NASA |access-date=12 June 2019 |archive-date=26 July 2020 |archive-url=https://web.archive.org/web/20200726111003/https://mars.nasa.gov/news/1461/remaining-martian-atmosphere-still-dynamic/ |url-status=live }}{{cite web |url=http://www.space.com/20560-mars-atmosphere-lost-curiosity-rover.html |title=Most of Mars' atmosphere is lost in space |last=Wall |first=Mike |date=8 April 2013 |website=Space.com |access-date=9 April 2013 |archive-date=30 January 2016 |archive-url=https://web.archive.org/web/20160130143450/http://www.space.com/20560-mars-atmosphere-lost-curiosity-rover.html |url-status=live }}]]

Argon is the third most abundant gas in the Martian atmosphere. It has a mean volume ratio of 1.9%. In terms of stable isotopes, Mars is enriched in ³⁸Ar relative to ³⁶Ar, which can be attributed to hydrodynamic escape.

One of Argon's isotopes, ⁴⁰Ar, is produced from the radioactive decay of ⁴⁰K. In contrast, ³⁶Ar is primordial: It was present in the atmosphere after the formation of Mars. Observations indicate that Mars is enriched in ⁴⁰Ar relative to ³⁶Ar, which cannot be attributed to mass-selective loss processes.{{Cite journal |last1=Mahaffy |first1=P.R. |last2=Webster |first2=C.R. |last3=Atreya |first3=S.K. |last4=Franz |first4=H. |last5=Wong |first5=M. |last6=Conrad |first6=P.G. |last7=Harpold |first7=D. |last8=Jones |first8=J.J. |last9=Leshin |first9=L.A. |date=2013-07-19 |title=Abundance and isotopic composition of gases in the Martian atmosphere from the Curiosity rover |journal=Science |volume=341 |issue=6143 |pages=263–266 |issn=0036-8075 |doi=10.1126/science.1237966 |pmid=23869014 |bibcode=2013Sci...341..263M |s2cid=206548973}} A possible explanation for the enrichment is that a significant amount of primordial atmosphere, including ³⁶Ar, was lost by impact erosion in the early history of Mars, while ⁴⁰Ar was emitted to the atmosphere after the impact.

File:Seasonal variations of oxygen at Gale crater 2012–2017.jpg]]

The estimated mean volume ratio of molecular oxygen (O₂) in the Martian atmosphere is 0.174%. It is one of the products of the photolysis of CO₂, water vapor, and ozone (O{{sub|3}}). It can react with atomic oxygen (O) to re-form ozone (O{{sub|3}}). In 2010, the Herschel Space Observatory detected molecular oxygen in the Martian atmosphere.{{cite journal |last1=Hartogh |first1=P. |last2=Jarchow |first2=C. |last3=Lellouch |first3=E. |last4=de Val-Borro |first4=M. |last5=Rengel |first5=M. |last6=Moreno |first6=R. |last7=Medvedev |first7=A.S. |last8=Sagawa |first8=H. |last9=Swinyard |first9=B.M. |last10=Cavalié |first10=T. |last11=Lis |first11=D.C. |last12=Błęcka |first12=M.I. |last13=Banaszkiewicz |first13=M. |last14=Bockelée-Morvan |first14=D. |last15=Crovisier |first15=J. |last16=Encrenaz |first16=T.|author16-link=Thérèse Encrenaz |last17=Küppers |first17=M. |last18=Lara |first18=L.-M. |last19=Szutowicz |first19=S. |last20=van den Bussche |first20=B. |last21=Bensch |first21=F. |last22=Bergin |first22=E.A. |last23=Billebaud |first23=F. |last24=Biver |first24=N. |last25=Blake |first25=G.A. |last26=Blommaert |first26=J.A.D.L. |last27=Cernicharo |first27=J. |last28=Decin |first28=L. |last29=Encrenaz |first29=P.|author29-link=Thérèse Encrenaz |last30=Feuchtgruber |first30=H. |display-authors=6 |year=2010 |title=Herschel / HIFI observations of Mars: First detection of O{{sub|2}} at sub-millimetre wavelengths and upper limits on HCL and H{{sub|2}}O{{sub|2}} |url=https://www.aanda.org/articles/aa/full_html/2010/13/aa15160-10/aa15160-10.html |journal=Astronomy and Astrophysics |volume=521 |pages=L49 |doi=10.1051/0004-6361/201015160 |bibcode=2010A&A...521L..49H |arxiv=1007.1301 |s2cid=119271891 |access-date=6 February 2019 |archive-date=7 February 2019 |archive-url=https://web.archive.org/web/20190207015254/https://www.aanda.org/articles/aa/full_html/2010/13/aa15160-10/aa15160-10.html |url-status=live }}

Atomic oxygen is produced by photolysis of CO₂ in the upper atmosphere and can escape the atmosphere via dissociative recombination or ion pickup. In early 2016, Stratospheric Observatory for Infrared Astronomy (SOFIA) detected atomic oxygen in the atmosphere of Mars, which has not been found since the Viking and Mariner mission in the 1970s.{{Cite web |url=https://www.nasa.gov/feature/ames/sofia/flying-observatory-detects-atomic-oxygen-in-martian-atmosphere |title=Flying Observatory Detects Atomic Oxygen in Martian Atmosphere – NASA |date=6 May 2016 |access-date=18 March 2017 |archive-date=8 November 2020 |archive-url=https://web.archive.org/web/20201108101219/https://www.nasa.gov/feature/ames/sofia/flying-observatory-detects-atomic-oxygen-in-martian-atmosphere/ |url-status=live }}

In 2019, NASA scientists working on the Curiosity rover mission, who have been taking measurements of the gas, discovered that the amount of oxygen in the Martian atmosphere rose by 30% in spring and summer.{{Cite web|url=https://www.bbc.com/news/science-environment-50419917|title=Nasa probes oxygen mystery on Mars|date=14 November 2019|website=BBC News|access-date=15 November 2019|archive-date=17 January 2020|archive-url=https://web.archive.org/web/20200117183603/https://www.bbc.com/news/science-environment-50419917|url-status=live}}

Similar to stratospheric ozone in Earth's atmosphere, the ozone present in the Martian atmosphere can be destroyed by catalytic cycles involving odd hydrogen species:

