Abstract
Perchlorate (ClO4−) and possibly chlorate (ClO3−) are considered to be ubiquitous on Mars1,2,3,4,5, and the ClO3−/ClO4− abundance ratio has critical implications for the redox conditions6,7, aqueous environments8,9 and habitability on Mars10. However, factors that control the ClO3−/ClO4− generation ratios are not well established. Here we expose mixtures of halite salt (NaCl) with Fe sulfates, Fe (hydr)oxides and Fe3+ montmorillonite to ultraviolet radiation or ozone in an Earth or CO2 atmosphere and show that Fe secondary mineralogy is the dominant factor controlling the ClO3−/ClO4− generation ratio: the sulfates and montmorillonite mixtures produce much higher yields of ClO4− than of ClO3−, whereas the opposite is true for the (hydr)oxide mixtures. Consistent with previous studies11,12,13,14,15,16,17,18, our results indicate that the physical state of chloride (Cl−) (that is, solid, liquid or gas) and the characteristics of the co-occurring minerals (for example, semiconductivity, surface area, acidity) have the greatest influence, whereas oxidation sources (ultraviolet radiation or ozone) and atmospheric composition induce only secondary effects. We conclude that, under the hyperarid climate and widespread Fe (hydr)oxide abundances prevailing on Mars since the Amazonian period19, Cl− oxidation should produce yields of ClO3− that are orders of magnitude higher than those of ClO4−, highlighting the importance of ClO3− in the surficial environments and habitability of modern Mars compared with ClO4−.
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Data availability
All data generated and analysed in this study are included in the Article and Supplementary Tables 1 and 2. A complete dataset for this study44 is also available at Science Data Bank at https://doi.org/10.11922/sciencedb.00914. Source data are provided with this paper.
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Acknowledgements
This work was supported by B-type Strategic Priority Program of the Chinese Academy of Sciences (grant no. XDB41000000) to Y.-Y.S.Z., X.-Y.L. and J.-Z.L.; the Key Research Program of the Institute of Geology & Geophysics CAS (grant no. IGGCAS-201905) to Y.-Y.S.Z. and C.Q.; the Pre-research Project on Civil Aerospace Technologies of the CNSA (grant no. D020102), the National Natural Science Foundation of China (grant no. 41673072) and the West Light Foundation of the CAS to Y.-Y.S.Z.; the National Key Scientific Instrument and Equipment Development Project (2012YQ090229) and Scientific Instrument Upgrading Project of Shandong Province (2012SGGZ18) to H.C.; the National Natural Science Foundation of China (grant nos 42173045 and 41573056) to Z.-C.W.; the International Partnership Program of the CAS (grant no. 121421KYSB20170020) to J.-H.W.; the National Natural Science Foundation of China (grant no. 41931077), the Pre-research Project on Civil Aerospace Technologies of CNSA (D020201) and the Youth Innovation Promotion Association of CAS (Y201867) to X.-Y.L.; and the Key Research Program of Frontier Sciences, CAS (grant no. QYZDY-SSW-DQC028) to J.-Z.L.
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S.-Y.Q. and Y.-Y.S.Z. designed the study and wrote a first draft of the manuscript. S.-Y.Q. conducted experiments and performed sample measurements and data analyses. H.C. and X.-Z.Y. performed analyses of oxychlorine species. W.A.J., X.N. and Z.-C.W. contributed to discussions on the pathways and mechanisms of perchlorate and chlorate formation. J.-H.W. and D.-S.Z. provided Mössbauer analysis and data interpretation of akaganeite samples. C.Q., X.-Y.L. and J.-Z.L. contributed to essential discussion on the design of this study and supervised the projects. All authors discussed the results, revised the manuscript and approved the final revisions.
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Extended data
Extended Data Fig. 1 Experimental apparatus used for UV and O3 experiments in this study.
(a) A 5-liter borosilicate reaction chamber was inserted with two 254 nm ultraviolet (UV) lamps (UVP Pen-ray PCQ Lamp, part number 90-0049-06). The lamp emitted 8.5 mW cm−2 at a 2-cm distance from the light source with 90% of the emission at λ = 254 nm and 10% of the emission at λ > 254 nm (up to 436 nm). Two equal aliquots of initial mineral mixtures were placed at the bottom of the reaction chamber. The water bath maintained 25 °C of the chamber, and simulant atmospheric gas was continuously flushing through the chamber at a rate of 0.7 L min−1. The chamber was covered entirely with heavy-duty aluminum foil during the experiment to eliminate UV leakage and prevent interference from other wavelength light sources. (b) The ozone reaction vessel was customized using quartz, and the initial solid or solution mixtures can be sealed in the vessel. During each experiment, 30 mg L−1 O3 gas from the ozone generator continuously flowed through the reaction vessel at a rate of 30 mL min−1 for 24 h. The vessel outlet was connected to the trapping flasks using Na2S2O3 and NaOH solutions to remove O3 and other potential reactive intermediate products in the exhaust gas.
