Transformation of jarosite to hematite in simulated Martian brines

The presence of jarosite – a hydrous iron sulfate – at Meridiani Planum supports the past existence of water on Mars surface. Thermodynamic calculations suggest that jarosite should transform to ferric oxyhydroxides in aqueous media. Here we show that jarosite can transform directly to hematite – the iron oxide identified at Meridiani Planum – in simulated Martian brines. Our results are consistent with a short aqueous history, but also indicate that jarosite can persist in highly saline Martian brines. The formation of goethite, a common iron oxyhydroxide on Earth, is prevented by the low water activity and/or high phosphate concentration of Martian brines.