: {{chem2|H + O3 ⟶ OH + O2}}

: {{chem2|O + OH ⟶ H + O2}}

: Net: {{chem2|O + O3 ⟶  2O2}}

Since water is an important source of these odd hydrogen species, higher abundance of ozone is usually observed in the regions with lower water vapor content.{{Cite journal|last=Krasnopolsky|first=Vladimir A.|date=2006-11-01|title=Photochemistry of the martian atmosphere: Seasonal, latitudinal, and diurnal variations|journal=Icarus|volume=185|issue=1|pages=153–170|doi=10.1016/j.icarus.2006.06.003|issn=0019-1035|bibcode=2006Icar..185..153K}} Measurements showed that the total column of ozone can reach 2–30 μm-atm around the poles in winter and spring, where the air is cold and has low water saturation ratio.{{Cite journal|last1=Perrier |first1=S. |last2=Bertaux |first2=J.L. |last3=Lefèvre |first3=F. |last4=Lebonnois |first4=S. |last5=Korablev|first5=O. |last6=Fedorova |first6=A. |last7=Montmessin |first7=F. |year=2006 |title=Global distribution of total ozone on Mars from SPICAM/MEX UV measurements|journal=Journal of Geophysical Research |series=Planets |volume=111 |issue=E9 |pages=E09S06 |doi=10.1029/2006JE002681 |issn=2156-2202 |bibcode=2006JGRE..111.9S06P |doi-access=free}} The actual reactions between ozone and odd hydrogen species may be further complicated by the heterogeneous reactions that take place in water-ice clouds.{{Cite journal |last1=Perrier |first1=Séverine |last2=Montmessin |first2=Franck |last3=Lebonnois |first3=Sébastien |last4=Forget |first4=François |last5=Fast |first5=Kelly |last6=Encrenaz |first6=Thérèse|author6-link=Thérèse Encrenaz |last7=Clancy|first7=R. Todd |last8=Bertaux |first8=Jean-Loup |last9=Lefèvre |first9=Franck |display-authors=6 |date=August 2008 |title=Heterogeneous chemistry in the atmosphere of Mars |journal=Nature |volume=454 |issue=7207 |pages=971–975 |doi=10.1038/nature07116 |pmid=18719584 |issn=1476-4687 |bibcode=2008Natur.454..971L |s2cid=205214046}}

It is thought that the vertical distribution and seasonality of ozone in the Martian atmosphere is driven by the complex interactions between chemistry and transport of oxygen-rich air from sunlit latitudes to the poles.{{cite journal |last1=Franck Lefèvre |last2=Montmessin |first2=Franck |date=November 2013 |title=Transport-driven formation of a polar ozone layer on Mars |journal=Nature Geoscience |volume=6 |issue=11 |pages=930–933 |doi=10.1038/ngeo1957 |issn=1752-0908 |bibcode=2013NatGe...6..930M}}{{cite web |url=http://sci.esa.int/mars-express/52881-a-seasonal-ozone-layer-over-the-martian-south-pole/ |title=A seasonal ozone layer over the Martian south pole |series=Mars Express |publisher=European Space Agency |website=sci.esa.int |access-date=2019-06-03 |archive-date=3 June 2019 |archive-url=https://web.archive.org/web/20190603213141/http://sci.esa.int/mars-express/52881-a-seasonal-ozone-layer-over-the-martian-south-pole/ |url-status=live }} The UV/IR spectrometer on Mars Express (SPICAM) has shown the presence of two distinct ozone layers at low-to-mid latitudes. These comprise a persistent, near-surface layer below an altitude of {{Convert|30|km|mi|abbr=on}}, a separate layer that is only present in northern spring and summer with an altitude varying from 30 to 60 km, and another separate layer that exists 40–60 km above the southern pole in winter, with no counterpart above the Mars's north pole.{{Cite journal |last1=Lebonnois |first1=Sébastien |last2=Quémerais |first2=Eric |last3=Montmessin |first3=Franck |last4=Lefèvre |first4=Franck |last5=Perrier |first5=Séverine |last6=Bertaux |first6=Jean-Loup |last7=Forget |first7=François |s2cid=55162288 |year=2006 |title=Vertical distribution of ozone on Mars as measured by SPICAM/Mars Express using stellar occultations |journal=Journal of Geophysical Research |series=Planets |volume=111 |issue=E9 |pages=E09S05 |doi=10.1029/2005JE002643 |issn=2156-2202 |bibcode=2006JGRE..111.9S05L |url=https://hal.archives-ouvertes.fr/hal-00109865/file/8fb300f315ff537a95f0f59c8d42b449f415.pdf |doi-access=free |access-date=30 August 2020 |archive-date=8 November 2020 |archive-url=https://web.archive.org/web/20201108170342/https://hal.archives-ouvertes.fr/hal-00109865/file/8fb300f315ff537a95f0f59c8d42b449f415.pdf |url-status=live }} This third ozone layer shows an abrupt decrease in elevation between 75 and 50 degrees south. SPICAM detected a gradual increase in ozone concentration at {{Convert|50|km|mi|abbr=on}} until midwinter, after which it slowly decreased to very low concentrations, with no layer detectable above {{Convert|35|km|mi|abbr=on}}.

{{See also|Water on Mars|Martian polar ice caps}}

File:Pia24622-curiosity 1-1041.jpg

Water vapor is a trace gas in the Martian atmosphere and has huge spatial, diurnal and seasonal variability.{{Cite journal |last=Titov |first=D.V. |date=2002-01-01 |title=Water vapour in the atmosphere of Mars |journal=Advances in Space Research |volume=29 |issue=2 |pages=183–191 |doi=10.1016/S0273-1177(01)00568-3 |issn=0273-1177 |bibcode=2002AdSpR..29..183T}}{{cite journal |last1=Whiteway |first1=J.A. |last2=Komguem |first2=L. |last3=Dickinson |first3=C. |last4=Cook |first4=C. |last5=Illnicki |first5=M. |last6=Seabrook |first6=J. |last7=Popovici |first7=V. |last8=Duck |first8=T. J.|last9=Davy |first9=R. |date=2009-07-03 |title=Mars Water-Ice Clouds and Precipitation |journal=Science |volume=325 |issue=5936 |pages=68–70 |doi=10.1126/science.1172344 |pmid=19574386 |issn=0036-8075 |bibcode=2009Sci...325...68W |s2cid=206519222}} Measurements made by Viking orbiter in the late 1970s suggested that the entire global total mass of water vapor is equivalent to about 1 to 2 km³ of ice.{{Cite journal |last1=Jakosky |first1=Bruce M. |last2=Farmer |first2=Crofton B. |year=1982 |title=The seasonal and global behavior of water vapor in the Mars atmosphere: Complete global results of the Viking Atmospheric Water Detector Experiment |journal=Journal of Geophysical Research |series=Solid Earth |volume=87 |issue=B4 |pages=2999–3019 |doi=10.1029/JB087iB04p02999 |issn=2156-2202 |bibcode=1982JGR....87.2999J}} More recent measurements by Mars Express orbiter showed that the globally annually-averaged column abundance of water vapor is about 10–20 precipitable microns (pr. μm).{{Cite journal |last1=Trokhimovskiy |first1=Alexander |last2=Fedorova |first2=Anna |last3=Korablev |first3=Oleg |last4=Montmessin |first4=Franck |last5=Bertaux |first5=Jean-Loup |last6=Rodin |first6=Alexander |last7=Smith |first7=Michael D. |date=2015-05-01 |title=Mars' water vapor mapping by the SPICAM IR spectrometer: Five martian years of observations |journal=Icarus |series=Dynamic Mars |volume=251 |pages=50–64 |doi=10.1016/j.icarus.2014.10.007 |issn=0019-1035 |bibcode=2015Icar..251...50T}}{{Cite web |url=https://www.sciencedaily.com/releases/2014/12/141222111603.htm |title=Scientists 'map' water vapor in Martian atmosphere |website=ScienceDaily |access-date=2019-06-08 |archive-date=8 June 2019 |archive-url=https://web.archive.org/web/20190608190846/https://www.sciencedaily.com/releases/2014/12/141222111603.htm |url-status=live }} Maximum abundance of water vapor (50-70 pr. μm) is found in the northern polar regions in early summer due to the sublimation of water ice in the polar cap.

