Estimatingcarbon in theNili Fossae plains region ofMars from orbiters (2 September 2015).
The formation ofcarbonates on Mars have been suggested based on evidence of the presence of liquid water and atmosphericcarbon dioxide in the planet's early stages.[1] Moreover, due to their utility in registering changes in environmental conditions such aspH,temperature, fluid composition,[2]carbonates have been considered as a primary target for planetary scientists' research.[1] However, since their first detection in 2008,[3] the large deposits of carbonates that were once[when?] expected onMars have not been found,[4] leading to multiple potential explanations that can explain why carbonates did not form massively on the planet.
Previously, mostremote sensing instruments such asOMEGA andTHEMIS—sensitive to infrared emissivity spectral features of carbonates—had not suggested the presence of carbonate outcrops,[5] at least at the 100 m or coarser spatial scales available from the returned data.[6]
Though ubiquitous, a 2003 study of carbonates on Mars showed that they are dominated bymagnesite (MgCO3) in Martian dust, had mass fractions less than 5%, and could have formed under current atmospheric conditions.[7] Furthermore, with the exception of the surface dust component, by 2007carbonates 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 ofHusband Hill inGusev crater.[8][9] (note: AnIAU naming convention within Gusev is not yet established).
The first successful identification of a strong infrared spectral signature fromsurficialcarbonate minerals of local scale (< 10 km2) was made by theMRO-CRISM team in 2008.[10] Spectral modeling in 2007 identified a key deposit inNili Fossae dominated by a single mineral phase that was spatially associated witholivine outcrops. The dominant mineral appeared to bemagnesite, while morphology inferred withHiRISE and thermal properties suggested that the deposit was lithic. Stratigraphically, this layer appeared betweenphyllosilicates below andmafic cap rocks above, temporally between theNoachian andHesperian eras. Even though infrared spectra are representative of minerals to less than ≈0.1 mm depths[11] (in contrast to gamma spectra which are sensitive to tens of cm depths),[12] stratigraphic,[clarification needed] morphologic,[clarification needed] and thermal properties are consistent with the existence of the carbonate as outcrop rather than alteration rinds.[clarification needed] 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 moderatepH and that the resulting carbonates were not exposed to sustained lowpH aqueous conditions even as recently as theHesperian.
When theThermal and Evolved Gas Analyzer (TEGA) andWCL experiments on the 2009Phoenix Mars lander found between 3–5wt% calcite (CaCO3) and an alkaline soil.[14] 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.[15]
AfterSpirit Rover stopped working scientists studied old data from the Miniature Thermal Emission Spectrometer, orMini-TES and confirmed the presence of large amounts ofcarbonate-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."[16][15]
Carbonates (calcium or iron carbonates) were discovered in a crater on the rim of Huygens Crater, located in theIapygia 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 theCompact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument on theMars 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 oflimestone.[17]
Geological and geomorphological evidence has reinforced the idea of the presence ofliquid water on early Mars.[4][18] Therefore, abundant precipitation ofcarbonates from atmospheric and water reactions is expected. However,spectral imaging has revealed only small amounts ofcarbonates, generating doubts about humans' understanding of geological processes onMars.[4] To overcome this problem, scientists have proposed explanations that reconcile the absence ofcarbonates with the presence of a CO2-richatmosphere and liquid water.
According to this explanation, the early Martian conditions are similar to those at present.[19] Essentially, it suggests thatcarbonates are absent because the planet never experienced conditions that included the presence ofliquid water and a CO2-rich thickatmosphere. Even if this explanation provides an insight in the reasons whycarbonates are not present, it is in disagreement with the geomorphological and mineralogical evidence supporting the existence ofliquid water on Mars's surface.[1][4][18]
According to this perspective, massivecarbonates 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 coveredcarbonate deposits, such aseolian deposition and latesedimentation processes.[24]
Finally, this hypothesis defends the idea thatcarbonates never precipitated because thepH conditions of the environment were too acidic to allowcarbonates to precipitate, or at leastsiderite, which is the primary carbonate mineral expected to precipitate first.[25] The acidic conditions are derived from the high partial pressures of atmosphericcarbon dioxide, as well as a persistentsulfate andiron enrichment that affect the optimal conditions forcarbonates to precipitate.[4]
Huygens Crater - circle shows location of carbonate deposit - representing a time when Mars had abundant liquid water on its surface (Scale bar = 259 km).
Nili Fossae on Mars - largest known carbonate deposit.
^Clark, B. C.; Arvidson, R. E.; Gellert, R.; Morris, R. V.; Ming, D. W.; Richter, L.; Ruff, S. W.; Michalski, J. R.; Farrand, W. H.; Yen, A.; Herkenhoff, K. E.; Li, R.; Squyres, S. W.; Schröder, C.; Klingelhöfer, G.; Bell, J. F. (June 2007). "Evidence for montmorillonite or its compositional equivalent in Columbia Hills, Mars".Journal of Geophysical Research: Planets.112 (E6).Bibcode:2007JGRE..112.6S01C.doi:10.1029/2006JE002756.hdl:1893/17119.
^Boynton, W. V.; Taylor, G. J.; Evans, L. G.; Reedy, R. C.; Starr, R.; Janes, D. M.; Kerry, K. E.; Drake, D. M.; Kim, K. J.; Williams, R. M. S.; Crombie, M. K.; Dohm, J. M.; Baker, V.; Metzger, A. E.; Karunatillake, S.; Keller, J. M.; Newsom, H. E.; Arnold, J. R.; Brückner, J.; Englert, P. A. J.; Gasnault, O.; Sprague, A. L.; Mitrofanov, I.; Squyres, S. W.; Trombka, J. I.; d'Uston, L.; Wänke, H.; Hamara, D. K. (December 2007)."Concentration of H, Si, Cl, K, Fe, and Th in the low- and mid-latitude regions of Mars".Journal of Geophysical Research: Planets.112 (E12).Bibcode:2007JGRE..11212S99B.doi:10.1029/2007JE002887.
^Boynton, W. V.; Ming, D. W.; Kounaves, S. P.; Young, S. M. M.; Arvidson, R. E.; Hecht, M. H.; Hoffman, J.; Niles, P. B.; Hamara, D. K.; Quinn, R. C.; Smith, P. H.; Sutter, B.; Catling, D. C.; Morris, R. V. (3 July 2009). "Evidence for Calcium Carbonate at the Mars Phoenix Landing Site".Science.325 (5936):61–64.Bibcode:2009Sci...325...61B.doi:10.1126/science.1172768.PMID19574384.S2CID26740165.
^abMorris, Richard V.; Ruff, Steven W.; Gellert, Ralf; Ming, Douglas W.; Arvidson, Raymond E.; Clark, Benton C.; Golden, D. C.; Siebach, Kirsten; Klingelhöfer, Göstar; Schröder, Christian; Fleischer, Iris; Yen, Albert S.; Squyres, Steven W. (23 July 2010). "Identification of Carbonate-Rich Outcrops on Mars by the Spirit Rover".Science.329 (5990):421–424.Bibcode:2010Sci...329..421M.doi:10.1126/science.1189667.PMID20522738.
