Impact origin of sediments at the Opportunity landing site on Mars

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Mars Exploration Rover Opportunity discovered sediments with layered structures thought to be unique to aqueous deposition and with minerals attributed to evaporation of an acidic salty sea. Remarkable iron-rich spherules were ascribed to later groundwater alteration, and the inferred abundance of water reinforced optimism that Mars was once habitable. The layered structures, however, are not unique to water deposition, and the scenario encounters difficulties in accounting for highly soluble salts admixed with less soluble salts, the lack of clay minerals from acid–rock reactions, high sphericity and near-uniform sizes of the spherules and the absence of a basin boundary. Here we present a simple alternative explanation involving deposition from a ground-hugging turbulent flow of rock fragments, salts, sulphides, brines and ice produced by meteorite impact. Subsequent weathering by intergranular water films can account for all of the features observed without invoking shallow seas, lakes or near-surface aquifers. Layered sequences observed elsewhere on heavily cratered Mars and attributed to wind, water or volcanism may well have formed similarly. If so, the search for past life on Mars should be reassessed accordingly.

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Figure 1: Nevada Test Site nuclear test explosion that produced crater Sedan.
Figure 2: Plots of surge deposit thickness versus radial distance.
Figure 3: Terrestrial surge deposits compared with cross-stratified martian deposits.
Figure 4: Martian strata compared with terrestrial surge strata.
Figure 5: Martian spherules compared with terrestrial accretionary lapilli and impact spherules.


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L.P.K. is supported by the NASA Exobiology Program. We thank C. Moore for supplying the iron condensation spherules from Meteor crater, and G. R. Osinski for comments on an early version of the manuscript. Author Contributions All authors contributed equally to the ideas and interpretations. L.P.K. wrote the initial draft and managed revisions.

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Correspondence to L. Paul Knauth.

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