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“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.

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.

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 universe. To find this new life form, she just needs to follow the sulphurous stench.

The diversity of life on Earth is astonishing. You don’t need a brain to be alive, or a heart, or a spine. You can survive without oxygen, without sunlight, even without two cells to rub together. You can live without feeling the effects of ageing, or in total isolation, or through centuries of hibernation. As soon as scientists come up with a definition of what makes a living creature, it seems that something comes along to challenge it.

What is life, anyway?

But in the absence of a robust definition, there are a few things that most biologists agree all life forms must have. They should all be capable of passing on their genetic material to a new generation of organisms through reproduction, as well as being able to grow and shed waste. But none of this would be possible without the most fundamental requirement of all: that they be capable of generating energy from their environment and putting it to use.

Organisms on Earth do this in three main ways. “You eat light, you eat organic matter or you eat rocks,” says Jennifer Macalady, an astrobiologist at Pennsylvania State University and leader of the Frasassi expedition. Plants and other organisms that photosynthesise are powered by the sun, while others get their energy from chemicals stored in other life forms or within the geology of our planet. “Piece by piece, we’ve uncovered all sorts of amazing metabolisms that we didn’t even imagine,” says Penelope Boston, director of NASA’s Astrobiology Institute in California.

These diverse metabolisms almost all rely on the same fundamental chemistry to generate energy. In each case, the power comes from the transfer of a single electron from one molecule to another. The molecule that donates the electron is said to be oxidised, while the electron receiver, paradoxically, is said to be reduced. These processes, known as redox reactions, can release energy and bring stability to the system.

One of the factors that may explain the importance of redox reactions to life is their extreme sluggishness. Reactions that take place more quickly, burning through all the available energy in one glorious dash towards chemical equilibrium, wouldn’t be able to sustain organisms with longer lifespans. “Life can only make money on reactions that are far from equilibrium,” says Macalady.

Of these, there are hundreds of different combinations of electron donor and acceptor molecules that could theoretically power life. The process of aerobic respiration that provides energy in our cells, for example, involves oxidising glucose to carbon dioxide and reducing oxygen to water. Photosynthesis, on the other hand, sees carbon dioxide reduced into sugars while water gets oxidised into oxygen.

Life’s periodic table

“By mixing and matching different electron donors and acceptors,” says Aronson, “you can start to see where and how feasible certain reactions might be.” Not all of these potential reactions are equally interesting, however. Some generate too little energy to power life, some involve elements too rare to be sustainable and others require pressures and temperatures not found on Earth.

So far, in fact, only a small fraction of the entries in this vast table of reactions have ever been found. That leaves open the possibility that a huge diversity of strange new metabolisms could be sustaining life in some hidden corner of the universe.

That corner could be surprisingly close to home. A , less than 10 million of which have been catalogued. All the rest – known colloquially as microbial dark matter – remain tantalisingly enigmatic. With so many unknown species waiting to be discovered, chances are that a totally new metabolism could be lurking under our feet.

For Macalady, the best chemical to start with is sulphur. “Sulphur is one of the most abundant elements in the universe,” she says. Sometimes it is in its raw elemental form, but it can also be bound with oxygen in a form known as sulphate (SO4) or in the molecule hydrogen sulphide (H₂S), known for its distinctive smell of rotten eggs. Certain organisms have already evolved to take advantage of sulphur’s abundance. In the same way that we breathe oxygen, there are certain microbes that rely on sulphate, says Daniel Jones, academic director of the National Cave and Karst Research Institute in New Mexico.

Some of the earliest identifiable life on Earth probably got its energy from sulphur. Organisms that reduced and oxidised elemental sulphur into hydrogen sulphide and sulphate have been traced back as far as , to the very beginning of fossil records.

So what about doing it the other way around? Just as photosynthesis in effect reverses the chemistry of aerobic respiration, does anything make a living by turning hydrogen sulphide and sulphate back into pure sulphur? No such organisms, known as sulphur comproportionators, have ever been found. But in theory, at least, the answer is yes. “It’s likely that in 3 billion plus years of evolution and the right pressures, some species have developed the ability to do this,” says Macalady.

They would need very specific conditions to pull it off: an extremely acidic environment high in sulphide and sulphate ions, and with a temperature somewhere between 0°C and 25°C. This is why Macalady and Aronson have come to Frasassi. Its chilly interior and distinctive smell make it an ideal place to go hunting for new sulphur-based life forms.

Thanks to the rope-rigging skills of two Italian cavers, we abseil down, slippery with mud, into a long cavern. The space adjoins a grey-black pool of water and the walls are covered with slimy, worm-like patterns called biovermiculations, created by slow-growing microbes. On the day I join them, Aronson’s mission is to collect clear, teardrop-shaped secretions that hang from the walls and ceilings of the cave. Geologists call these snottites, and their resemblance to the dripping tip of a runny nose is uncanny. Because the snottites are full of bacteria and extremely acidic, Aronson hopes they will contain the sulphur producers.

She has good reason for optimism. In the 15 years or so that Macalady and Jones have been coming to these caves, they have found that Frasassi snottites contain multiple strains of bacteria that we have yet to grow in the lab. “We suspect that there might be some new tricks that life knows that we haven’t really seen before,” says Macalady.

Aronson collects samples with tweezers, dropping globules onto a strip of pH paper to test their acidity. The pH comes up as 0. They could be even more acidic, says Aronson, but that is as low as the paper can measure. For reference, the acids our stomachs use to break down food score somewhere between 1.5 and 3.5 on the pH scale. This environment would be inhospitable for us, but the life she is hunting revels in the extreme acidity. Aronson won’t know what she has found for sure until she gets her samples genetically sequenced, but her results could have major repercussions for the search for life on even more hostile terrain.

One reason why Aronson’s work is so exciting is that niches resembling the Frasassi caves are thought to have existed on Mars. Acid sulphate environments there, and associated volcanic emissions containing sulphide, would have provided the optimal conditions for sulphur-producing life, says Macalady.

Europa, the smallest of Jupiter’s moons, is also a candidate. Its ocean probably contains sulphate and possibly sulphuric acid, says Aronson. If we discover sulphide there as well, she says, “it might make sense to begin considering sulphur comproportionation as a potential metabolism”.

But such organisms may ultimately seem almost normal. More extreme redefinitions of life might yet be possible. (see “Does life need water?“), or with a chemistry built on something other than carbon (see “Does life need carbon?“). Moreover, alien life forms might be able to harness electrons from the environment rather than from redox chemistry inside cells, or harvest energy directly from electromagnetic radiation. That would make their biology totally unique. Of course, says Roger Buick at the University of Washington, Seattle, “these are all very speculative ideas”.

Some are more speculative than others. Macalady points to an unusual breed of microbes on Earth, the haloarchaea, which use proteins called rhodopsins to feed on light. “Some of those can absorb a photon and directly pump a proton across a membrane,” she says. They do this to produce adenosine triphosphate, a molecule that carries energy within cells, and don’t require the transport of electrons. If an organism were able to extract all of its energy from such processes, it would have no need for the redox reactions that dictated Aronson’s search.

“It’s always possible that you could have life that has a totally different way of surviving,” says Laurie Barge at NASA’s Jet Propulsion Laboratory in California. “But it’s really hard to make predictions when you don’t have any examples.”

Back in the cave, the rhythmic dripping of water and the metal clinks of climbing gear echo around, and my toes are going numb in my boots. As we eventually make our way out towards the sunshine and leave the subterranean darkness behind, I can’t help wondering if I brushed shoulders with life of the kind that might flourish on Mars. For the moment, though, I’m just grateful to breathe air that doesn’t stink of science.