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Life on Mars?

Geological and biological processes observed on Earth provide hunky-dory explanations for methane on Mars

Reports of methane on Mars first seeped out in 2004. Three separate groups had detected traces of the greenhouse gas in the red planet's atmosphere. One group relied on spectrometry readings from the European Space Agency's orbiting Mars Express. Two others pulled their data from powerful telescopes on Earth.

click for full image and caption
Much of Mars’s carbon dioxide atmosphere...

When the news reached Mukul Sharma, the Dartmouth geochemist immediately thought through all of the obvious possible sources. There were comet or meteor impacts to be considered, as well as magmatic activity. And then of course there were the most tantalizing explanations of all, capable of changing the way we view the universe and our place in it—those that signaled past or even present life. Bacteria produce most of the methane found on Earth and thus could be a subtle marker for life on Mars.

Sharma was familiar with the slate of suspects, having taught for several years a class called "Life on Mars?" But he believed the simplest of all these potential explanations to be an inorganic chemical reaction known as serpentinization.

Here on Earth, "there are several places on the continents and in the ocean basins where abiotic methane is being produced by serpentinization reactions," Sharma says. The process requires the mineral olivine, water, carbon dioxide and some catalysts—all well documented to be present on Mars.

Serpentinization had already entered the flurry of possibilities that scientists put forward, but no one had worked out how the reaction could produce the levels of methane that had been observed in the Martian atmosphere.

So, Sharma and a colleague, Dartmouth postdoctoral fellow Chris Oze, set out to calculate just how easy it would be for serpentinization to produce the gas.

Olivine hides out in what geochemists call ultramafic rock—rock high in magnesium- and iron-containing olivine and pyroxenes, which are silicate minerals. During serpentinization, Sharma explains, water attacks olivine and alters it to another mineral, called serpentine. At the same time, the hydrogen molecules are cleaved from the water. In the presence of certain catalysts, those hydrogen molecules combine with the carbon from carbon dioxide to form methane (CH4).

For the reaction to occur, the water must not be frozen, so serpentinization could not take place on the surface of Mars today. But Sharma said subsurface hydrothermal activity is a possibility. "Chris and I reasoned that the reactions could occur below the surface, such as close to the bottom of Helas basin, where the normal thermal gradient of the planet would predict the temperatures to be high enough for the water to flow."

Temperatures there could not heat the water to the mark, roughly 300 degrees Celsius, at which serpentinization is most efficient. But Sharma says that should not matter—the reaction can take place at room temperature and would still spit out enough methane to sustain the levels that had been detected.

A key consideration is that methane on Mars must be replenished by a current or recent source because it's an unstable gas broken down by ultraviolet radiation. On Mars, methane molecules typically survive about 340 years. Achieving a balance between the rate of methanogenesis and the rate that methane breaks down would be the crux of any calculation.

Writing in Geophysical Research Letters in May, Sharma and Oze determined that it would take just 80,000 tons of olivine each year to sustain the amount of methane observed in the Martian atmosphere. Orbiter studies suggest that there are huge amounts of olivine on Mars, more than enough to replenish the gas at the required rate.

Although Sharma and Oze's work shows that the mere presence of methane is not enough to justify claims of life on Mars—some of them shouted prematurely from media accounts that amplified the initial methane detections—neither does the adequacy of serpentinization rule out biogenic sources. After all, the methanogens that generate the stuff are found in virtually every place on Earth where oxygen is not—from the intestines of cows to the hot springs of Yellowstone to the farthest depths of glacial ice.

It was this last location where Berkeley physicist Buford Price joined the search for the source of methane on Mars.

Years ago, Price said, "I became fascinated by the question, ‘How can microorganisms live for hundreds of thousands of years while frozen in deep ice?" To answer that question, he and colleagues determined the metabolic rate for microbes trapped in a 3,053-meter-deep ice core pulled from the Greenland Ice Shelf. The core  revealed at its greatest depths striking variations in concentrations of methane. Clusters of the ancient organisms called archaea were found "at exactly the depths where there was excess methane," Price explained.

As soon as Price read that methane had been detected on Mars, he "got very excited" and immediately began calculating whether the metabolic rate he had established for ice-locked methanogens on Earth might apply to the conditions as scientists understand them to be on Mars, thereby contributing to the necessary atmospheric balance of methane. In a paper that appeared in Proceedings of the National Academy of Sciences in December 2005, Price posited that it could.

He observed that the metabolic rate on Earth rises with temperature, which increases with depth, both on Earth and on Mars. The concentration of microbes and the thickness of ice would also vary the rate of methane production.

All of that means that there are different plausible biogenic scenarios in play, but Price picked a favorite: "In my opinion, if the methane is biogenic, the methanogens are likely to be at a depth of hundreds or even a thousand meters, where they have access to ice that is warm enough to contain aqueous veins," Price said. The temperature would be somewhere between -10 and -40 degrees.

Sharma and Price both point out that the best way to determine whether the methane on Mars is biogenic or abiogenic would be to measure the ratio of carbon-12 to carbon-13 found in methane. Methanogens produce a gas much higher in carbon-12 than that produced by serpentinization, and this distinctive isotopic composition persists throughout the life of individual methane molecules.

But reading that signature will have to wait until a future mission ferries to Mars a mass spectrometer equipped to do so.

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