Guest “geologizing” by David Middleton
Substance found in Antarctic ice may solve a martian mystery
By Tess Joosse Jan. 26, 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.
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.
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.”
The terrestrial jarosite was discovered in an ice core from Talos Dome, Antarctica:
Many interpretations have been proposed to explain the presence of jarosite within Martian surficial sediments, including the possibility that it precipitated within paleo-ice deposits owing to englacial weathering of dust. However, until now a similar geochemical process was not observed on Earth nor in other planetary settings. We report a multi-analytical indication of jarosite formation within deep ice. Below 1000 m depth, jarosite crystals adhering on residual silica-rich particles have been identified in the Talos Dome ice core (East Antarctica) and interpreted as products of weathering involving aeolian dust and acidic atmospheric aerosols. The progressive increase of ice metamorphism and re-crystallization with depth, favours the relocation and concentration of dust and the formation of acidic brines in isolated environments, allowing chemical reactions and mineral neo-formation to occur. This is the first described englacial diagenetic mechanism occurring in deep Antarctic ice and supports the ice-weathering model for jarosite formation on Mars, highlighting the geologic importance of paleo ice-related processes on this planet. Additional implications concern the preservation of dust-related signals in deep ice cores with respect to paleoclimatic reconstructions and the englacial history of meteorites from Antarctic blue ice fields.Baccolo et al., 2021
Baccolo, G., Delmonte, B., Niles, P.B. et al. Jarosite formation in deep Antarctic ice provides a window into acidic, water-limited weathering on Mars. Nat Commun 12, 436 (2021). https://doi.org/10.1038/s41467-020-20705-z