It's All About the Sulphides

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Science  13 Jul 2012:
Vol. 337, Issue 6091, pp. 134
DOI: 10.1126/science.337.6091.134-d

Both hydrated sulphates and carbonates have been observed on the surface of Mars. However, these two types of minerals form under very different conditions, with hydrated sulphates requiring acidic conditions that are incompatible with the formation or preservation of carbonates. Dehouck et al. explored the alteration of terrestrial basaltic minerals, similar to those found on Mars, under a simulated Mars-like atmosphere. After a 4-year-long exposure to a CO2-dominated humid atmosphere, samples without iron sulphide suffered only minor alteration, whereas mixtures of silicates and sulphides led to acidic conditions that favored the precipitation of hydrated sulphates. Infrared reflectance spectra of the samples showed good agreement with data from Mars. Thus, rather than being the result of planet-wide acidic conditions, as has been widely believed, martian hydrated sulphates may have formed from sulphide-rich basalts that produced locally acidic environments. With regional variations in bedrock composition controlling the distribution of alteration minerals on the surface of Mars, rather than a global change of atmospheric chemistry, it is possible for the carbonates to form coevally under the same atmospheric conditions.

Geochim. Cosmochim. Acta 90, 47 (2012).

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