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A volcanic environment for bedrock diagenesis at Meridiani Planum on Mars


Exposed bedrocks at Meridiani Planum on Mars display chemical and mineralogical evidence suggesting interaction with liquid water1,2,3,4,5,6. On the basis of morphological observations as well as high abundances of haematite and sulphate minerals, the rocks have been interpreted as sediments that were deposited in a shallow body of briny water with subsequent evaporation leaving behind the sulphate minerals1,2. The iron-sulphur mineralization at Meridiani has also been inferred to be analogous to that produced during oxidative weathering of metal sulphide minerals, such as occurs at acid mine drainage sites7. Neither of these interpretations, however, is consistent with the chemical composition of the rocks. Here we propose an alternative model for diagenesis of Meridiani bedrock that involves deposition of volcanic ash followed by reaction with condensed sulphur dioxide- and water-bearing vapours emitted from fumaroles. This scenario does not require prolonged interaction with a standing body of surface water and may have occurred at high temperatures. Consequently, the model invokes an environment considerably less favourable for biological activity on Mars than previously proposed interpretations.

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Figure 1: Ternary diagram comparing the molar chemical compositions of Meridiani bedrocks analysed by Opportunity 4 with typical martian basalts.
Figure 2: Comparison of fine-scale morphologic features of bedrock from Meridiani Planum (a) with a terrestrial volcaniclastic deposit (b).


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This research was supported in part by the NASA Astrobiology Institute.

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Correspondence to Thomas M. McCollom.

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McCollom, T., Hynek, B. A volcanic environment for bedrock diagenesis at Meridiani Planum on Mars. Nature 438, 1129–1131 (2005).

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