Published online 27 March 2009 | Nature | doi:10.1038/news.2009.197

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Methane-producing mineral discovered on Mars

But it may not explain the presence of the gas on the Red Planet today.

Nili FossaeTraces of serpentine found at Nili Fossae on Mars.NASA/JPL/University of Arizona

Surprises keep coming from Nili Fossae, a long, deep scar in the surface of Mars. In December last year, scientists reported evidence there for carbonates — minerals that typically form in the presence of water1. Then, in January, reports came that there was a large plume of methane in the area. On Earth the gas is made mostly by animals as a by-product — although it can also be produced naturally in the absence of life2.

Now Bethany Ehlmann, a PhD student at Brown University in Providence, Rhode Island, is finding evidence at Nili Fossae for a mineral that could tie all these observations together. Ehlmann used a spectrometer on the Mars Reconnaissance Orbiter to identify two small outcrops of a mineral called serpentine. Serpentine arises from another mineral, olivine, in a hydrothermal process in which hydrogen gas is produced — a potential energy source for microbes that could in turn produce methane.

The process of serpentinization also produces methane itself, without the need for life. "It was a past source of methane, for sure," says Ehlmann, who announced the result on Wednesday at the Lunar and Planetary Science Conference at The Woodlands, near Houston, Texas. Serpentine can also be altered, in lower temperature water, into carbonate.

However, the finding does not rule out life on Mars today. That depends on whether the presence of serpentine has anything to do with the apparent production of present-day methane. "It's certainly an intriguing coincidence that one of the major regions in which we find these minerals has been highlighted as a possible source region of methane," says Ehlmann. "But there's this timing problem."

The problem of timing arises because serpentine on Mars is ancient, about 3.8 billion years old, whereas the reports of methane gas are contemporary. Yet it is possible, Ehlmann says, that fractures deep underground could be providing the necessary water and heat for serpentine to be formed today, and for methane to percolate up.

Happy landings

Regardless, Ehlmann would like to see the region, with its evidence for hydrothermal activity, put back on the shortlist of landing sites for the US$2.3-billion Mars Science Laboratory rover, which, after launching in 2011, will explore Mars and search for evidence of past or present life. Nili Fossae was on an earlier list of potential landing sites, but was cut when the list narrowed to four sites, most of which appear to be ancient lake or riverine environments within craters.

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But Matthew Golombek, of the Jet Propulsion Laboratory in Pasadena, California, notes that the Nili Fossae region is large, and the reported sites of carbonate and serpentine are far apart. Furthermore, the evidence for methane suggests a large generalized plume that cannot be assigned to a specific spot at all. "All of these things are not available in one landing ellipse," says Golombek, who is co-chair of the landing-site committee for the rover. He says that Nili Fossae was removed from the list mostly because of landing concerns.

The Mars Science Laboratory needs a landing site at a low elevation so that the parachutes have time to slow the craft as it plummets through the planet's thin air. Nili Fossae itself sits relatively high, but some of the nearby reported sites for carbonate and serpentine are at lower elevations.

Golombek says the committee will open up the process to adding new sites again in the autumn. Ehlmann says she'll be ready. 

  • References

    1. Ehlmann, B. L. et al. Science 322, 1828–1832 (2008). | Article | ChemPort |
    2. Mumma, M. J. et al. Science 323, 1041–1045 (2009). | Article | ChemPort |
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