Down desolation road

IT DOES not rain much in the Atacama desert. A 1,000km strip of land running along the Chilean coast, it is Earth’s driest desert outside its poles. Average annual rainfall in certain parts can be as low as a millimetre or two a year, and some Atacaman weather stations have never seen a drop of water.

Yet it does rain occasionally. And as Dirk Schulze-Makuch, an astrobiologist at the Technical University of Berlin, and his colleagues report in Proceedings of the National Academy of Sciences, a desert downpour in 2015 offered the perfect natural experiment for probing the limits of what sorts of conditions life can tolerate.

The Atacama is not quite lifeless. A few specialised animals and plants scrape a living in the less arid parts. And scientists have found evidence of microbial life even in the very driest areas. What is less clear, though, is whether those microbes are natives able to endure such extreme aridity, perhaps by becoming dormant, or whether they are merely the dead remains of interlopers, blown in on the wind but unable to survive in their harsh new environment.

The rains offered a chance to check. If the microbes were alive, or dormant, then the rain should have ushered in a brief golden age of growth, as the scarcest resource in the ecosystem—water—became briefly abundant. If the desert was merely strewn with corpses, though, then it would have made no difference.

Shortly after the rains in 2015, therefore, Dr Schulze-Makuch and his colleagues dug trenches in the desert and took samples from the soils at various depths. They returned in 2016 and 2017 to get more samples. The ones from 2015 showed evidence of microbial metabolism, including molecules like adenosine triphosphate, or ATP, which is used by all cells to store energy, as well as enzymes, fatty acids and the by-products of biochemical processes. Crucially, when they turned to the later samples, all those chemicals had become dramatically scarcer. Levels of ATP in 2017 were around a thousandth of their value in 2015. There were similar falls for other biological molecules, too. Those results were bolstered by others showing that different kinds of bacteria and fungi were present in different layers of desert soil. That suggests organised ecosystems rather than mere bacterial boneyards.

The driest soils on Earth, in other words, do indeed seem to play host to microbial ecosystems that can flourish when the rains eventually come. As a demonstration of life’s resilience, that is interesting enough. But it may have broader implications. Astrobiologists such as Dr Schulze-Makuch see the Atacama as the closest Earthly analogue for the surface of Mars. There is plenty of geological evidence to suggest that the Mars of billions of years ago was much wetter than it is today. And results from orbiting probes suggest that dribbles of water occasionally appear on the modern planet’s surface, possibly caused by the intermittent melting of subterranean ice. If life did arise on Mars, then it may still be clinging on, lying dormant in the regolith and waiting for the next brief pulse of meltwater.

Admittedly, Mars is even less hospitable than the Atacama. Besides the aridity, the Martian surface is bitterly cold and blasted by solar radiation. That radiation creates powerful oxidants that would likely destroy any living cells. If anything does survive, it will be deeply buried. That is one reason why the ExoMars rover—built by Roscosmos and the European Space Agency, and due to blast off in 2020—carries a drill capable of digging almost two metres below the surface.