View source for Carbonates on Mars

{{short description|Overview of the presence of carbonates on Mars}}

[[file:PIA19816-Mars-EstimatingCarbon-Orbiters-20150902.jpg|thumb|400px|right|{{center|Estimating [[carbon]] in the [[Nili Fossae]] plains region of [[Mars]] from orbiters (2 September 2015).}}]]

The formation of '''[[carbonate]]s on [[Mars]]''' have been suggested based on evidence of the presence of liquid water and atmospheric [[carbon dioxide]] in the planet's early stages.<ref name=":02">{{Cite journal |last1=Niles |first1=Paul B. |last2=Catling |first2=David C. |last3=Berger |first3=Gilles |last4=Chassefière |first4=Eric |last5=Ehlmann |first5=Bethany L. |last6=Michalski |first6=Joseph R. |last7=Morris |first7=Richard |last8=Ruff |first8=Steven W. |last9=Sutter |first9=Brad |date=January 2013 |title=Geochemistry of Carbonates on Mars: Implications for Climate History and Nature of Aqueous Environments |url=http://link.springer.com/10.1007/s11214-012-9940-y |journal=Space Science Reviews |volume=174 |issue=1–4 |pages=301–328 |doi=10.1007/s11214-012-9940-y |bibcode=2013SSRv..174..301N |issn=0038-6308}}</ref> Moreover, due to their utility in registering changes in environmental conditions such as [[pH]], [[temperature]], fluid composition,<ref>{{Citation |last1=Bridges |first1=John C. |title=Chapter 5 - Carbonates on Mars |date=2019-01-01 |work=Volatiles in the Martian Crust |pages=89–118 |editor-last=Filiberto |editor-first=Justin |url=https://www.sciencedirect.com/science/article/pii/B9780128041918000052 |access-date=2024-04-30 |publisher=Elsevier |doi=10.1016/b978-0-12-804191-8.00005-2 |isbn=978-0-12-804191-8 |last2=Hicks |first2=Leon J. |last3=Treiman |first3=Allan H. |editor2-last=Schwenzer |editor2-first=Susanne P.}}</ref> [[carbonate]]s have been considered as a primary target for planetary scientists' research.<ref name=":02"/> However, since their first detection in 2008,<ref>{{cite journal |last1=Ehlmann |first1=Bethany L. |last2=Mustard |first2=John F. |last3=Murchie |first3=Scott L. |last4=Poulet |first4=Francois |last5=Bishop |first5=Janice L. |last6=Brown |first6=Adrian J. |last7=Calvin |first7=Wendy M. |last8=Clark |first8=Roger N. |last9=Marais |first9=David J. Des |last10=Milliken |first10=Ralph E. |last11=Roach |first11=Leah H. |last12=Roush |first12=Ted L. |last13=Swayze |first13=Gregg A. |last14=Wray |first14=James J. |date=19 December 2008 |title=Orbital Identification of Carbonate-Bearing Rocks on Mars |url=https://resolver.caltech.edu/CaltechAUTHORS:20121016-112524059 |journal=Science |volume=322 |issue=5909 |pages=1828–1832 |bibcode=2008Sci...322.1828E |doi=10.1126/science.1164759 |pmid=19095939 |s2cid=1190585}}</ref> the large deposits of carbonates that were once{{when|date=September 2024}} expected on Mars have not been found,<ref name=":12">{{Cite journal |last1=Fairén |first1=Alberto G. |last2=Fernández-Remolar |first2=David |last3=Dohm |first3=James M. |last4=Baker |first4=Victor R. |last5=Amils |first5=Ricardo |date=September 2004 |title=Inhibition of carbonate synthesis in acidic oceans on early Mars |url=https://www.nature.com/articles/nature02911 |journal=Nature |volume=431 |issue=7007 |pages=423–426 |doi=10.1038/nature02911 |pmid=15386004 |bibcode=2004Natur.431..423F |issn=1476-4687}}</ref> leading to multiple potential explanations that can explain why carbonates did not form massively on the planet.

