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Nasa's Curiosity rover found methane at about 1 part per billion in Mars's atmosphere, 4,000 times less than in Earth's air. Photograph: Nasa/JPL-Caltech/MSSS/EPA
Nasa's Curiosity rover found methane at about 1 part per billion in Mars's atmosphere, 4,000 times less than in Earth's air. Photograph: Nasa/JPL-Caltech/MSSS/EPA

Methane on Mars: does it mean the Curiosity rover has found life?

This article is more than 9 years old
science editor
Nasa’s announcement of the discovery of methane – which on Earth comes largely from life – has sparked speculation

The results are certainly tantalising. From time to time, Mars belches out clouds of methane, a gas that on Earth comes largely from life. When animals and other organisms eat food they produce methane as a waste gas. From one end or the other, that gas ultimately finds its way out into the air.

Nasa’s announcement on Tuesday that its Curiosity rover had detected wafts of methane in the Martian air was met with immediate speculation that life might be the source. It might. Communities of microbes could be living under the Martian surface and churning out the gas. Perhaps the corpses of long-extinct bugs are being heated in the Martian interior and vaporised into methane. But any number of other processes that involve nothing as spectacular as life can and do make methane too. The problem is that detecting methane alone is never enough to answer the question of whether or not we are alone.

“You need to know a lot more about what’s going on right at the source,” said Michael New, an astrobiologist at Nasa’s headquarters in Washington DC. “You need to know the context. It’s very hard to look at methane alone and say it came from life.”

Nasa made its first definitive detection of methane on Mars in 2009. Telescopes picked out spectral features of the gas, and revealed plumes of methane pouring from the northern hemisphere. The finding was evidence for a replenishing supply of the gas on the planet, because methane molecules, on average, last only 340 years in Martian air before they are broken down by sunlight.

The prospect that life forms were responsible for the 2009 methane plumes (and that humanity’s first encounter with aliens might be through a fog of their waste gases) was fiercely debated at the time. The latest results from Curiosity, based on measurements on the Martian surface, are a much-needed confirmation. They show that the gas is present at about 1 part per billion in the Martian atmosphere, or 4,000 times less than in Earth’s air.

But still the caveats remain. Methane is given off by any number of processes. Rocks bearing the mineral olivine have been found on Mars. They can react with water, also spotted on Mars, to produce methane. Another source could be icy subsurface clathrates, or molecular cages that harbour methane but can release it in bursts over time. The gas can be made in other ways too, such as reactions between cosmic dust and ultraviolet light from the sun.

Nasa’s Curiosity rover was not sent to Mars to find signs of life. Its mission was to scour the environment and work out whether it was once habitable or inhabited. In the coming months, the rover will analyse soils and rocks on the planet, to learn more about organics found in those, but a direct detection of life is almost impossible.

“In the best of all possible worlds, you would crack open a Martian rock and there would be eyes staring back at you. Or at least endolithic communities [organisms that live in the cracks of rocks such as moss and lichen], which you can find living inside rocks in the desert and in Antarctica. The rocks can provide protection, but sometimes they are using the chemistry of the rocks for energy as well. So if you cracked open a rock and saw a band of green or orange, then that could mean life. That would be great, but we can’t expect it to happen,” said New.

Nasa is already planning a follow-up mission to Curiosity that could provide the next leap forward in understanding whether Mars is or was ever inhabited. Based on the Curiosity rover, the Mars 2020 mission is part of a long-term campaign to bring Martian rocks home. “Once you have samples on Earth, you can do a lot of very deep analyses we cannot put on spacecraft,” New said. “You can look inside rocks for organic matter and if it’s there, what does it look like? Is it dots of carbon, or is it something else?”

But even bringing Mars rocks back to break open on Earth might not be enough. The Allan Hills meteorite, probably the oldest Martian meteorite ever found, was picked up in Antarctica in 1984. Since then, it has been studied extensively, but the results from years of tests are still controversial. “While most people don’t believe there’s evidence for life in there, it’s still not an open-and-shut case,” said New.

Stronger evidence might come from a joint mission under way at the European and Russian space agencies. The ExoMars rover and a Mars trace gas orbiter are due to be launched in 2018 with the express goal of searching for signatures of life past or present. Good evidence for a biological origin for methane on Mars could come from measurements of the isotopes of carbon and hydrogen that make up the methane molecules. On Earth, at least, life tends to use lighter isotopes, so more carbon-12 than carbon-13.

More on this story

More on this story

  • Why methane on Mars has reignited our quest for life on other planets

  • Curiosity rover's discovery of methane ‘spikes’ fuels speculation of life on Mars

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