As I step out of the lift shaft I’m hit by a roaring, warm, dry wind. The air tastes salty and, as my eyes adjust to the orange glow of artificial lights, I see I’m standing at the crossroads of a network of cavernous tunnels, each stretching as far as the eye can see. Thick layers of creamy coloured dust drape the tunnel walls, shifting slightly when a truck rumbles by, before settling back.
Right now I’m at Boulby potash mine. At 1.3 kilometres below ground it is the UK’s deepest mine.This hot, dark, bone-dry environment is the last place you would expect to find any signs of life, but today I’m with scientists who are going to introduce me to some of the weirdest creatures in the world. Charles Cockell, an astrobiologist from Edinburgh University, and his team are studying microbes that have been found in the mine’s warmest and saltiest niches. In contrast to the life forms we know, these microbes don’t need oxygen or sunlight.
Although it seems strange to me, this subsurface life turns out to be far from unusual. “All over the world the underground is thoroughly inhabited, with organisms living in cracks and fractures in the rock down to depths of more than five kilometres,” says Penelope Boston, associate director of the National Cave and Kart Research Institute in Carlsbad, New Mexico. From a frigid lake a kilometre under the ice of Antarctica to hot, acidic caverns beneath Mexico and radioactive gold mines in South Africa, life finds a way to survive and thrive in extreme and seemingly hostile locations.
Ever since “extremophiles” were first discovered in the 1960s scientists have been trying to understand how they survive. But these life-forms are far more than a curiosity – they could help us find the answer to a compelling mystery, “is there life beyond our planet?” Some say Earth is the planet that won the lottery – the only one in the Universe able to support life. Others argue that life is likely to have evolved many times on other planets. By exploring the extremes of where life exists on Earth, Cockell and his colleagues believe they can narrow down where else in the Universe life could be found.
“Even when the surface of a planet appears barren it is plausible that there is life underground,” says Sam Payler, a PhD student in Cockell’s team at Edinburgh. “These deep subsurface environments could act like a ‘bomb shelter’ for a planet.”
In particular, the warm, dark and salty environment at Boulby mine shares similarities with the kind of habitat believed to exist deep below the rusty red surface of Mars.
“There is no such thing as the perfect analogue for Mars, but we do know that there are salt deposits there, and there is evidence of water on the surface in the past,” says Cockell.
Today the atmosphere of Mars is very thin and anything living on the surface would be bombarded with high levels of radiation. But the red planet is considered to have been, at some time in the past, more hospitable. If that is the case there’s a chance that there is still life there — or its fossil remnants — down in the “bomb shelter”.
As we clomp down one of the dusty tunnels Cockell and Payler explain what captivates them about the search for life in unpleasant places. “These organisms tell us about the boundaries of where life can exist and we also find out where not to look — where we think life can’t exist,” says Payler. And as if these organisms don’t have a hard enough life already, Cockell has found ways to torture them even further. The smiling astrobiologist, who began his research career with NASA in the mid-1990s, has literally hung them out to dry, in space, on rocks exposed outside the International Space Station, to see if they could survive.
Remarkably, one species did.
By the time we reach the underground lab the extremes of this environment are taking their toll. I’m starting to feel faint and have to sit down next to an air-conditioning unit to recover. Luckily Cockell and Payler are made of stronger stuff, because collecting the Boulby life forms is a tough task. “We drive in trucks for up to 12 kilometres out underneath the North Sea and then we sample the brine flows, which look like water oozing out of the rock wall,” explains Payler. Huge fans blast cooler air throughout the mine, but the rock temperatures can often be around 50°C.
Wearing protective clothing and using sterile tubes, the scientists scoop up some of this gloop and bring it back to the underground lab to take a closer look. By extracting and analysing DNA the scientists try to understand which trunk of the tree of life the organisms they find belong to. So far they have shown that many of the organisms down here are archaea — single celled micro-organisms that were once thought to be bacteria.
“The ones we find at Boulby come in a wide variety of shapes, from rods to spheres and even squares,” says Payler. But while they look pretty much the same under the microscope, archaea are fundamentally different from bacteria.
That was discovered in 1977 when American microbiologist Carl Woese read some of their DNA. Archaea are now considered to represent a third kingdom of life alongside the kingdoms of the bacteria and eukaryotes — creatures that package their cells’ nuclei in membranes, like we do.
Bacteria and archaea have been here for eons. In Western Australia’s Pilbara region evidence of bacteria goes back some 3.5 billion years, in sedimentary rocks that carry cabbage-like imprints of ancient bacterial communities called stromatolites.
