Why the hunt for alien life is under way far beneath Earth’s surface

Microbes that breathe sulphur could redefine what it means to be alive and provide clues about what organisms may lurk in the cosmos

By Donna Lu

Graham Carter

“IT’S the smell of science!” says Heidi Aronson, her face dimly lit by the beam of her head torch. In that case, science smells like an egg sandwich that’s been left out past its use-by date and then rolled in mud.

Some 300 metres above our heads is a bucolic Italian landscape of rolling sunflower fields, Verdicchio wineries and winding mountain roads. Here in the Frasassi caves, the air reeks of hydrogen sulphide and the walls are slimy with slow-growing microbial deposits. My mind keeps drifting upwards, but there is nowhere Aronson, a researcher at the University of Southern California, would rather be. Down here, far from the light, she is hunting aliens.

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Virtually anywhere you look on Earth, you find life. It can be in sites dominated by heavy metals that are toxic to humans or on the plateau of the Atacama desert, where soils are so dry they are Mars-like. It can be found feeding on nuclear waste, as well as at both extremes of temperature and pH.

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But if Aronson is right, then the Frasassi system could be crawling with life unlike anything we’ve ever seen: microbes that gulp down sulphur compounds the way we breathe air. These would be evidence of a biology radically different from all other life on Earth.

Such a discovery would have dramatic consequences. An organism capable of generating energy in this way would not only shed light on the origins of life on our own planet, it could also hint at the nature of life elsewhere in the …