Unlike in Earth's atmosphere, liquid-water clouds cannot exist in the Martian atmosphere; this is because of the low atmospheric pressure. Cirrus-like water-ice clouds have been observed by the cameras on Opportunity rover and Phoenix lander.{{Cite web |url=https://mars.nasa.gov/mer/ |title=Mars Exploration Rover |last1=mars.nasa.gov |publisher=NASA |department=Jet Propulsion Laboratory |website=mars.nasa.gov |access-date=2019-06-08 |archive-date=8 August 2012 |archive-url=https://web.archive.org/web/20120808162327/http://marsrovers.jpl.nasa.gov/home/ |url-status=live }}{{cite AV media |url=https://www.nasa.gov/mission_pages/phoenix/images/press/15777.html |title=Ice Clouds in Martian Arctic |medium=accelerated movie |publisher=NASA |website=www.nasa.gov |access-date=2019-06-08 |archive-date=3 January 2019 |archive-url=https://web.archive.org/web/20190103235734/https://www.nasa.gov/mission_pages/phoenix/images/press/15777.html |url-status=dead }} Measurements made by the Phoenix lander showed that water-ice clouds can form at the top of the planetary boundary layer at night and precipitate back to the surface as ice crystals in the northern polar region.{{Cite journal |last1=Montmessin |first1=Franck |last2=Forget |first2=François |last3=Millour |first3=Ehouarn |last4=Navarro |first4=Thomas |last5=Madeleine |first5=Jean-Baptiste |last6=Hinson |first6=David P. |last7=Spiga |first7=Aymeric |date=September 2017 |title=Snow precipitation on Mars driven by cloud-induced night-time convection |journal=Nature Geoscience |volume=10 |issue=9 |pages=652–657 |doi=10.1038/ngeo3008 |issn=1752-0908 |bibcode=2017NatGe..10..652S |s2cid=135198120}}

file:Mars Viking 21i093.png, the water ice precipitated by adhering to dry ice (observed by the Viking 2 lander)]]

{{Main|Methane on Mars}}

As a volcanic and biogenic species, methane is of interest to geologists and astrobiologists. However, methane is chemically unstable in an oxidizing atmosphere with UV radiation. The lifetime of methane in the Martian atmosphere is about 400 years.{{Cite web|url=https://www.esa.int/Our_Activities/Human_and_Robotic_Exploration/Exploration/ExoMars/The_methane_mystery|title=The methane mystery|last=esa|website=European Space Agency|access-date=2019-06-07|archive-date=2 June 2019|archive-url=https://web.archive.org/web/20190602161707/http://www.esa.int/Our_Activities/Human_and_Robotic_Exploration/Exploration/ExoMars/The_methane_mystery|url-status=live}} The detection of methane in a planetary atmosphere may indicate the presence of recent geological activities or living organisms.{{Cite web|url=http://www.nasa.gov/press-release/nasa-finds-ancient-organic-material-mysterious-methane-on-mars|title=NASA Finds Ancient Organic Material, Mysterious Methane on Mars|last=Potter|first=Sean|date=2018-06-07|website=NASA|access-date=2019-06-06|archive-date=8 June 2019|archive-url=https://web.archive.org/web/20190608040839/https://www.nasa.gov/press-release/nasa-finds-ancient-organic-material-mysterious-methane-on-mars/|url-status=live}}{{Cite journal |last=Witze |first=Alexandra |date=2018-10-25 |title=Mars scientists edge closer to solving methane mystery |journal=Nature |volume=563 |issue=7729 |pages=18–19 |bibcode=2018Natur.563...18W |doi=10.1038/d41586-018-07177-4 |doi-access=free |pmid=30377322}} Since 2004, trace amounts of methane (range from 60 ppb to under detection limit (< 0.05 ppb)) have been reported in various missions and observational studies.{{Cite journal |last1=Formisano |first1=Vittorio |last2=Atreya |first2=Sushil |last3=Encrenaz |first3=Thérèse|author3-link=Thérèse Encrenaz |last4=Ignatiev |first4=Nikolai |last5=Giuranna |first5=Marco |date=2004-12-03 |title=Detection of Methane in the Atmosphere of Mars |journal=Science |volume=306 |issue=5702 |pages=1758–1761 |s2cid=13533388 |doi=10.1126/science.1101732 |pmid=15514118 |issn=0036-8075 |bibcode=2004Sci...306.1758F|doi-access=free }}{{Cite journal |last1=Krasnopolsky |first1=Vladimir A. |last2=Maillard |first2=Jean Pierre |last3=Owen |first3=Tobias C. |date=December 2004 |title=Detection of methane in the martian atmosphere: evidence for life? |journal=Icarus |volume=172 |issue=2 |pages=537–547 |doi=10.1016/j.icarus.2004.07.004 |bibcode=2004Icar..172..537K}}{{Cite journal |last1=Geminale |first1=A. |last2=Formisano |first2=V. |last3=Giuranna |first3=M.|date=July 2008 |title=Methane in Martian atmosphere: Average spatial, diurnal, and seasonal behaviour |journal=Planetary and Space Science |volume=56 |issue=9 |pages=1194–1203 |doi=10.1016/j.pss.2008.03.004 |bibcode=2008P&SS...56.1194G}}{{Cite journal |last1=Mumma|first1=M.J. |last2=Villanueva|first2=G.L. |last3=Novak |first3=R.E. |last4=Hewagama|first4=T. |last5=Bonev |first5=B.P. |last6=DiSanti |first6=M.A. |last7=Mandell |first7=A.M. |last8=Smith |first8=M.D. |s2cid=25083438|date=2009-02-20|title=Strong Release of Methane on Mars in Northern Summer 2003 |journal=Science |volume=323 |issue=5917 |pages=1041–1045 |doi=10.1126/science.1165243 |pmid=19150811 |issn=0036-8075 |bibcode=2009Sci...323.1041M|doi-access=free }}{{Cite journal |last1=Fonti |first1=S. |last2=Marzo |first2=G.A. |date=March 2010 |title=Mapping the methane on Mars|journal=Astronomy and Astrophysics |volume=512|pages=A51|doi=10.1051/0004-6361/200913178|issn=0004-6361 |bibcode=2010A&A...512A..51F|doi-access=free}}{{Cite journal |last1=Geminale |first1=A. |last2=Formisano|first2=V.|last3=Sindoni|first3=G.|date=2011-02-01|title=Mapping methane in Martian atmosphere with PFS-MEX data|journal=Planetary and Space Science|series=Methane on Mars: Current Observations, Interpretation and Future Plans|volume=59|issue=2|pages=137–148|doi=10.1016/j.pss.2010.07.011|issn=0032-0633|bibcode=2011P&SS...59..137G}}{{Cite journal|last1=Vasavada|first1=Ashwin R.|last2=Zurek|first2=Richard W.|last3=Sander|first3=Stanley P.|last4=Crisp|first4=Joy|last5=Lemmon|first5=Mark|last6=Hassler|first6=Donald M.|last7=Genzer|first7=Maria|last8=Harri|first8=Ari-Matti|last9=Smith|first9=Michael D.|date=2018-06-08|title=Background levels of methane in Mars' atmosphere show strong seasonal variations|journal=Science|volume=360|issue=6393|pages=1093–1096|doi=10.1126/science.aaq0131|issn=0036-8075|pmid=29880682|bibcode=2018Sci...360.1093W|doi-access=free}}{{Cite journal|last1=Amoroso|first1=Marilena|last2=Merritt|first2=Donald|last3=Parra|first3=Julia Marín-Yaseli de la|last4=Cardesín-Moinelo|first4=Alejandro|last5=Aoki|first5=Shohei|last6=Wolkenberg|first6=Paulina|last7=Alessandro Aronica|last8=Formisano|first8=Vittorio|last9=Oehler|first9=Dorothy|date=May 2019|title=Independent confirmation of a methane spike on Mars and a source region east of Gale Crater|journal=Nature Geoscience|volume=12|issue=5|pages=326–332|doi=10.1038/s41561-019-0331-9|issn=1752-0908|bibcode=2019NatGe..12..326G|s2cid=134110253}} The source of methane on Mars and the explanation for the enormous discrepancy in the observed methane concentrations are still under active debate.{{Cite journal|last1=Zahnle|first1=Kevin|last2=Freedman|first2=Richard S.|last3=Catling|first3=David C.|date=2011-04-01|title=Is there methane on Mars?|journal=Icarus|volume=212|issue=2|pages=493–503|doi=10.1016/j.icarus.2010.11.027|issn=0019-1035|url=https://zenodo.org/record/1259041|bibcode=2011Icar..212..493Z|access-date=4 July 2019|archive-date=1 October 2020|archive-url=https://web.archive.org/web/20201001023932/https://zenodo.org/record/1259041|url-status=live}} In 2024, NASA reported that the only place on Mars where methane has been found is Gale Crater.{{cite web | url=https://www.jpl.nasa.gov/news/why-is-methane-seeping-on-mars-nasa-scientists-have-new-ideas/ | title=Why is Methane Seeping on Mars? NASA Scientists Have New Ideas | website=Jet Propulsion Laboratory }}

