Live microbial filaments can become fossils when coated with minerals.
Credit: David Fernàndez-Remolar
Fossil microbes found along an iron-rich river in Spain reveal how signs of life could be preserved in minerals found on Mars. The discovery may help to equip the next generation Mars rover with the tools it would need to find evidence of past life on the planet.
The Rio Tinto arises from springs west of Seville. These springs percolate up through iron ores that were deposited by geothermal activity more than 200 million years ago. Spring water dissolves iron sulfide minerals from the ores, and this stains the river red. The iron sulfide minerals also dissociate to form sulfuric acid.
With a pH between 1.5 and 3, Rio Tinto is as sour as vinegar, yet it supports a surprising variety of life. Bacteria, algae, single-celled organisms called protists and fungi all thrive in the acid headwaters.
Rio Tinto has attracted the attention of exobiologists because this environment can create the iron mineral hematite, which has been found on Mars. On Earth, hematite only forms with liquid water. Since liquid water is seen as a prerequisite for life elsewhere, the mineral's presence on Mars tantalizes those who hope to find signs of life, past or present, on our neighboring planet.
By examining incipient fossils along Rio Tinto's shores and comparing them with much older fossils left on terraces now high above the river, David Fern?ndez-Remolar of the Astrobiology Center in Torrej?n de Ardoz, Spain and Andrew Knoll of Harvard University hope to better understand how similar minerals may have preserved a record of life on Mars.
Pools at the edge of the river evaporate in the heat of the Spanish summer and leave behind mineral deposits. Over the years, as the river cuts down into the valley it creates rock terraces. The oldest and highest terraces formed 2 million years ago while the youngest are just a few centimeters above the surface.
When Fern?ndez-Remolar and Knoll looked at those evaporating pools, they saw microbes that had become coated with nanoparticles of iron minerals that had precipitated out of the water. The most common mineral they observed was a rust-like iron oxide called goethite. Layers of fine-grained goethite surrounded the youngest fossil microbes, preserving the rod-like shapes of individual bacteria as well as filaments formed by bacterial colonies.
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's 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."
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," Fern?ndez-Remolar says.
Planetary scientist Carol Stoker of NASA's Ames Research Center at Moffett Field, California, 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.
Fern?ndez-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.