Jarosite
Jarosite is a potassium iron sulfate that forms in acidic water, found in some Martian meteorites and on Mars itself.
Jarosite is a yellow-brown sulfate of potassium and iron containing hydroxyl. On Earth it forms where acidic, sulfur-rich water evaporates, for example in mine waste and around hot springs.
Jarosite has been identified in the Martian nakhlite meteorite MIL 03346, found in Antarctica, where it lies in veins along with other alteration minerals. NASA's Opportunity rover also found jarosite in rocks at Meridiani Planum in 2004, which was strong evidence that acidic water once soaked parts of the Martian surface.
Because jarosite forms only from acidic, oxidising water, finding it in a Martian meteorite supports the idea that such water acted near the Martian surface at times in the planet's history. Jarosite also contains potassium, which in principle allows it to be dated by the argon–argon method. Jarosite is a reminder that meteorites from Mars carry a record of its watery past.
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