On Mars, clay minerals are widespread in terrains that date back to the Noachian period (4.1 billion to 3.7 billion years ago)1,2,3,4,5. It is thought that the Martian basaltic crust reacted with liquid water during this time to form hydrated clay minerals3,6. Here we propose, however, that a substantial proportion of these clays was formed when Mars’ primary crust reacted with a dense steam or supercritical atmosphere of water and carbon dioxide that was outgassed during magma ocean cooling7,8,9. We present experimental evidence that shows rapid clay formation under conditions that would have been present at the base of such an atmosphere and also deeper in the porous crust. Furthermore, we explore the fate of a primordial clay-rich layer with the help of a parameterized crustal evolution model; we find that the primordial clay is locally disrupted by impacts and buried by impact-ejected material and by erupted volcanic material, but that it survives as a mostly coherent layer at depth, with limited surface exposures. These exposures are similar to those observed in remotely sensed orbital data from Mars1,2,3,4,5. Our results can explain the present distribution of many clays on Mars, and the anomalously low density of the Martian crust in comparison with expectations.
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Thanks to M. Rutherford, T. Hiroi and J. Bosenberg for assistance with experiments and instrumental measurements. Discussions with R. Milliken were helpful in identifying clay minerals in the altered samples.
The authors declare no competing financial interests.
Reviewer Information Nature thanks F. McCubbin, L. Schaefer and the other anonymous reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
Electron-probe microanalyser (EPMA) chemical maps, with silicon mapped as red, aluminium as green and calcium as blue. a, An unaltered sample, where plagioclase appears yellow, pyroxene is purple, olivine is red and glass is orange. b, A sample altered in the liquid field (325 °C, 300 bar), where a thick rind of altered material (arrows) is observed (blue phase, portlandite: a calcium hydroxide). c, A sample altered in the supercritical field with H2O only (425 °C, 300 bar). Alteration can be seen to extend into the interior of the grain (arrows).
The blue dashed line represents a sawtooth LHB with a dynamic instability occurring at 4.1 Ga (ref. 20); the solid red line represents an accretionary tail bombardment. The accretionary tail curve was found by fitting an exponential function through the N50 ages of the five large basins from ref. 35, then scaling to reproduce the correct number of craters for our model grid size (see Methods). The LHB curve was modelled after that in ref. 20. Here, dC50/dt refers to the number of craters of diameter 50 km or more that are formed per time step over the model grid.
The areal clay coverage represents the fraction of surface grid cells with more than 10% clay, assumed to be detectable by orbital remote sensing. The background clay content is the median clay content in surface grid cells located outside of detections. The dashed line at 3.4% areal clay coverage corresponds to the estimate in ref. 4. The light shaded region represents background clay contents of less than 1%, consistent with the lack of detectable clay in X-ray diffraction measurements from Gale crater soils22, and with the lack of clays in Martian regolith breccias23. The dark shaded region represents the confluence of these two constraints, such that model results within or near this region are more consistent with observed clay distributions on the Martian surface.
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Cannon, K., Parman, S. & Mustard, J. Primordial clays on Mars formed beneath a steam or supercritical atmosphere. Nature 552, 88–91 (2017). https://doi.org/10.1038/nature24657
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