== Mars probes ==

Previously, most [[space probe|remote sensing instruments]] such as [[Mars Express|OMEGA]] and [[Mars Odyssey|THEMIS]]—sensitive to infrared emissivity spectral features of carbonates—had not suggested the presence of carbonate outcrops,<ref name=Bibring2006>{{cite journal |last1=Bibring |first1=Jean-Pierre |last2=Langevin |first2=Yves |last3=Mustard |first3=John F. |last4=Poulet |first4=François |last5=Arvidson |first5=Raymond |last6=Gendrin |first6=Aline |last7=Gondet |first7=Brigitte |last8=Mangold |first8=Nicolas |last9=Pinet |first9=P. |last10=Forget |first10=F. |last11=Berthé |first11=Michel |last12=Bibring |first12=Jean-Pierre |last13=Gendrin |first13=Aline |last14=Gomez |first14=Cécile |last15=Gondet |first15=Brigitte |last16=Jouglet |first16=Denis |last17=Poulet |first17=François |last18=Soufflot |first18=Alain |last19=Vincendon |first19=Mathieu |last20=Combes |first20=Michel |last21=Drossart |first21=Pierre |last22=Encrenaz |first22=Thérèse |last23=Fouchet |first23=Thierry |last24=Merchiorri |first24=Riccardo |last25=Belluci |first25=GianCarlo |last26=Altieri |first26=Francesca |last27=Formisano |first27=Vittorio |last28=Capaccioni |first28=Fabricio |last29=Cerroni |first29=Pricilla |last30=Coradini |first30=Angioletta |last31=Fonti |first31=Sergio |last32=Korablev |first32=Oleg |last33=Kottsov |first33=Volodia |last34=Ignatiev |first34=Nikolai |last35=Moroz |first35=Vassili |last36=Titov |first36=Dimitri |last37=Zasova |first37=Ludmilla |last38=Loiseau |first38=Damien |last39=Mangold |first39=Nicolas |last40=Pinet |first40=Patrick |last41=Douté |first41=Sylvain |last42=Schmitt |first42=Bernard |last43=Sotin |first43=Christophe |last44=Hauber |first44=Ernst |last45=Hoffmann |first45=Harald |last46=Jaumann |first46=Ralf |last47=Keller |first47=Uwe |last48=Arvidson |first48=Ray |last49=Mustard |first49=John F. |last50=Duxbury |first50=Tom |last51=Forget |first51=François |last52=Neukum |first52=G. |title=Global Mineralogical and Aqueous Mars History Derived from OMEGA/Mars Express Data |journal=Science |date=21 April 2006 |volume=312 |issue=5772 |pages=400–404 |doi=10.1126/science.1122659 |pmid=16627738 |bibcode=2006Sci...312..400B |s2cid=13968348 }}</ref> at least at the 100 m or coarser spatial scales available from the returned data.<ref name=Catling2007>{{cite journal |last1=Catling |first1=David C. |title=Ancient fingerprints in the clay |journal=Nature |date=July 2007 |volume=448 |issue=7149 |pages=31–32 |doi=10.1038/448031a |pmid=17611529 |s2cid=4387261 }}</ref>

Though ubiquitous, a 2003 study of carbonates on Mars showed that they are dominated by [[magnesite]] (MgCO<sub>3</sub>) in Martian dust, had mass fractions less than 5%, and could have formed under current atmospheric conditions.<ref name=Bandfield2003>{{cite journal |last1=Bandfield |first1=Joshua L. |last2=Glotch |first2=Timothy D. |last3=Christensen |first3=Philip R. |title=Spectroscopic Identification of Carbonate Minerals in the Martian Dust |journal=Science |date=22 August 2003 |volume=301 |issue=5636 |pages=1084–1087 |doi=10.1126/science.1088054 |pmid=12934004 |bibcode=2003Sci...301.1084B |s2cid=38721436 }}</ref> Furthermore, with the exception of the surface dust component, by 2007 [[carbonates]] had not been detected by any in situ mission, even though mineralogic modeling did not preclude small amounts of calcium carbonate in Independence class rocks of [[Husband Hill]] in [[Gusev crater]].<ref>{{cite journal |last1=Tarnas |first1=J. D. |last2=Stack |first2=K. M. |last3=Parente |first3=M. |last4=Koeppel |first4=A. H. D. |last5=Mustard |first5=J. F. |last6=Moore |first6=K. R. |last7=Horgan |first7=B. H. N. |last8=Seelos |first8=F. P. |last9=Cloutis |first9=E. A. |last10=Kelemen |first10=P. B. |last11=Flannery |first11=D. |last12=Brown |first12=A. J. |last13=Frizzell |first13=K. R. |last14=Pinet |first14=P. |title=Characteristics, Origins, and Biosignature Preservation Potential of Carbonate-Bearing Rocks Within and Outside of Jezero Crater |journal=Journal of Geophysical Research: Planets |date=November 2021 |volume=126 |issue=11 |pages=e2021JE006898 |doi=10.1029/2021JE006898 |pmid=34824965 |pmc=8597593 |bibcode=2021JGRE..12606898T }}</ref><ref name=Clark2007>{{cite journal |last1=Clark |first1=B. C. |last2=Arvidson |first2=R. E. |last3=Gellert |first3=R. |last4=Morris |first4=R. V. |last5=Ming |first5=D. W. |last6=Richter |first6=L. |last7=Ruff |first7=S. W. |last8=Michalski |first8=J. R. |last9=Farrand |first9=W. H. |last10=Yen |first10=A. |last11=Herkenhoff |first11=K. E. |last12=Li |first12=R. |last13=Squyres |first13=S. W. |last14=Schröder |first14=C. |last15=Klingelhöfer |first15=G. |last16=Bell |first16=J. F. |title=Evidence for montmorillonite or its compositional equivalent in Columbia Hills, Mars |journal=Journal of Geophysical Research: Planets |date=June 2007 |volume=112 |issue=E6 |doi=10.1029/2006JE002756 |bibcode=2007JGRE..112.6S01C }}</ref> (note: An [[International Astronomical Union|IAU]] naming convention within Gusev is not yet established).

