What Red Planet fossils might look like

By Susan Brown

updated 3:05 p.m. ET May 1, 2008

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.

David Fernàndez-Remolar A cap of iron-rich rock preserves microbes that lived in Rio Tinto about 2 million years ago.

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 Fernàndez-Remolar of the Astrobiology Center in Torrejón 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.

Washed up
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 Fernàndez-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.

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