Direct evidence for life on Mars may remain as elusive as ever, but if something is living there, it is probably lurking beneath the surface, according to scientists.
A study has found that martian meteorites contain pockets of methane gas, hinting that methane-eating microbes might be able to thrive in the planet’s soil in a “deep biosphere similar to that on Earth”.
Nigel Blamey, who led the research at Brock University in Ontario, said: “We must be clear that we have not detected life. However, if life exists on Mars, then we should be focusing on the subsurface.”
Last year, Nasa’s Mars Curiosity rover observed “wafts” of methane coming from beneath the planet’s Gale Crater, suggesting that the gas is still being produced on the planet today.
The surface of Mars, which is bombarded by radiation and where temperatures plummet to -90C, is known to be extremely hostile to life. The latest findings suggest that conditions beneath the surface could be more favourable – at least for microbes like those found in some extreme environments on Earth, which use methane to respire instead of oxygen.
The bacteria, which thrive in methane-rich sludge at the bottoms of rivers and lakes, can live in oxygen-free environments that would be toxic to most other life.
Professor Monica Grady, a planetary scientist at the Open University, said: “If true, the results indicates that the martian subsurface could be capable of supporting life.”
However, she added that scientists remained divided about the significance of methane on Mars. On Earth, much of the methane in the atmosphere is produced by life, including both microbes, known as methanogens, and animals. An alternative hypothesis would be that methane in the meteorites could have been produced by microbes more than a billion years ago at a time when liquid water flowed across the surface of the red planet. “Some microbes produce methane, some microbes eat methane, so we can’t be sure,” said Grady.
The six meteorites, taken from various museum collections, are the result of asteroid collisions with Mars millions of years ago. When the asteroids hit the planet, they blasted off pieces of surface, some of which made it to Earth as meteorites.
When the scientists crushed samples weighing around one-quarter of a gram, taken from the interior of each of the meteorites, all of them released methane gas, which the scientists believe is probably held in small pockets between grains in the minerals.
The study, published in the journal Nature Communications, showed that the most pristine of the meteorites contained the highest methane concentrations, suggesting that the gas had not simply been introduced as the samples degraded on Earth. Tests of two non-martian meteorites also found far lower methane concentrations.
Professor Andrew Coates, a planetary scientist at University College London’s Mullard Space Science Laboratory, said: “The most likely type of life on Mars was primitive forms emerging 3.8 billion years ago, when Mars was very different to now, with water on the surface, a thick atmosphere and a magnetic field. At the same time primitive life was emerging on Earth.”
The European Space Agency’s planned ExoMars mission may provide more answers when it drills up to two metres beneath the surface after its arrival in 2019, he predicted.