== Remote sensing data ==

The first successful identification of a strong infrared spectral signature from [[surficial]] [[carbonate]] minerals of local scale (< 10&nbsp;km<sup>2</sup>) was made by the [[Mars Reconnaissance Orbiter|MRO]]-[[CRISM]] team in 2008.<ref>{{cite journal |last1=Ehlmann |first1=Bethany L. |last2=Mustard |first2=John F. |last3=Murchie |first3=Scott L. |last4=Poulet |first4=Francois |last5=Bishop |first5=Janice L. |last6=Brown |first6=Adrian J. |last7=Calvin |first7=Wendy M. |last8=Clark |first8=Roger N. |last9=Marais |first9=David J. Des |last10=Milliken |first10=Ralph E. |last11=Roach |first11=Leah H. |last12=Roush |first12=Ted L. |last13=Swayze |first13=Gregg A. |last14=Wray |first14=James J. |title=Orbital Identification of Carbonate-Bearing Rocks on Mars |journal=Science |date=19 December 2008 |volume=322 |issue=5909 |pages=1828–1832 |doi=10.1126/science.1164759 |pmid=19095939 |bibcode=2008Sci...322.1828E |s2cid=1190585 |url=https://resolver.caltech.edu/CaltechAUTHORS:20121016-112524059 }}</ref> Spectral modeling in 2007 identified a key deposit in [[Nili Fossae]] dominated by a single mineral phase that was spatially associated with [[olivine]] outcrops. The dominant mineral appeared to be [[magnesite]], while morphology inferred with [[HiRISE]] and thermal properties suggested that the deposit was lithic. Stratigraphically, this layer appeared between [[phyllosilicate]]s below and [[mafic]] cap rocks above, temporally between the [[Noachian]] and [[Hesperian]] eras. Even though infrared spectra are representative of minerals to less than ≈0.1&nbsp;mm depths<ref>{{cite journal |last1=Poulet |first1=F. |last2=Gomez |first2=C. |last3=Bibring |first3=J.-P. |last4=Langevin |first4=Y. |last5=Gondet |first5=B. |last6=Pinet |first6=P. |last7=Belluci |first7=G. |last8=Mustard |first8=J. |title=Martian surface mineralogy from Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité on board the Mars Express spacecraft (OMEGA/MEx): Global mineral maps |journal=Journal of Geophysical Research: Planets |date=August 2007 |volume=112 |issue=E8 |doi=10.1029/2006JE002840 |bibcode=2007JGRE..112.8S02P |url=https://hal.science/hal-04439179/file/Journal%20of%20Geophysical%20Research%20Planets%20-%202007%20-%20Poulet%20-%20Martian%20surface%20mineralogy%20from%20Observatoire%20pour%20la%20Min%20ralogie.pdf }}</ref> (in contrast to gamma spectra which are sensitive to tens of cm depths),<ref>{{cite journal |last1=Boynton |first1=W. V. |last2=Taylor |first2=G. J. |last3=Evans |first3=L. G. |last4=Reedy |first4=R. C. |last5=Starr |first5=R. |last6=Janes |first6=D. M. |last7=Kerry |first7=K. E. |last8=Drake |first8=D. M. |last9=Kim |first9=K. J. |last10=Williams |first10=R. M. S. |last11=Crombie |first11=M. K. |last12=Dohm |first12=J. M. |last13=Baker |first13=V. |last14=Metzger |first14=A. E. |last15=Karunatillake |first15=S. |last16=Keller |first16=J. M. |last17=Newsom |first17=H. E. |last18=Arnold |first18=J. R. |last19=Brückner |first19=J. |last20=Englert |first20=P. A. J. |last21=Gasnault |first21=O. |last22=Sprague |first22=A. L. |last23=Mitrofanov |first23=I. |last24=Squyres |first24=S. W. |last25=Trombka |first25=J. I. |last26=d'Uston |first26=L. |last27=Wänke |first27=H. |last28=Hamara |first28=D. K. |title=Concentration of H, Si, Cl, K, Fe, and Th in the low- and mid-latitude regions of Mars |journal=Journal of Geophysical Research: Planets |date=December 2007 |volume=112 |issue=E12 |doi=10.1029/2007JE002887 |bibcode=2007JGRE..11212S99B |url=https://digitalcommons.lsu.edu/geo_pubs/1175 }}</ref> stratigraphic,{{clarify|date=August 2019}} morphologic,{{clarify|date=August 2019}} and thermal properties are consistent with the existence of the carbonate as outcrop rather than alteration rinds.{{clarify|date=August 2019}} Nevertheless, the morphology was distinct from typical terrestrial sedimentary carbonate layers suggesting formation from local aqueous alteration of olivine and other igneous minerals. However, key implications were that the alteration would have occurred under moderate [[pH]] and that the resulting carbonates were not exposed to sustained low [[pH]] aqueous conditions even as recently as the [[Hesperian]].