See also the section "detection of methane" for more details.

Sulfur dioxide (SO₂) in the atmosphere would be an indicator of current volcanic activity. It has become especially interesting due to the long-standing controversy of methane on Mars. If volcanoes have been active in recent Martian history, it would be expected to find SO₂ together with methane in the current Martian atmosphere.{{Cite journal|last=Krasnopolsky|first=Vladimir A.|date=2005-11-15|title=A sensitive search for SO2 in the martian atmosphere: Implications for seepage and origin of methane|journal=Icarus|series=Jovian Magnetospheric Environment Science|volume=178|issue=2|pages=487–492|doi=10.1016/j.icarus.2005.05.006|issn=0019-1035|bibcode=2005Icar..178..487K}}{{Cite web|url=https://www.newscientist.com/article/dn8256-volcanoes-ruled-out-for-martian-methane/|title=Volcanoes ruled out for Martian methane|last=Hecht|first=Jeff|website=www.newscientist.com|access-date=2019-06-08|archive-date=8 June 2019|archive-url=https://web.archive.org/web/20190608052038/https://www.newscientist.com/article/dn8256-volcanoes-ruled-out-for-martian-methane/|url-status=live}} No SO₂ has been detected in the atmosphere, with a sensitivity upper limit set at 0.2 ppb.{{cite journal|last1=Krasnopolsky|first1=Vladimir A|year=2012|title=Search for methane and upper limits to ethane and SO2 on Mars|journal=Icarus|volume=217|issue=1|pages=144–152|bibcode=2012Icar..217..144K|doi=10.1016/j.icarus.2011.10.019}}{{cite journal|last1=Encrenaz|first1=T.|author1-link=Thérèse Encrenaz|last2=Greathouse|first2=T. K.|last3=Richter|first3=M. J.|last4=Lacy|first4=J. H.|last5=Fouchet|first5=T.|last6=Bézard|first6=B.|last7=Lefèvre|first7=F.|last8=Forget|first8=F.|last9=Atreya|first9=S. K.|year=2011|title=A stringent upper limit to SO2 in the Martian atmosphere|journal=Astronomy and Astrophysics|volume=530|page=37|bibcode=2011A&A...530A..37E|doi=10.1051/0004-6361/201116820|doi-access=free}} However, a team led by scientists at NASA Goddard Space Flight Center reported detection of SO₂ in Rocknest soil samples analyzed by the Curiosity rover in March 2013.McAdam, A. C.; Franz, H.; Archer, P. D.; Freissinet, C.; Sutter, B.; Glavin, D. P.; Eigenbrode, J. L.; Bower, H.; Stern, J.; Mahaffy, P. R.; Morris, R. V.; Ming, D. W.; Rampe, E.; Brunner, A. E.; Steele, A.; Navarro-González, R.; Bish, D. L.; Blake, D.; Wray, J.; Grotzinger, J.; MSL Science Team (2013). "Insights into the Sulfur Mineralogy of Martian Soil at Rocknest, Gale Crater, Enabled by Evolved Gas Analyses". 44th Lunar and Planetary Science Conference, held 18–22 March 2013 in The Woodlands, Texas. LPI Contribution No. 1719, p. 1751

Carbon monoxide (CO) is produced by the photolysis of CO₂ and quickly reacts with the oxidants in the Martian atmosphere to re-form CO₂. The estimated mean volume ratio of CO in the Martian atmosphere is 0.0747%.

