The formation of carbonates on Mars have been suggested based on evidence of the presence of liquid water and atmospheric carbon dioxide in the planet's early stages.[1] Moreover, due to their utility in registering changes in environmental conditions such as pH, 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 on Mars have not been found,[4] leading to multiple potential explanations that can explain why carbonates did not form massively on the planet.
Previously, most remote sensing instruments such as OMEGA and THEMIS—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 by magnesite (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 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.[8][9] (note: An IAU naming convention within Gusev is not yet established).
The first successful identification of a strong infrared spectral signature from surficialcarbonate minerals of local scale (< 10 km2) was made by the MRO-CRISM team in 2008.[10] 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 phyllosilicates 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 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 moderate pH and that the resulting carbonates were not exposed to sustained low pH aqueous conditions even as recently as the Hesperian.
When the Thermal and Evolved Gas Analyzer (TEGA) and WCL experiments on the 2009 Phoenix 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]
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."[16][15]
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.[17]
Geological and geomorphological evidence has reinforced the idea of the presence of liquid water on early Mars.[4][18] Therefore, abundant precipitation of carbonates from atmospheric and water reactions is expected. However, spectral imaging has revealed only small amounts of carbonates, generating doubts about humans' understanding of geological processes on Mars.[4] To overcome this problem, scientists have proposed explanations that reconcile the absence of carbonates with the presence of a CO2-rich atmosphere and liquid water.
According to this explanation, the early Martian conditions are similar to those at present.[19] Essentially, it suggests that carbonates are absent because the planet never experienced conditions that included the presence of liquid water and a CO2-rich thick atmosphere. Even if this explanation provides an insight in the reasons why carbonates are not present, it is in disagreement with the geomorphological and mineralogical evidence supporting the existence of liquid water on Mars' surface.[1][4][18]
According to this perspective, massive carbonates 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 eolian deposition and late sedimentation processes.[24]
Finally, this hypothesis defends the idea that carbonates never precipitated because the pH conditions of the environment were too acidic to allow carbonates to precipitate, or at least siderite, which is the primary carbonate mineral expected to precipitate first.[25] 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 carbonates 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.