Evidence for widespread presence of carbonates began to increase in 2009, when low levels (<10%) of Mg-rich carbonates were found across the Martian area of [[Syrtis Major Planum|Syrtis Major]], [[Margaritifer Terra]], [[Lunae Palus quadrangle|Lunae Planum]], [[Elysium Planitia]], as reported from analysis of data acquired by the [[Planetary Fourier Spectrometer|Planetary Fourier Spectrometer (PFS)]] on board the [[Mars Express]] spacecraft.<ref>{{cite journal |last1=Palomba |first1=Ernesto |last2=Zinzi |first2=Angelo |last3=Cloutis |first3=Edward A. |last4=D’Amore |first4=Mario |last5=Grassi |first5=Davide |last6=Maturilli |first6=Alessandro |title=Evidence for Mg-rich carbonates on Mars from a 3.9 μm absorption feature |journal=Icarus |date=September 2009 |volume=203 |issue=1 |pages=58–65 |doi=10.1016/j.icarus.2009.04.013 |bibcode=2009Icar..203...58P |s2cid=121348711 |url=https://hal.archives-ouvertes.fr/hal-00567275/file/PEER_stage2_10.1016%252Fj.icarus.2009.04.013.pdf }}</ref>

When the [[Thermal and Evolved Gas Analyzer]] (TEGA) and [[Phoenix (spacecraft)#Wet Chemistry Laboratory (WCL)|WCL]] experiments on the 2009 [[Phoenix (spacecraft)|Phoenix]] Mars lander found between 3–5wt% calcite (CaCO<sub>3</sub>) and an alkaline soil.<ref>{{cite journal |last1=Boynton |first1=W. V. |last2=Ming |first2=D. W. |last3=Kounaves |first3=S. P. |last4=Young |first4=S. M. M. |last5=Arvidson |first5=R. E. |last6=Hecht |first6=M. H. |last7=Hoffman |first7=J. |last8=Niles |first8=P. B. |last9=Hamara |first9=D. K. |last10=Quinn |first10=R. C. |last11=Smith |first11=P. H. |last12=Sutter |first12=B. |last13=Catling |first13=D. C. |last14=Morris |first14=R. V. |title=Evidence for Calcium Carbonate at the Mars Phoenix Landing Site |journal=Science |date=3 July 2009 |volume=325 |issue=5936 |pages=61–64 |doi=10.1126/science.1172768 |pmid=19574384 |bibcode=2009Sci...325...61B |s2cid=26740165 }}</ref> In 2010 analyses by the Mars Exploration Rover Spirit, identified outcrops rich in magnesium-iron carbonate (16–34 wt%) in the Columbia Hills of Gusev crater, most likely precipitated from carbonate-bearing solutions under hydrothermal conditions at near-neutral pH in association with volcanic activity during the Noachian era.<ref name="Morris Ruff Gellert et al 2010">{{cite journal |last1=Morris |first1=Richard V. |last2=Ruff |first2=Steven W. |last3=Gellert |first3=Ralf |last4=Ming |first4=Douglas W. |last5=Arvidson |first5=Raymond E. |last6=Clark |first6=Benton C. |last7=Golden |first7=D. C. |last8=Siebach |first8=Kirsten |last9=Klingelhöfer |first9=Göstar |last10=Schröder |first10=Christian |last11=Fleischer |first11=Iris |last12=Yen |first12=Albert S. |last13=Squyres |first13=Steven W. |title=Identification of Carbonate-Rich Outcrops on Mars by the Spirit Rover |journal=Science |date=23 July 2010 |volume=329 |issue=5990 |pages=421–424 |doi=10.1126/science.1189667 |pmid=20522738 |bibcode=2010Sci...329..421M }}</ref>

