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Year in Review: Methane shortage on Mars

Trace of gas not enough to be sign of life

A LITTLE GAS  The concentration of methane in the Martian atmosphere measured by the rover Curiosity is one ten-thousandth that on Earth. High levels of methane on Mars would have been a compelling indication of life there.

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After months of searching, the NASA rover Curiosity detected no appreciable methane in Mars’ atmosphere, disappointing scientists who had hoped to find a strong sign of life on the Red Planet.

On Earth, microbes have churned out as much as 95 percent of all atmospheric methane, so finding that gas in Mars’ air would have been solid circumstantial evidence of life. Instead, the rover measured no more than a trace of methane, with an average concentration of a mere 0.18 parts per billion (SN: 10/19/13, p. 7).

Many planetary scientists had expected that Curiosity would catch a whiff of methane. Over the last decade, researchers have measured fluctuating methane levels on Mars using Earth-based instruments and spacecraft. Some scientists have proposed that microbes buried beneath the planet’s frozen ground produced the gas, which could have been unleashed during a seasonal thaw and then somehow quickly removed from the atmosphere.

With more time, Curiosity might still discover such a methane release, says Michael Mumma of NASA’s Goddard Spaceflight Center in Greenbelt, Md., who is not a member of the Curiosity team.

But even if such a discovery never happens, that wouldn’t necessarily mean that Mars is barren. Micro-organisms that don’t generate methane might dwell on Mars, and the planet might have hosted now-extinct life at some time in the past.

In March, NASA announced that Curiosity had uncovered evidence of an ancient, hospitable aquatic environment, home to energy-rich minerals that could have fueled life-forms (SN Online: 3/12/13). Those hospitable conditions lasted millions of years and ended as recently as 3.5 billion years ago, researchers reported in December (SN Online: 12/9/13).

Although the findings are enticing, the rover has yet to find organic compounds, the building blocks of life.

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