Noble gases, other than helium and argon, are present at trace levels (neon at 2.5 ppmv, krypton at 0.3 ppmv and xenon at 0.08 ppmv) in the Martian atmosphere. The concentration of helium, neon, krypton and xenon in the Martian atmosphere has been measured by different missions.{{Cite journal|last1=Owen|first1=T.|last2=Biemann|first2=K.|last3=Rushneck|first3=D. R.|last4=Biller|first4=J. E.|last5=Howarth|first5=D. W.|last6=Lafleur|first6=A. L.|date=1976-12-17|title=The Atmosphere of Mars: Detection of Krypton and Xenon|journal=Science|volume=194|issue=4271|pages=1293–1295|doi=10.1126/science.194.4271.1293|pmid=17797086|issn=0036-8075|bibcode=1976Sci...194.1293O|s2cid=37362034}}{{Cite journal|last1=Owen|first1=Tobias|last2=Biemann|first2=K.|last3=Rushneck|first3=D. R.|last4=Biller|first4=J. E.|last5=Howarth|first5=D. W.|last6=Lafleur|first6=A. L.|date=1977|title=The composition of the atmosphere at the surface of Mars|journal=Journal of Geophysical Research|volume=82|issue=28|pages=4635–4639|doi=10.1029/JS082i028p04635|issn=2156-2202|bibcode=1977JGR....82.4635O}}{{Cite journal|last1=Krasnopolsky|first1=Vladimir A.|last2=Gladstone|first2=G. Randall|date=2005-08-01|title=Helium on Mars and Venus: EUVE observations and modeling|journal=Icarus|volume=176|issue=2|pages=395–407|doi=10.1016/j.icarus.2005.02.005|issn=0019-1035|bibcode=2005Icar..176..395K}}{{Cite journal|last1=Conrad|first1=P. G.|last2=Malespin|first2=C. A.|last3=Franz|first3=H. B.|last4=Pepin|first4=R. O.|last5=Trainer|first5=M. G.|last6=Schwenzer|first6=S. P.|last7=Atreya|first7=S. K.|last8=Freissinet|first8=C.|last9=Jones|first9=J. H.|date=2016-11-15|title=In situ measurement of atmospheric krypton and xenon on Mars with Mars Science Laboratory|journal=Earth and Planetary Science Letters|volume=454|pages=1–9|doi=10.1016/j.epsl.2016.08.028|issn=0012-821X|url=http://oro.open.ac.uk/47798/1/conrad%2B%2B2016_accepted.pdf|bibcode=2016E&PSL.454....1C|osti=1417813 |access-date=4 July 2019|archive-date=19 July 2018|archive-url=https://web.archive.org/web/20180719154553/http://oro.open.ac.uk/47798/1/conrad%2B%2B2016_accepted.pdf|url-status=live}} The isotopic ratios of noble gases reveal information about the early geological activities on Mars and the evolution of its atmosphere.{{Cite web |url=http://www.jpl.nasa.gov/news/news.php?feature=6631 |title=Curiosity finds evidence of Mars crust contributing to atmosphere |publisher=NASA |department=JPL |access-date=2019-06-08 |archive-date=9 March 2020 |archive-url=https://web.archive.org/web/20200309140512/https://www.jpl.nasa.gov/news/news.php?feature=6631 |url-status=live }}

Molecular hydrogen (H₂) is produced by the reaction between odd hydrogen species in the middle atmosphere. It can be delivered to the upper atmosphere by mixing or diffusion, decompose to atomic hydrogen (H) by solar radiation and escape the Martian atmosphere.{{Cite journal|last=Krasnopolsky|first=V. A.|date=2001-11-30|title=Detection of Molecular Hydrogen in the Atmosphere of Mars|journal=Science|volume=294|issue=5548|pages=1914–1917|doi=10.1126/science.1065569|pmid=11729314|bibcode=2001Sci...294.1914K|s2cid=25856765}} Photochemical modeling estimated that the mixing ratio of H₂ in the lower atmosphere is about 15 ±5 ppmv.

File:Mars annotated-vertical-profiles.png

The vertical temperature structure of the Martian atmosphere differs from Earth's atmosphere in many ways. Information about the vertical structure is usually inferred by using the observations from thermal infrared soundings, radio occultation, aerobraking, landers' entry profiles.{{Cite journal|last=Smith|first=Michael D.|s2cid=102489157|date=May 2008|title=Spacecraft Observations of the Martian Atmosphere|journal=Annual Review of Earth and Planetary Sciences|volume=36|issue=1|pages=191–219|doi=10.1146/annurev.earth.36.031207.124334|issn=0084-6597|bibcode=2008AREPS..36..191S}}{{Cite journal|last1=Withers|first1=Paul|last2=Catling|first2=D. C.|s2cid=26311417|date=December 2010|title=Observations of atmospheric tides on Mars at the season and latitude of the Phoenix atmospheric entry|journal=Geophysical Research Letters|volume=37|issue=24|pages=n/a|doi=10.1029/2010GL045382|bibcode=2010GeoRL..3724204W|doi-access=free}} Mars's atmosphere can be classified into three layers according to the average temperature profile:

