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charon writes:

The Spirit rover on Mars found some interesting rock formations in the Gusev crater. A NASA scientist has just published a paper suggesting that these silica deposits could have been formed by biological activity [gizmodo.com].

"This mineral, opaline silica, can form in various ways," said Steve Ruff, the planetary scientist at Arizona State University who led the recent study. "It can form around a hot spring or geyser, or in fumaroles," he added, referring to the steaming vents around volcanoes that spew hot, sulfur-rich gases into the air.

Initially, Ruff and his colleagues suspected Spirit's opaline silica deposits formed billions of years ago, from basaltic rocks that were leached by sulfuric acid pouring out of fumaroles. But as they continued to analyze Spirit's data, the scientists began to favor another possibility: opaline silicate precipitating out of hot, mineral-rich waters. After Spirit became stuck in a rut in 2009, and died in 2010, there was no way prove one scenario or the other.

A few years back, Ruff got a new lead. Reading a volcanology paper, he came across a reference to El Tatio, a vast Chilean hydrothermal system located 14,000 feet above sea level, where hot spring and geyser channels contain deposits of opaline silica. Excitingly, many of the silica deposits at El Tatio bore striking similarity to those in Gusev crater, and the cold, arid environment seemed pretty Mars-like, too.

To learn more about what's shaping opaline silica minerals on Earth, Ruff and his colleague Jack Farmer traveled to El Tatio to survey the environment and collect samples for spectral analysis and high-resolution imaging. They learned that silica minerals at El Tatio form in shallow, hydrothermal waters—and that the deposits most closely resembling the Martian ones occur in the presence of microbes.

Full Paper: Silica deposits on Mars with features resembling hot spring biosignatures at El Tatio in Chile [nature.com] DOI: 10.1038/ncomms13554

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