Chemical signatures in 2.7 billion-year-old shale rocks also provide evidence of methane-producing archaea. After 40 years of studying the Pilbara rocks, amidst counter-claims that the “cabbages” were merely mineral oddities, Malcom Walter, the founding director of the Australian Centre for Astrobiology at the University of NSW, has been vindicated. “The evidence for early life in the Pilbara gets better and better”, he says.
These ancient rocks tell us that bacteria and archaea evolved when our planet was a most un-Earthlike place. It was a watery world, oppressively warm thanks to an atmosphere with almost no oxygen, but rich in carbon dioxide. While today’s microbes are happily adapted to life on Earth today, some appear to have retained their ancestors’ capacity to live in extreme locales. Some modern day bacteria can also do this, but when it comes to thriving in unearthly places, the archaea take the prize. And so one of the best clues for what life may look like on Mars may lie with the archaea in the Boulby mine.
That’s why Cockell and his team want to know how they survive. One way of doing this is to read their DNA and compare it with DNA from surface-dwelling microbes, to identify shared genes. In particular they are interested in “functional” genes – the ones that code for essential processes such as digestion and respiration. If they can identify these then they can start to understand how the organisms survive. For one thing, where do these carbon-based life forms get their carbon?
Cockell has a number of hypotheses. “We think they get their carbon from diverse sources, including carbon dioxide in the atmosphere and carbon in the rocks.” To test these ideas Payler is nurturing different batches of microbes in flasks with different environments, trying to find out what aspects of the mine’s environment are essential for survival.
Near Tapijulapa Mexico another group of researchers is braving the deadly cave known as Cueva de Villa Luz. Led by Penelope Boston, they are searching for organisms that could survive here. Boston, with her shock of curly blonde ringlets, was a young undergraduate when NASA’s Viking landers arrived on Mars in the mid-1970s. With that first glimpse of the Martian landscape she was hooked. “It took what had been a tiny little astronomical object in the sky and turned it completely into a landscape. As that very first primitive picture came rastering across the screen Mars became a destination and altered, really, the course of my life”, she said in a TED talk in 2006.
Ever since, Boston has been dedicated to the search for life on Mars. She has explored extreme environments – the Arctic, the Antarctic, and high and low deserts – to get a notion of what life might be like on Mars. And she has helped design tools for exploring Mars, working with the NASA Institute for Advanced Concepts to scope out what sort of equipment would be needed to set up a manned base in a lava-tube cave. Such a location would shield astronauts from intense solar radiation and is also where they’d want to start searching for Martian life. For that, she’s collaborated with Steve Dubowsky at Massachusetts Institute of Technology, developing tennis ball-sized, bouncing, cave-exploring robots.
But when it comes to exploring Earth’s extreme habitats, Cueva de Villa Luz tops her list.
“We have to wear masks to screen out the poisonous hydrogen sulfide, and it is so hot (50°C) that we have to wear ice pack vests and helmets and can only stay in there for a maximum of 30 minutes at a time,” she says. And yet, this hellish environment is bursting with life. Hanging from the dark walls are gooey blobs – communities of microbes – which Boston and her colleagues nickname “snottites”.
And feeding from the slimy snottites are hosts of midges. In this case the microbes are getting their energy by feasting on hydrogen sulphide. Meanwhile, in neighbouring caves Boston has found similar microbes that have adapted to oxidise the iron or manganese they slurp up from the cave walls – extracting the little packets of energy in the process. “There is a surprisingly large menu of things to support life in these caves,” says Boston.
Both Boston and Cockell argue that it is possible that life took refuge in Martian caves when the planet lost both its atmosphere and surface water, around three billion years ago.
So are the Boulby microbes the kinds of things we should expect to find on Mars? To put this to the test Cockell and his colleagues have constructed a mini-version of Mars in the labs at Edinburgh University. A shiny silver cylinder about the size of an oil drum, it is covered in dials, protuberances and wires and is designed to simulate a range of Martian environments. “We can test the Boulby organisms to see if they survive with a carbon dioxide atmosphere, high levels of radiation and an acidic environment for example,” says Cockell.
Even if the scientists can’t find organisms that could survive on Mars today, by twisting a few dials on the chamber they can try out the comparatively more pleasant conditions of the planet’s past. If that’s successful it could help future Martian explorers find fossils of now-extinct life.
