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Mars is a large enough planet that astrobiologists looking for life need to narrow the parameters of the search to those environments most conducive to habitability.

NASA's Mars Curiosity mission is exploring such a spot right now at its landing site around Gale Crater, where the rover has found extensive evidence of past water and is gathering information on methane in the atmosphere, a possible signature of microbial activity.

But where would life most likely gain energy from its surroundings? One possibility is in an environment that includes "green rust," a partially oxidized iron mineral. A fully oxidized iron "rust"—one exposed to oxidation for long enough—turns orangey-red, similar to the color of Mars' regolith. When oxidization is incomplete, however, the iron rust is greenish.

This means that there are two different "redox states," or types of iron with different numbers of electrons in the same mineral. This difference between the two iron redox states could allow the mineral to take in or give up electrons and thus act as a catalyst, said Laurie Barge, a planetary scientist at NASA's Jet Propulsion Laboratory. She studies hydrothermal vents, an area where chemical contrasts also fuel life.

"From an environmental science perspective, green rust can absorb and concentrate nutrients, and can also accept and donate electrons for life," said Barge.

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