Extended Data Fig. 2 Initial mineral mixtures used in the UV and O3 experiments.
Each Fe mineral, except for amorphous Fe3+-sulfate gel and akaganeite, was mixed with halite (mass ratio 7:1) and continuously shaken for 12 h for homogeneity. Amorphous Fe3+-sulfate gel was first mixed with a saturated NaCl solution with the same amount of initial Cl− as the halite salt, and then the mixture was air-dried in a fume hood before use. Akaganeite contained chlorine in its structure, so no additional NaCl was added. The mixture of halite-ferricopipiate-rhomboclase presents a partial solution after shaking due to crystalline water release from the ferricopipiate and rhomboclase (see Extended Data Figure 5 for details).
Extended Data Fig. 3 Correlations of ClO3−/ClO4− molar ratios versus ClO3− or ClO4− concentrations.
Data symbols represent the yielded ClO3− and ClO4− and their corresponding ClO3−/ClO4− molar ratios of the UV + CO2, UV + Earth, and mineral-O3 experiments. Variations of ClO3−/ClO4− are moderately correlated to the ClO3− concentrations (R2 = 0.7834) but not correlated to the ClO4− concentrations (R2 = 0.0251). Therefore, changes in ClO3− concentrations are likely responsible for the variation of ClO3−/ClO4− ratios. The uncertainties of ClO4− molar yields and ClO3−/ClO4− molar ratios are smaller than the symbols and thus are not shown.
Extended Data Fig. 4 Correlations between total ClOx− yields and specific surface area of minerals.
In (a) and (b), Fe (hydr)oxides (magnetite, goethite, akaganeite, and ferrihydrite) are shown as squares, and Fe montmorillonite is shown as a triangle. (a) In the mineral-O3 experiments, a strong correlation presents between the ClOx− yields and surface area of the four Fe (hydr)oxides (R2 = 0.7852), while Fe montmorillonite does not follow the trend. (b) Under UV + Earth conditions, ClOx− yields and specific surface area are strongly correlated for magnetite, goethite, and akaganeite (R2 = 0.9276), while ferrihydrite and Fe montmorillonite are not correlated. The absent correlation might be due to the poor crystallinity of 2-line ferrihydrite since the photocatalytic efficiency is related to the crystallinity of the catalyst. (c) Neither Fe (hydr)oxides nor Fe montmorillonite under UV + CO2 conditions shows apparent correlations between the ClOx− yields and the specific surface area of the minerals. (d) In the NaCl solution-O3 experiments, minerals (quartz, ferrihydrite, and magnetite) mixed in the NaCl solution demonstrate a strong correlation between the ClOx− yields and specific surface area of the minerals, regardless of the types of the minerals. In the solution, the mineral surface mainly provides an interface for the O3 oxidation processes. Uncertainties of ClOx− molar yields cannot be shown on the log coordinate in subfigure (c). Uncertainties that are smaller than the space occupied by the symbols are not shown in subfigures (a), (b) and (d).
Extended Data Fig. 5 A synthetic mixture of ferricopiapite and rhomboclase showed partial dissolution and dehydration during the sample preparation and subsequent experiments.
(a) Synthetic ferricopiapite and rhomboclase; (b) ferricopiapite and rhomboclase mixture was mixed and shaken with NaCl salt for 12 h, and resulted in the partial dissolution of the mixture. (c), (d) after UV and O3 experiments, the mineral mixture became dry again. The presence of partial solution in the initial salt mixture before the experiments may account for the increase of ClO3− yields both in the UV and O3 experiments. The water originated from the sulfate mixture, as the ferricopiapite and rhomboclase contain nominally 30.72% and 25.25% of crystalline water, respectively.
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Supplementary Tables 1 and 2.
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Data table for plotting Fig. 2.
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Source Data Extended Data Fig. 3
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Source Data Extended Data Fig. 4
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Qu, SY., Zhao, YY.S., Cui, H. et al. Preferential Formation of Chlorate over Perchlorate on Mars Controlled by Iron Mineralogy. Nat Astron 6, 436–441 (2022). https://doi.org/10.1038/s41550-021-01588-6
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DOI: https://doi.org/10.1038/s41550-021-01588-6
