There is debate whether the surface of Mars was in fact ever much warmer than today or whether the geomorphological and mineralogical evidence for the early action of liquid water can be explained by erosion and chemical weathering on the surface and within the crust of a cold, dry planet. To address this, I have used carbonate 'clumped isotope' thermometry to constrain the formation temperature of small carbonate concretions in the oldest available martian rock, the 4.1 billion-year-old meteorite ALH84001. In combination with traditional oxygen and carbon isotope analysis, these measurements showed that the carbonates formed at a temperature of ~20°C, when an aqueous solution derived from the martian atmosphere evaporated [Halevy et al., PNAS, 2011].

Figure 5: Carbonate concretions in the martian meteorite ALH84001.

Testing models of ancient geochemical cycles is often challenging. The environments that they represent are long gone and the products of their operation are not necessarily preserved or easily observed. One way to overcome this is with experiments that simulate small, simplified parts of the system.

A set of mineral precipitation experiments have showed as little as 1 to 10 parts-per-billion SO₂ in an otherwise pure CO₂ atmosphere are enough to inhibit the precipitation of calcium carbonate [Halevy and Schrag, Geophys. Res. Lett., 2009]. The importance of this is in providing a possible explanation for the rarity of carbonate minerals on early martian surfaces. Importantly, inhibition of carbonates occurs at near-neutral pH—conditions that are suitable for formation of clay minerals, which are widely observed on the surface of Mars.

Figure 6: The results of mineral precipitation experiments in O2-poor solutions saturated with both calcite (CaCO3) and hannebachite (CaSO3·0.5H2O). Hannebachite precipitates at the expense of calcite even at SO2 abundances (relative to CO2) much lower than predicted by thermodynamics. The only CaCO3 that finally forms is vaterite, the least stable (most soluble) CaCO3 polymorph, suggesting that sulfite or one of its aqueous derivatives is inhibiting the precipitation of the more stable forms (calcite and aragonite).

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