Jarosite was first spotted on Mars in 2004, when the NASA Opportunity rover rolled over fine-grained layers of it. The discovery made headlines because jarosite needs water to form, along with iron, sulfate, potassium, and acidic conditions. The work suggests jarosite forms the same way on Mars, says Megan Elwood Madden, a geochemist at the University of Oklahoma who was not involved with the research. But she wonders whether the process can explain the huge abundance of jarosite on Mars. “On Mars, this is not just some thin film,” she says. “These are meters-thick deposits.”

[Giovanni Baccolo, a geologist at the University of Milan-Bicocca] concedes that the ice core contained only small amounts of jarosite, particles smaller than an eyelash or a grain of sand. But he explains that there’s much more dust on Mars than in Antarctica, which only receives small amounts of airborne ash and dirt from northern continents. “Mars is such a dusty place — everything is covered in dust,” Baccolo says. More ash would favor more jarosite formation under the right conditions, he says. Baccolo wants to use Antarctic cores to investigate whether ancient martian ice deposits were cauldrons for the formation of other minerals. He says jarosite shows how glaciers weren’t just land carving machines, but might have contributed to Mars’s chemical makeup. “This is just the first step in linking deep Antarctic ice with the martian environment.” The researchers reported their findings this month in Nature Communications.

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