Down in the caverns of Cueva de Villa Luz, Boston and her colleagues have found some clues as to what these fossils might look like. This particular community of microorganisms make crazy patterns across the cave walls, winding hither and thither like snail tracks. And when the communities die they leave their patterns behind. This is what bouncing mini-robots might search for if they make it into the Martian lava-tube caves.
Thinking along similar lines, Cockell and his team are using their Mars simulation chamber to compress millions of years into a few days. By locking out oxygen and cranking up the pressure and temperature they plan to mimic the fossilisation process and turn the Boulby microorganisms to stone. “We can investigate their potential for fossilisation on Mars and their detectability,” says Cockell.
If we find life on Mars it seems plausible the Universe could be teeming with it. When it comes to the basic requirements, most scientists agree the essentials are water, a source of energy, and the life-building elements: carbon, hydrogen, phosphorus, sulfur, oxygen and nitrogen. It is fair to say there are probably multiple locations in the Universe that could have provided these basic requirements at some point. “The problem is that we just don’t know exactly how life gets started in the first place. Do habitable conditions make the emergence of life inevitable, or is life on Earth really a freak occurrence?” says Cockell.
Within our own Solar System, Europa and Ganymede (two of Jupiter’s moons), and Enceladus and Titan (two of Saturn’s moons), are potential candidates for extra-terrestrial life. “Enceladus is very active geologically and it would be fascinating to sample the jets of fluid that it blows out to see if they contain any fragments of biomolecules or isotopic hints of life,” says Boston. And venturing further afield the odds for life seem promising. There are 100 billion stars in the Milky Way and 100 billion galaxies in the observable Universe, “so there is a great deal of ‘real estate’ out there. Most life forms will be microbial, but I’m sure that more complex life forms will have developed somewhere at some point”, says Boston.
But not everyone agrees our Universe is so habitable. David Waltham, a geologist from Royal Holloway, University of London, argues that Earth is an exceptionally rare planet. In his recent book, Lucky Planet, he describes the chance series of events that have made Earth “just right” by giving it four billion years of stable climate, enabling life to get started, persist and flourish for so long.
For starters Earth has a prime location within our galaxy. “If Earth were closer to the centre of the galaxy then life would be obliterated by the regular supernovae, but if we were further away we wouldn’t have enough of the elements essential for life (which are created by supernovae). We sit in a perfect sweet spot,” explains Waltham. And it isn’t only the location within the galaxy that matters. It also turns out that our Sun orbits the galaxy at about the same rate as the spiral arms of the galaxy, which means we don’t travel through the spiral arms as often as we might otherwise, further reducing our chances of encountering supernovae.
Then there is our position within the Solar System. Not only is Earth the perfect distance from our Sun, but the arrangement of the other planets in our Solar System help to keep us at that perfect distance. “Observations of extra-solar systems indicate that the orbits of planets in our Solar System are more circular than is usual, which enables Earth to maintain a more stable climate than it would otherwise,” says Waltham.
Adding to that stability is our Moon, just the right size to hold Earth’s axis steady and prevent complete climate chaos.
And down on the Earth, the constant restless dance of the tectonic plates helps to recycle elements, maintain water on the surface and keep the climate in a steady state. Other planets (including Mars) show signs of having had plate tectonics in the past, but it appears that it is rare for a planet to maintain its plate tectonics for billions of years as Earth has done.
All of these factors helped life to gain a foothold on Earth, but even more luck has enabled life to persist as long as it has. “Some of the key steps in life’s evolution happened at the perfect time,” says Waltham. In particular, photosynthetic cyanobacteria which sucked up carbon dioxide, converting it into sugar and releasing oxygen as a by-product, evolved around two billion years ago and caused a dramatic cooling on Earth. “If they hadn’t come at this time then the Earth would have overheated and life would have been wiped out,” says Waltham.
Taken individually, each of Earth’s “lucky” features is not so remarkable, but for Waltham it is the sum and timing of these features that makes the Earth truly special. “The evidence points towards Earth being a very peculiar place, perhaps the only highly-habitable planet we will ever find,” he says. Nevertheless, Waltham agrees that simple life forms could be out there somewhere, and that Mars is a good place to start looking.
Indeed, it could even be that our own origins lie on Mars. Assuming that tough microbes, such as those found at Boulby, could survive the levels of radiation in outer space, it is plausible that life evolved on Mars and then travelled to Earth on a meteorite.
“There is absolutely no evidence for it but as a highly exploratory thought, you could go to Mars and look for Martian life and find that we have been Martians all along,” says Cockell.
Kate Ravilious is a freelance science journalist, based in York, UK.
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