After [[Spirit Rover]] stopped working scientists studied old data from the Miniature Thermal Emission Spectrometer, or [[Mini-TES]] and confirmed the presence of large amounts of [[carbonate]]-rich rocks, which means that regions of the planet may have once harbored water. The carbonates were discovered in an outcrop of rocks called "Comanche."<ref>{{cite press release |title=Outcrop of long-sought rare rock on Mars found |url=https://www.sciencedaily.com/releases/2010/06/100603140959.htm |work=ScienceDaily |publisher=Arizona State University |date=4 June 2010 }}</ref><ref name="Morris Ruff Gellert et al 2010"/>

[[Carbonates]] (calcium or iron carbonates) were discovered in a crater on the rim of Huygens Crater, located in the [[Iapygia quadrangle]]. The impact on the rim exposed material that had been dug up from the impact that created Huygens. These minerals represent evidence that Mars once had a thicker carbon dioxide atmosphere with abundant moisture. These kind of carbonates only form when there is a lot of water. They were found with the [[Compact Reconnaissance Imaging Spectrometer for Mars]] (CRISM) instrument on the [[Mars Reconnaissance Orbiter]]. Earlier, the instrument had detected clay minerals. The carbonates were found near the clay minerals. Both of these minerals form in wet environments. It is supposed that billions of years age Mars was much warmer and wetter. At that time, carbonates would have formed from water and the carbon dioxide-rich atmosphere. Later the deposits of carbonate would have been buried. The double impact has now exposed the minerals. Earth has vast carbonate deposits in the form of [[limestone]].<ref>{{cite web|url=http://www.jpl.nasa.gov/news/news.cfm?release=2011-071|title=Some of Mars' Missing Carbon Dioxide May be Buried|website=NASA/JPL|url-status=live|archive-url=https://web.archive.org/web/20111205163915/http://www.jpl.nasa.gov/news/news.cfm?release=2011-071|archive-date=2011-12-05}}</ref>
{| class="wikitable"
|+Carbonates found on Mars
!
!Name
!Mission
!
|-
|MgCO3
|magnesite
|remote sensing, MRO-CRISM
|2008
|-
|MgCO3
|magnesite
|remote sensing Mars Express-PFS
|2009
|-
|CaCO3
|calcite
|Phoenix
|2009
|-
|FeCO3
|siderite
|Curiosity
|2020
|}

== Absence of Carbonates on Mars ==
Geological and geomorphological evidence has reinforced the idea of the presence of [[Liquid water on Mars|liquid water on early Mars]].<ref name=":12"/><ref name=":2">{{Cite journal |last=Wordsworth |first=Robin D. |date=2016-06-29 |title=The Climate of Early Mars |url=https://www.annualreviews.org/doi/10.1146/annurev-earth-060115-012355 |journal=Annual Review of Earth and Planetary Sciences |volume=44 |issue=1 |pages=381–408 |doi=10.1146/annurev-earth-060115-012355 |issn=0084-6597|arxiv=1606.02813 |bibcode=2016AREPS..44..381W }}</ref> Therefore, abundant precipitation of [[carbonate]]s from atmospheric and water reactions is expected. However, [[spectral imaging]] has revealed only small amounts of [[carbonate]]s, generating doubts about humans' understanding of geological processes on [[Mars]].<ref name=":12" /> To overcome this problem, scientists have proposed explanations that reconcile the absence of [[carbonate]]s with the presence of a CO<sub>2</sub>-rich [[atmosphere]] and liquid water.

