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Electrochemistry could explain Mars’s organics

Brines and metal-rich minerals could reduce CO₂ to make alcohols and more

by Sam Lemonick
November 2, 2018

A photograph of NASA's Curiosity rover on Mars.
Credit: NASA/JPL-Caltech/MSSS
The National Aeronautics & Space Administration's Curiosity rover found deposits of organic molecules in rocks on Mars.

Scientists have identified aliphatic and aromatic organic molecules on the surface of Mars and in martian meteorites over the last several decades. The source of these molecules is unknown, but some scientists suggest they formed during meteorite impacts or through geological processes. Now one team proposes a novel explanation: As salty liquids flowed through microscopic fissures in metal-rich minerals on the red planet, electrochemical reduction of carbon dioxide (ERC) produced small organic molecules (Sci. Adv. 2018, DOI: 10.1126/sciadv.aat5118). Chemists are intrigued by the mechanism, but some say they want to see more evidence that the chemistry is plausible.

Andrew Steele of the Carnegie Institution for Science has been studying martian organics when he noticed signs of corrosion in meteorites in which the organic molecules had been found. The corrosion made him wonder if an electrochemical process could have created the molecules. That led him to ERC—a process that chemists have used in which an electrochemical potential drives reduction of CO2 to yield CO and simple organics.

Credit: Andrew Steele
Scientists propose that the minerals titano-magnetite (a) and pyrite (b) sandwiching brine in cracks between them (c)--like in this slice of a martian meteorite--could form galvanic cells that electrochemically reduce carbon dioxide to organic molecules.

He and his colleagues propose that neighboring micrometer-thick layers of iron-rich and iron-poor minerals could act as electrodes in a galvanic cell, with a brine moving through cracks in the rock serving as the electrolyte. At low pH, Steele says, electrochemical reactions could reduce CO2 dissolved in the brine to make organic molecule like methane and formate.

“This inspiring study shows how materials complexity can translate into very interesting localized reaction conditions,” electrochemist Frank Marken of the University of Bath says. He adds that the process could be the first step in developing a chemical environment that could be compatible with the beginnings of life. And Kirsten Siebach, a planetary geologist at Rice University, says the process could have led to organics on Earth too.

But Marken and others aren’t ready to say the researchers’ explanation is complete. They are skeptical that the proposed system has enough energy to reduce CO2 and suggest another source, such as light, might be needed. David Fermin, an electrochemist at the University of Bristol, says one way to test the energetics of the system is to replicate it in a lab.

Steele says he’s working on such a demonstration, and says he hasn’t been able to model the system computationally because many variables are still unknown. But he explains that electrical potentials can increase dramatically in nanoscale galvanic cells. William Zimmerman, who studies microfluidic systems, agrees that these ERC reactions are “perfectly plausible” at the proposed scales.


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