What Red Planet fossils might look like

The minerals surrounding the fossils changed as the sediments cemented to form rock. Finely grained minerals encased fossils found in the youngest terrace, but those from a rock layer 700 to 800 years old had larger crystals. Over time, the minerals altered chemically as well. Rustlike goethite slowly loses hydrogen and oxygen atoms to become more stable hematite over time. In fossils from the oldest terraces, hematite had begun to replace the goethite. These findings were recently reported in the journal Icarus.

The iron-rich rocks of Mars' Meridiani Planum, where the rover Opportunity explores, may have formed through roughly similar geochemical processes, says planetary geologist Timothy Glotch of the State University of New York in Stony Brook.

"Rio Tinto is a decent analog for what we see on Mars," Glotch said, noting that spectral analyses suggest Martian hematite originally formed as goethite or a similar mineral that was later altered to hematite. "It's a story similar to what they see in Rio Tinto."

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The Martian hematite rocks are far older than the Rio Tinto rocks. They may date back to as much as 3 to 4 billion years ago, a time that coincides with the earliest evolution of life on Earth. A lack of tectonic activity on Mars is likely to have left them relatively untransformed. For that reason, "Mars would be a very good place to look for preservation of microbial structures," Fernàndez-Remolar says.

Planetary scientist Carol Stoker of NASA's Ames Research Center at Moffett Field, Calif., agrees that if life was abundant when the Meridiani sediments formed, the fossils would likely be similarly preserved. But she isn't holding out much hope for any rover to find fossils. Successful identification of fossil life requires careful field work by geologists who select many of the most promising samples to analyze, she says.

David Fernàndez-Remolar Translucent orange goethite encloses fossil bacterial filaments

Fernàndez-Remolar and Knoll thinly sliced the Rio Tinto rocks to see the microbial structures. Although future rovers could be equipped with more powerful micro-imagers, they still wouldn't be able to peer inside the rocks. The next generation rover is expected to pulverize samples and to analyze the dust, a process that would obliterate the shape of anything that happened to be preserved.

Even missions designed to bring samples of Martian rocks back to Earth are unlikely to be able to select and ship back enough rocks to make detection of fossils probable, says Stoker. "The missions most likely to find definitive evidence of fossil life on Mars will be those conducted by human crews," she claims.

This report originally appeared in Astrobiology Magazine.

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