=== Cold and Dry Early Mars Environments ===
According to this explanation, the early Martian conditions are similar to those at present.<ref>{{Cite journal |last1=Carr |first1=Michael H. |last2=Head |first2=James W. |date=May 2003 |title=Oceans on Mars: An assessment of the observational evidence and possible fate |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2002JE001963 |journal=Journal of Geophysical Research: Planets |volume=108 |issue=E5 |page=5042 |doi=10.1029/2002JE001963 |bibcode=2003JGRE..108.5042C |issn=0148-0227}}</ref> Essentially, it suggests that [[carbonate]]s are absent because the planet never experienced conditions that included the presence of [[Extraterrestrial liquid water|liquid water]] and a CO<sub>2</sub>-rich thick [[atmosphere]]. Even if this explanation provides an insight in the reasons why [[carbonate]]s are not present, it is in disagreement with the geomorphological and mineralogical evidence supporting the existence of [[liquid water on Mars]]' surface.<ref name=":02"/><ref name=":12" /><ref name=":2" />

=== Inability of Detection with Current Technology ===
This hypothesis establishes that the [[Thermal Emission Spectrometer]] (TES) aboard the [[Mars Global Surveyor]] spacecraft and the [[Thermal Emission Imaging System]] (THEMIS) on board the [[2001 Mars Odyssey|Mars Odyssey]] spacecraft are unable to detect [[carbonate]]s.<ref name=":3">{{Cite journal |last1=Kirkland |first1=Laurel E. |last2=Herr |first2=Kenneth C. |last3=Adams |first3=Paul M. |date=December 2003 |title=Infrared stealthy surfaces: Why TES and THEMIS may miss some substantial mineral deposits on Mars and implications for remote sensing of planetary surfaces |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2003JE002105 |journal=Journal of Geophysical Research: Planets |volume=108 |issue=E12 |page=5137 |doi=10.1029/2003JE002105 |bibcode=2003JGRE..108.5137K |issn=0148-0227}}</ref> According to this notion, the [[carbonate]]s indeed formed and are still exist on [[Mars]], but they remain undetected due to the limited sensitivity of the current tools used for mineralogical detection on the planet.<ref name=":3" />

=== Secondary Chemical Alteration ===
This concept involves the potential for secondary chemical alteration of ancient [[carbonate]]s on Mars, due to the formation of [[acid rain]]<ref>{{Cite journal |last1=Craddock |first1=Robert A. |last2=Howard |first2=Alan D. |date=November 2002 |title=The case for rainfall on a warm, wet early Mars |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2001JE001505 |journal=Journal of Geophysical Research: Planets |volume=107 |issue=E11 |page=5111 |doi=10.1029/2001JE001505 |bibcode=2002JGRE..107.5111C |issn=0148-0227}}</ref> resulting from the combination of water vapor and [[sulfate]]s. The consequence of this process implies the [[chemical decomposition]] of superficial [[carbonate]]s layers, as [[carbonate]]s are not resistant to acidic [[pH]] conditions; acid-fog [[weathering]]; and [[Photodecomposition|photo-decomposition]].<ref>{{Cite journal |last=Huguenin |first=Robert L. |date=1974-09-10 |title=The formation of goethite and hydrated clay minerals on Mars |url=http://doi.wiley.com/10.1029/JB079i026p03895 |journal=Journal of Geophysical Research |volume=79 |issue=26 |pages=3895–3905 |doi=10.1029/JB079i026p03895|bibcode=1974JGR....79.3895H }}</ref><ref>{{Cite journal |last1=Mukhin |first1=L. M. |last2=Koscheevi |first2=A. P. |last3=Dikov |first3=Yu P. |last4=Huth |first4=J. |last5=Wänke |first5=H. |date=January 1996 |title=Experimental simulations of the photodecomposition of carbonates and sulphates on Mars |url=https://www.nature.com/articles/379141a0 |journal=Nature |volume=379 |issue=6561 |pages=141–143 |doi=10.1038/379141a0 |pmid=8538763 |bibcode=1996Natur.379..141M |issn=1476-4687}}</ref>

