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Nature 443, E2 (7 September 2006) | doi:10.1038/nature05213; Published online 6 September 2006
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Planetary science: Bedrock formation at Meridiani Planum (Reply)
Thomas M. McCollom1,2 & Brian M. Hynek2
Abstract
Squyres et al.1 contend that our proposed volcanic origin for Meridiani Planum2 is inconsistent with more recently obtained data3. But although the new data reveal some variation in chemical composition, this variation is small (Fig. 1a) and mainly due to modest variations in magnesium and sulphur, with concentrations of the other elements remaining essentially constant3. In a volcanic model, this variation can be readily explained by mobilization of highly soluble magnesium sulphate salts during the later stages of alteration and diagenesis (Fig. 1a), as in the sedimentary/evaporite model in which sediments that were initially deposited with uniform composition are subsequently modified3, 4, 5. Although morphological features in the bedrock may be consistent with aeolian and fluvial origins6, this interpretation is not unique, particularly as features with similar grain size, sorting and morphology are seen in base surge deposits2, 7, 8, 9. Neither chemical nor morphological data therefore preclude a volcanic origin.
Figure 1: Ternary diagrams showing relative molar abundances of major elements for Meridiani bedrock.
Diagrams include data discussed by Squyres et al.1 and typical martian basalts2. a, In a volcanic scenario2, bedrock compositions are attributable to reaction of basaltic ash with sulphuric acid from volcanic vapours. Minor scattering among compositions can be accounted for by mobilization of magnesium sulphate salts in the later stages of alteration (arrows). b, In a sedimentary/evaporite scenario, extrapolation from bedrock compositions to remove sulphate salts would require the original siliciclastic component to be substantially depleted in divalent cations and enriched in Si + Al relative to martian basalt12.
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