Paul Davies asks if there's life on Mars

“That Mars is inhabited by beings of one sort or other is as certain as it is uncertain what those beings may be.” With these dramatic words, the American astronomer Percival Lowell informed the world about a network of canals he thought existed on the Red Planet. Lowell conjectured that Mars was a dying, drying planet, whose inhabitants built straight channels to bring melt water from the polar caps to the arid equatorial regions. He produced elaborate maps of a canal network to support his theory.

That was in 1906, when the idea of life on Mars seemed entirely plausible. Then in the 1960s space probes sent to Mars failed to reveal any sign of the much-discussed canals. In 1977 two spacecraft called Viking landed on Mars and found a freeze-dried desert bathed in deadly ultra-violet rays. The craft were equipped with robot arms that scooped up dirt and then analysed it for microbes. Nothing was found. Mars looked completely dead.

Recently, however, opinion has begun to shift. Photographs of the surface show what look like dried-up river channels and lake beds. There are hints of an ancient ocean. Evidently Mars was once warm and wet and maybe not unlike our own planet. Could life have flourished there in the remote past? Might it still be clinging on today in some obscure niche?

There is a good chance we shall learn the answers to these questions in the coming decades. Mars is the one planet in our solar system that is just about accessible to human exploration, and the motivation to go there is strong. It could be our only chance of finding a second genesis – another location in the universe where life has emerged from nonlife. The consequences for science and philosophy are incalculable. The discovery of a second tree of life would transform our view, not only of the nature of life, but of our own place in the universe.

Long ago, Mars must have had a much thicker atmosphere laden with greenhouse gases such as carbon dioxide. This would have elevated the temperature and provided sufficient pressure for liquid water to reside on the surface. Estimates suggest this “Garden of Eden” phase ended as long ago as 3.5 billion years, although sporadic warming may have occurred since.

The best hope for life on Mars today is the subsurface zone. Over the past twenty years scientists have been astonished to find microbes dwelling deep inside the Earth’s crust. The depth of this hidden biosphere extends in places to several kilometres. Because temperature rises with depth (the Earth has a hot interior) deep-living organisms are heat-loving too. In some cases they thrive at temperatures above the normal boiling point of water. The energy source for many species of subsurface life derives not from sunlight but from chemical energy. Some organisms can take gases and minerals percolating up from the Earth’s crust and turn them directly into biomass.

The discovery of subsurface life capable of sustaining itself without sunlight greatly boosted the hopes for life on Mars. The Red Planet has a hot interior too. This is evidenced by extensive volcanoes, some of which may still be active. There are undoubtedly hot spots underground on Mars, where volcanic heat has melted the permafrost, providing liquid reservoirs that could harbour primitive microbial life. Subsurface Martian life might reveal its presence through gases such as methane seeping to the surface.

To properly study any deep-living Martian microbes it will be necessary to probe beneath the surface. Planned missions to Mars, such as the European Space Agency’s Beagle 2, will incorporate penetrators and drills, but they are unlikely to burrow deep enough to reach any living organisms.

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