• Troposphere (≈0–40 km): The layer where most of the weather phenomena (e.g. convection and dust storms) take place. Its dynamics is heavily driven by the daytime surface heating and the amount of suspended dust. Mars has a higher scale height of 11.1 km than Earth (8.5 km) because of its weaker gravity. The theoretical dry adiabatic lapse rate of Mars is 4.3 °C km⁻¹,{{Cite journal|last=Leovy|first=Conway|date=July 2001|title=Weather and climate on Mars|journal=Nature|volume=412|issue=6843|pages=245–249|doi=10.1038/35084192|pmid=11449286|bibcode=2001Natur.412..245L|s2cid=4383943|issn=1476-4687}} but the measured average lapse rate is about 2.5 °C km⁻¹ because the suspended dust particles absorb solar radiation and heat the air. The planetary boundary layer can extend to over 10 km thick during the daytime.{{Cite journal|last1=Petrosyan|first1=A.|last2=Galperin|first2=B.|last3=Larsen|first3=S. E.|last4=Lewis|first4=S. R.|last5=Määttänen|first5=A.|last6=Read|first6=P. L.|last7=Renno|first7=N.|last8=Rogberg|first8=L. P. H. T.|last9=Savijärvi|first9=H.|date=2011-09-17|journal=Reviews of Geophysics|volume=49|issue=3|pages=RG3005|doi=10.1029/2010RG000351|issn=8755-1209|bibcode=2011RvGeo..49.3005P|title=The Martian Atmospheric Boundary Layer|hdl=2027.42/94893|s2cid=37493454|hdl-access=free}} The near-surface diurnal temperature range is huge (60 °C) due to the low thermal inertia. Under dusty conditions, the suspended dust particles can reduce the surface diurnal temperature range to only 5 °C.{{Cite book|title=Atmospheric evolution on inhabited and lifeless worlds|last=Catling|first=David C.|date=13 April 2017|others=Kasting, James F.|isbn=9780521844123|location=Cambridge|oclc=956434982|bibcode=2017aeil.book.....C}} The temperature above 15 km is controlled by radiative processes instead of convection. Mars is also a rare exception to the "0.1-bar tropopause" rule found in the other atmospheres in our solar system.{{Cite journal|last1=Robinson|first1=T. D.|last2=Catling|first2=D. C.|date=January 2014|title=Common 0.1 bar tropopause in thick atmospheres set by pressure-dependent infrared transparency|journal=Nature Geoscience|volume=7|issue=1|pages=12–15|arxiv=1312.6859|doi=10.1038/ngeo2020|issn=1752-0894|bibcode=2014NatGe...7...12R|s2cid=73657868}}
• Mesosphere (≈40–100 km): The layer that has the lowest temperature. CO₂ in the mesosphere acts as a cooling agent by efficiently radiating heat into space. Stellar occultation observations show that the mesopause of Mars locates at about 100 km (around 0.01 to 0.001 Pa level) and has a temperature of 100–120 K.{{Cite journal|last1=Forget|first1=François|last2=Montmessin|first2=Franck|last3=Bertaux|first3=Jean-Loup|last4=González-Galindo|first4=Francisco|last5=Lebonnois|first5=Sébastien|last6=Quémerais|first6=Eric|last7=Reberac|first7=Aurélie|last8=Dimarellis|first8=Emmanuel|last9=López-Valverde|first9=Miguel A.|date=2009-01-28|title=Density and temperatures of the upper Martian atmosphere measured by stellar occultations with Mars Express SPICAM|journal=Journal of Geophysical Research|volume=114|issue=E1|pages=E01004|doi=10.1029/2008JE003086|issn=0148-0227|url=https://hal.archives-ouvertes.fr/hal-00357038/file/Forget_et_al-2009-Journal_of_Geophysical_Research__Solid_Earth_%281978-2012%29.pdf|bibcode=2009JGRE..114.1004F|s2cid=2660831 |access-date=24 June 2019|archive-date=3 May 2019|archive-url=https://web.archive.org/web/20190503133316/https://hal.archives-ouvertes.fr/hal-00357038/file/Forget_et_al-2009-Journal_of_Geophysical_Research__Solid_Earth_%281978-2012%29.pdf|url-status=live}} The temperature can sometimes be lower than the frost point of CO₂, and detections of CO₂ ice clouds in the Martian mesosphere have been reported.
• Thermosphere (≈100–230 km): The layer is mainly controlled by extreme UV heating. The temperature of the Martian thermosphere increases with altitude and varies by season. The daytime temperature of the upper thermosphere ranges from 175 K (at aphelion) to 240 K (at perihelion) and can reach up to 390 K,{{Cite journal|last1=Bougher|first1=S. W.|last2=Pawlowski|first2=D.|last3=Bell|first3=J. M.|last4=Nelli|first4=S.|last5=McDunn|first5=T.|last6=Murphy|first6=J. R.|last7=Chizek|first7=M.|last8=Ridley|first8=A.|date=February 2015|title=Mars Global Ionosphere-Thermosphere Model: Solar cycle, seasonal, and diurnal variations of the Mars upper atmosphere: BOUGHER ET AL.|journal=Journal of Geophysical Research: Planets|volume=120|issue=2|pages=311–342|doi=10.1002/2014JE004715|hdl=2027.42/110830|s2cid=91178752|hdl-access=free}}{{Cite journal|last1=Bougher|first1=Stephen W.|last2=Roeten|first2=Kali J.|last3=Olsen|first3=Kirk|last4=Mahaffy|first4=Paul R.|last5=Benna|first5=Mehdi|last6=Elrod|first6=Meredith|last7=Jain|first7=Sonal K.|last8=Schneider|first8=Nicholas M.|last9=Deighan|first9=Justin|date=2017|title=The structure and variability of Mars dayside thermosphere from MAVEN NGIMS and IUVS measurements: Seasonal and solar activity trends in scale heights and temperatures|journal=Journal of Geophysical Research: Space Physics|volume=122|issue=1|pages=1296–1313|doi=10.1002/2016JA023454|issn=2169-9402|bibcode=2017JGRA..122.1296B|doi-access=free|hdl=2027.42/136242|hdl-access=free}} but it is still significantly lower than the temperature of Earth's thermosphere. The higher concentration of CO₂ in the Martian thermosphere may explain part of the discrepancy because of the cooling effects of CO₂ in high altitude. It is thought that auroral heating processes is not important in the Martian thermosphere because of the absence of a strong magnetic field in Mars, but the MAVEN orbiter has detected several aurora events.{{Cite web|url=http://www.nasa.gov/image-feature/goddard/maven-captures-aurora-on-mars|title=MAVEN Captures Aurora on Mars|last=Zell|first=Holly|date=2015-05-29|website=NASA|access-date=2019-06-05|archive-date=31 July 2020|archive-url=https://web.archive.org/web/20200731174049/https://www.nasa.gov/image-feature/goddard/maven-captures-aurora-on-mars/|url-status=live}}{{Cite web|url=http://www.nasa.gov/feature/jpl/large-solar-storm-sparks-global-aurora-and-doubles-radiation-levels-on-the-martian-surface|title=NASA Missions See Effects at Mars From Large Solar Storm|last=Greicius|first=Tony|date=2017-09-28|website=NASA|access-date=2019-06-05|archive-date=8 June 2019|archive-url=https://web.archive.org/web/20190608041335/https://www.nasa.gov/feature/jpl/large-solar-storm-sparks-global-aurora-and-doubles-radiation-levels-on-the-martian-surface/|url-status=live}}

