28.01.2021

Researchers have discovered a common martian mineral deep within an ice core from Antarctica. The find suggests the mineral—a brittle, yellow-brown substance known as jarosite—was forged the same way on both Earth and Mars: from dust trapped within ancient ice deposits. It also reveals how important these glaciers were on the Red Planet: Not only did they carve valleys, the researchers say, but they also helped create the very stuff Mars is made of.

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.

These requirements aren’t easily satisfied on Mars, and scientists began to theorize how the mineral could have become so abundant. Some thought it may have been left behind by the evaporation of small amounts of salty, acidic water. But the alkaline basalt rocks in Mars’s crust would have neutralized the acidic moisture, says Giovanni Baccolo, a geologist at the University of Milan-Bicocca and lead author on the new study.

Another idea was that the jarosite was born within massive ice deposits that might have blanketed the planet billions of years ago. As ice sheets grew over time, dust would have accumulated within the ice—and may have been transformed into jarosite within slushy pockets between ice crystals. But the process had never been observed anywhere in the Solar System.

On Earth, jarosite can be found in piles of mining waste that have been exposed to air and rain, but it’s not common. No one expected to find it in Antarctica, and Baccolo wasn’t hunting for it. Instead, he was searching for minerals that might indicate ice age cycles within the layers of a 1620-meter-long ice core,which record thousands of years of Earth’s history. But in the core’s deepest ice, he came across strange dust particles that he thought might be jarosite.

To confirm the mineral’s identity, Baccolo and his collaborators measured how it absorbed x-rays. They also examined grains under powerful electron microscopes, confirming it was jarosite. The particles were also noticeably cracked and devoid of sharp edges, a sign that they had formed and eroded from chemical assaults in pockets within the ice, the researchers report this month in Nature Communications.

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.”

Baccolo 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.”

Quelle: AAAS