=== Hidden Carbonate Deposits ===
According to this perspective, massive [[carbonate]]s deposits formed but are hidden beneath several layers of secondary alteration rocks, preventing their identification on the surface. Other alternatives to this hypothesis include: Masking of carbonates as a consequence of the abundant soils on Mars; and resurfacing processes that have covered [[carbonate]] deposits, such as [[Aeolian processes|eolian]] deposition and late [[sedimentation]] processes.<ref>{{Cite journal |last=Baker |first=Victor R. |date=July 2001 |title=Water and the martian landscape |url=https://www.nature.com/articles/35084172 |journal=Nature |volume=412 |issue=6843 |pages=228–236 |doi=10.1038/35084172 |pmid=11449284 |bibcode=2001Natur.412..228B |issn=1476-4687}}</ref>

=== Inhibition due to Acidic Conditions ===
Finally, this hypothesis defends the idea that [[carbonate]]s never precipitated because the [[pH]] conditions of the environment were too acidic to allow [[carbonate]]s to precipitate, or at least [[siderite]], which is the primary carbonate mineral expected to precipitate first.<ref>{{Cite journal |last=Catling |first=David C. |date=1999-07-25 |title=A chemical model for evaporites on early Mars: Possible sedimentary tracers of the early climate and implications for exploration |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/1998JE001020 |journal=Journal of Geophysical Research: Planets |volume=104 |issue=E7 |pages=16453–16469 |doi=10.1029/1998JE001020 |bibcode=1999JGR...10416453C |issn=0148-0227}}</ref> The acidic conditions are derived from the high partial pressures of atmospheric [[carbon dioxide]], as well as a persistent [[sulfate]] and [[iron]] enrichment that affect the optimal conditions for [[carbonate]]s to precipitate.<ref name=":12" />

==Gallery==
<gallery>
Image:Huygens Crater.jpg|[[Huygens Crater]] - circle shows location of carbonate deposit - representing a time when Mars had abundant liquid water on its surface (Scale bar = 259&nbsp;km).
Image:PIA19817-Mars-NiliFossae-CarbonateRichDeposit-20150902.jpg|[[Nili Fossae]] on Mars - largest known carbonate deposit.
</gallery>

== See also ==

* {{annotated link|Areography (geography of Mars)}}
* {{annotated link|Chemical gardening}}
* {{annotated link|Chloride-bearing deposits on Mars}}
* {{annotated link|Composition of Mars}}
* {{annotated link|Elysium Planitia}}
* {{annotated link|Fretted terrain}}
* {{annotated link|Geology of Mars}}
* {{annotated link|Glaciers on Mars}}
* {{annotated link|Gravity of Mars}}
* {{annotated link|Groundwater on Mars}}
* {{annotated link|Hecates Tholus}}
* {{annotated link|Iapygia quadrangle}}
* {{annotated link|Lakes on Mars}}
* {{annotated link|Life on Mars}}
* {{annotated link|List of quadrangles on Mars}}
* {{annotated link|List of rocks on Mars}}
* {{annotated link|Magnetic field of Mars}}
* {{annotated link|Mars Geyser Hopper}}
* {{annotated link|Martian craters}}
* {{annotated link|Martian dichotomy}}
* {{annotated link|Martian geyser}}
* {{annotated link|Martian gullies}}
* {{annotated link|Martian soil}}
* {{annotated link|Mineralogy of Mars}}
* {{annotated link|Ore resources on Mars}}
* {{annotated link|Scientific information from the Mars Exploration Rover mission}}
* {{annotated link|Seasonal flows on warm Martian slopes}}
* {{annotated link|Vallis (planetary geology)}}
* {{annotated link|Water on Mars}}
* {{annotated link|Mars carbonate catastrophe}}

==References==
{{Reflist}}
{{Mars}}
{{Portal bar|Solar System}}

{{DEFAULTSORT:Carbonates On Mars}}
[[Category:Rocks on Mars]]
[[Category:Mars]]
[[Category:Planetary science]]