Mars does not have a persistent stratosphere due to the lack of shortwave-absorbing species in its middle atmosphere (e.g. stratospheric ozone in Earth's atmosphere and organic haze in Jupiter's atmosphere) for creating a temperature inversion.{{Cite web|url=https://marsed.asu.edu/mep/atmosphere|title=Mars Education {{!}} Developing the Next Generation of Explorers|website=marsed.asu.edu|access-date=2019-06-03|archive-date=3 June 2019|archive-url=https://web.archive.org/web/20190603213152/https://marsed.asu.edu/mep/atmosphere|url-status=live}} However, a seasonal ozone layer and a strong temperature inversion in the middle atmosphere have been observed over the Martian south pole.{{Cite journal|last1=McCleese|first1=D. J.|last2=Schofield|first2=J. T.|last3=Taylor|first3=F. W.|last4=Abdou|first4=W. A.|last5=Aharonson|first5=O.|last6=Banfield|first6=D.|last7=Calcutt|first7=S. B.|last8=Heavens|first8=N. G.|last9=Irwin|first9=P. G. J.|s2cid=128907168|date=November 2008|title=Intense polar temperature inversion in the middle atmosphere on Mars|journal=Nature Geoscience|volume=1|issue=11|pages=745–749|doi=10.1038/ngeo332|issn=1752-0894|bibcode=2008NatGe...1..745M}} The altitude of the turbopause of Mars varies greatly from 60 to 140 km, and the variability is driven by the CO₂ density in the lower thermosphere.{{Cite journal|last1=Slipski|first1=M.|last2=Jakosky|first2=B. M.|last3=Benna|first3=M.|last4=Elrod|first4=M.|last5=Mahaffy|first5=P.|last6=Kass|first6=D.|last7=Stone|first7=S.|last8=Yelle|first8=R.|date=2018|title=Variability of Martian Turbopause Altitudes|journal=Journal of Geophysical Research: Planets|volume=123|issue=11|pages=2939–2957|doi=10.1029/2018JE005704|issn=2169-9100|bibcode=2018JGRE..123.2939S|doi-access=free}} Mars also has a complicated ionosphere that interacts with the solar wind particles, extreme UV radiation and X-rays from Sun, and the magnetic field of its crust.{{Cite web|url=http://sci.esa.int/mars-express/58554-mars-ionosphere-shaped-by-crustal-magnetic-fields/|title=Mars' ionosphere shaped by crustal magnetic fields|website=sci.esa.int|access-date=2019-06-03|archive-date=3 June 2019|archive-url=https://web.archive.org/web/20190603213142/http://sci.esa.int/mars-express/58554-mars-ionosphere-shaped-by-crustal-magnetic-fields/|url-status=live}}{{Cite web|url=http://sci.esa.int/mars-express/51056-new-views-of-the-martian-ionosphere/|title=New views of the Martian ionosphere|website=sci.esa.int|access-date=2019-06-03|archive-date=11 November 2013|archive-url=https://web.archive.org/web/20131111200330/http://sci.esa.int/mars-express/51056-new-views-of-the-martian-ionosphere/|url-status=live}} The exosphere of Mars starts at about 230 km and gradually merges with interplanetary space.File:Solar Wind Strips the Martian Atmosphere.webm accelerates ions from Mars's upper atmosphere into space
(video (01:13); 5 November 2015)|none]]

{{See also|mineral dust|Martian soil#Atmospheric dust}}Under sufficiently strong wind (> 30 ms⁻¹), dust particles can be mobilized and lifted from the surface to the atmosphere. Some of the dust particles can be suspended in the atmosphere and travel by circulation before falling back to the ground. Dust particles can attenuate solar radiation and interact with infrared radiation, which can lead to a significant radiative effect on Mars. Orbiter measurements suggest that the globally-averaged dust optical depth has a background level of 0.15 and peaks in the perihelion season (southern spring and summer).{{Cite journal |last=Smith |first=Michael D. |date=2004-01-01 |title=Interannual variability in TES atmospheric observations of Mars during 1999–2003 |journal=Icarus|series=Special Issue on DS1 / Comet Borrelly |volume=167 |issue=1 |pages=148–165 |doi=10.1016/j.icarus.2003.09.010 |issn=0019-1035 |bibcode=2004Icar..167..148S}} The local abundance of dust varies greatly by seasons and years.{{Cite journal |last1=Montabone |first1=L. |last2=Forget |first2=F. |last3=Millour |first3=E. |last4=Wilson |first4=R.J. |last5=Lewis |first5=S.R. |last6=Cantor |first6=B. |last7=Kass |first7=D. |last8=Kleinböhl |first8=A. |last9=Lemmon |first9=M.T. |display-authors=6 |date=2015-05-01 |title=Eight-year climatology of dust optical depth on Mars |journal=Icarus |series=Dynamic Mars |volume=251 |pages=65–95 |doi=10.1016/j.icarus.2014.12.034 |issn=0019-1035|arxiv=1409.4841 |bibcode=2015Icar..251...65M |s2cid=118336315}} During global dust events, Mars surface assets can observe optical depth that is over 4.{{Cite web|url=https://mars.nasa.gov/resources/21917/atmospheric-opacity-from-opportunitys-point-of-view|title=Atmospheric opacity from Opportunity's point of view|last=NASA/JPL-Caltech/TAMU|website=NASA's Mars Exploration Program|date=12 June 2018 |access-date=2019-06-09|archive-date=9 June 2019|archive-url=https://web.archive.org/web/20190609014006/https://mars.nasa.gov/resources/21917/atmospheric-opacity-from-opportunitys-point-of-view/|url-status=live}}{{Cite journal |last1=Lemmon |first1=Mark T. |last2=Wolff |first2=Michael J. |last3=Bell |first3=James F. |last4=Smith |first4=Michael D. |last5=Cantor |first5=Bruce A. |last6=Smith |first6=Peter H. |date=2015-05-01 |title=Dust aerosol, clouds, and the atmospheric optical depth record over 5 Mars years of the Mars Exploration Rover mission |journal=Icarus|series=Dynamic Mars |volume=251 |pages=96–111 |issn=0019-1035 |arxiv=1403.4234 |s2cid=5194550 |doi=10.1016/j.icarus.2014.03.029 |bibcode=2015Icar..251...96L}} Surface measurements also showed the effective radius of dust particles ranges from 0.6 μm to 2 μm and has considerable seasonality.{{Cite journal |last1=Chen-Chen |first1=H. |last2=Pérez-Hoyos |first2=S. |last3=Sánchez-Lavega |first3=A. |date=2019-02-01 |title=Dust particle size and optical depth on Mars retrieved by the MSL navigation cameras |journal=Icarus |volume=319 |pages=43–57 |doi=10.1016/j.icarus.2018.09.010 |issn=0019-1035|bibcode=2019Icar..319...43C |arxiv=1905.01073 |s2cid=125311345}}{{Cite journal|last1=Vicente-Retortillo |first1=Álvaro |last2=Martínez |first2=Germán M. |last3=Renno |first3=Nilton O. |last4=Lemmon |first4=Mark T. |last5=de la Torre-Juárez |first5=Manuel |date=2017 |title=Determination of dust aerosol particle size at Gale Crater using REMS UVS and Mastcam measurements |journal=Geophysical Research Letters |volume=44 |issue=8 |pages=3502–3508 |doi=10.1002/2017GL072589 |issn=1944-8007 |bibcode=2017GeoRL..44.3502V |doi-access=free|hdl=2027.42/137189 |hdl-access=free }}

Dust has an uneven vertical distribution on Mars. Apart from the planetary boundary layer, sounding data showed that there are other peaks of dust mixing ratio at the higher altitude (e.g. 15–30 km above the surface).{{Cite journal |last1=McCleese |first1=D.J. |last2=Heavens |first2=N.G. |last3=Schofield |first3=J.T. |last4=Abdou |first4=W.A. |last5=Bandfield |first5=J.L. |last6=Calcutt |first6=S.B. |last7=Irwin |first7=P.G.J. |last8=Kass |first8=D.M. |last9=Kleinböhl |first9=A. |display-authors=6 |year=2010 |title=Structure and dynamics of the Martian lower and middle atmosphere as observed by the Mars Climate Sounder: Seasonal variations in zonal mean temperature, dust, and water ice aerosols |journal=Journal of Geophysical Research |series=Planets |volume=115 |issue=E12 |page=E12016 |doi=10.1029/2010JE003677 |bibcode=2010JGRE..11512016M |s2cid=215820851 |issn=2156-2202 |url=https://authors.library.caltech.edu/22051/1/McCleese2010p12537J_Geophys_Res-Planet.pdf |access-date=24 June 2019 |archive-date=15 August 2017 |archive-url=https://web.archive.org/web/20170815155905/http://authors.library.caltech.edu/22051/1/McCleese2010p12537J_Geophys_Res-Planet.pdf |url-status=live }}{{Cite journal |last1=Guzewich |first1=Scott D. |last2=Talaat |first2=Elsayed R. |last3=Toigo |first3=Anthony D. |last4=Waugh |first4=Darryn W. |last5=McConnochie |first5=Timothy H. |year=2013 |title=High-altitude dust layers on Mars: Observations with the Thermal Emission Spectrometer |journal=Journal of Geophysical Research |series=Planets |volume=118 |issue=6 |pages=1177–1194 |doi=10.1002/jgre.20076 |issn=2169-9100 |bibcode=2013JGRE..118.1177G |doi-access=free}}

File:Seasonal variations in oxygen and methane at Gale crater 2012–2017.jpg]]

{{Further|Climate of Mars#Dust storms|Dust storms#On Mars}}

File:2005-1103mars-full.jpg

File:Dust storm co.tif

File:Dust clouds over Mars ESA384856.jpg

File:PIA03170 fig1duststroms.jpg

Local and regional dust storms are not rare on Mars. Local storms have a size of about 10³ km² and occurrence of about 2000 events per Martian year, while regional storms of 10⁶ km² large are observed frequently in southern spring and summer. Near the polar cap, dust storms sometimes can be generated by frontal activities and extra-tropical cyclones.{{Cite journal|last1=Read|first1=P L|last2=Lewis|first2=S R|last3=Mulholland|first3=D P|date=2015-11-04|title=The physics of Martian weather and climate: a review|journal=Reports on Progress in Physics|volume=78|issue=12|pages=125901|doi=10.1088/0034-4885/78/12/125901|pmid=26534887|issn=0034-4885|url=http://oro.open.ac.uk/44802/1/__userdata_documents9_srl89_Desktop_readlewismulholland2015iop.pdf|bibcode=2015RPPh...78l5901R|s2cid=20087052 |access-date=24 June 2019|archive-date=20 July 2018|archive-url=https://web.archive.org/web/20180720203925/http://oro.open.ac.uk/44802/1/__userdata_documents9_srl89_Desktop_readlewismulholland2015iop.pdf|url-status=live}}

Global dust storms (area > 10⁶ km² ) occur on average once every 3 Martian years. Observations showed that larger dust storms are usually the result of merging smaller dust storms, but the growth mechanism of the storm and the role of atmospheric feedbacks are still not well understood. Although it is thought that Martian dust can be entrained into the atmosphere by processes similar to Earth's (e.g. saltation), the actual mechanisms are yet to be verified, and electrostatic or magnetic forces may also play in modulating dust emission. Researchers reported that the largest single source of dust on Mars comes from the Medusae Fossae Formation.{{cite journal|last1=Ojha|first1=Lujendra|last2=Lewis|first2=Kevin|last3=Karunatillake|first3=Suniti|last4=Schmidt|first4=Mariek|date=20 July 2018|title=The Medusae Fossae Formation as the single largest source of dust on Mars|journal=Nature Communications|volume=9|pages=2867|number=2867 (2018)|bibcode=2018NatCo...9.2867O|doi=10.1038/s41467-018-05291-5|pmid=30030425|pmc=6054634}}

{{Main article|2018 Mars global dust storm}}

On 1 June 2018, NASA scientists detected signs of a dust storm (see image) on Mars which resulted in the end of the solar-powered Opportunity rover's mission since the dust blocked the sunlight (see image) needed to operate. By 12 June, the storm was the most extensive recorded at the surface of the planet, and spanned an area about the size of North America and Russia combined (about a quarter of the planet). By 13 June, Opportunity rover began experiencing serious communication problems due to the dust storm.{{cite web|url=https://www.scientificamerican.com/article/as-massive-storm-rages-on-mars-opportunity-rover-falls-silent/|title=As Massive Storm Rages on Mars, Opportunity Rover Falls Silent – Dust clouds blotting out the sun could be the end of the solar-powered probe|last=Malik|first=Tariq|date=13 June 2018|work=Scientific American|access-date=13 June 2018|archive-date=13 June 2018|archive-url=https://web.archive.org/web/20180613180214/https://www.scientificamerican.com/article/as-massive-storm-rages-on-mars-opportunity-rover-falls-silent/|url-status=live}}{{Cite web |last=Wall |first=Mike |date=12 June 2018 |title=NASA's Curiosity Rover Is Tracking a Huge Dust Storm on Mars (Photo) |url=https://www.space.com/40867-nasa-curiosity-rover-mars-dust-storm.html |url-status=live |archive-url=https://web.archive.org/web/20201221104118/https://www.space.com/40867-nasa-curiosity-rover-mars-dust-storm.html |archive-date=21 December 2020 |access-date=13 June 2018 |website=Space.com}}{{cite web|url=https://www.jpl.nasa.gov/news/news.php?feature=7158|title=NASA to Hold Media Teleconference on Martian Dust Storm, Mars Opportunity Rover|last1=Good|first1=Andrew|last2=Brown|first2=Dwayne|date=12 June 2018|work=NASA|access-date=12 June 2018|last3=Wendell|first3=JoAnna|archive-date=21 June 2018|archive-url=https://web.archive.org/web/20180621193755/https://www.jpl.nasa.gov/news/news.php?feature=7158|url-status=live}}{{cite web|url=https://www.jpl.nasa.gov/news/news.php?feature=7160|title=NASA Encounters the Perfect Storm for Science|last=Good|first=Andrew|date=13 June 2018|work=NASA|access-date=14 June 2018|archive-date=25 June 2018|archive-url=https://web.archive.org/web/20180625144655/https://www.jpl.nasa.gov/news/news.php?feature=7160|url-status=live}}{{cite web|url=https://www.youtube.com/watch?v=fIKxdRFx2Wo|title=Mars Dust Storm News – Teleconference – audio (065:22)|author=NASA Staff|date=13 June 2018|work=NASA|access-date=13 June 2018|archive-date=13 June 2018|archive-url=https://web.archive.org/web/20180613173946/https://www.youtube.com/watch?v=fIKxdRFx2Wo|url-status=live}} File:PIA22737-Mars-2018DustStorm-MCS-MRO-Animation-20181030.webm – May to September 2018
(Mars Climate Sounder; Mars Reconnaissance Orbiter)
(1:38; animation; 30 October 2018; file description)]]

{{Main articles|Martian dust devils}}

File:PIA24039-MarsCuriosityRover-DustDevil-20200809.gif on Mars – viewed by the Curiosity rover – (August 9, 2020)