See more in our gallery: “The quest to drill the world’s deepest hole“
AN UNLIKELY explorer is floating off the east coast of Japan. At first glance, the ship resembles a rather strange oil tanker. It is colossal: perched on deck are a helipad, cranes and a scaffold tower around 30 storeys high.
In the control room, a supervisor monitors the screens, before setting the tall scaffold in motion. “Confirm the hole position,” he says. Inside the tower, machinery whirs as the world’s longest drill is lowered towards the ocean floor. Its ultimate destination, when it gets there, will be uncharted territory.
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So goes a typical day on board . Today, it is aiming for the fault that caused last year’s Tohoku earthquake to reconstruct its causes, but the ship has a much more ambitious goal in sight. Geologists are planning to use Chikyu to drill all the way through the crust and into the mantle to fetch a cache of rock samples. This feat has never been done before – in fact, no one has even come close.
If the project gets the go-ahead, it will be one of earth science’s most spectacular ventures. Comparable to a moonshot, it could transform our understanding of our planet’s evolution, and challenge the fundamental paradigms of earth science. There is even a chance that we will find something unusual lurking down there, something few would have thought possible until recently.
This is not the first time geologists have yearned to explore the deep Earth. In 1909, Croatian meteorologist Andrija Mohorovicic discovered that seismic waves, triggered by earthquakes, travelled significantly faster below a depth of 30 kilometres than they did higher up, hinting that these deep rocks had different compositions and physical properties. With this discovery, Mohorovicic secured his place in the annals of science. This step change in seismic velocity was named the Mohorovicic discontinuity – aka the Moho – and marks the upper boundary of the mantle.
Geologists now know that the top of the mantle lies 30 to 60 kilometres beneath the surface of thick continental crust, and as little as around 5 km below the seabed at points where the crust is at its thinnest. What happens at that depth shifts tectonic plates, moulds the land we stand on, and unleashes the fury of earthquakes and volcanoes. It has therefore shaped all life on the planet – including us.
Yet it wasn’t until the late 1950s that scientists felt the urge to investigate the mantle. At the time, the idea of plate tectonics was still hotly debated. Harry Hess, and other proponents of the theory, claimed that hot convective currents from deep within the mantle were driving floating tectonic plates around the planet’s surface. Hess and colleague Walter Munk felt hampered by the lack of physical evidence for the theory, and turned to some of their drinking buddies from the US National Academy of Sciences. At a wine-fuelled breakfast in California in April 1957, the so-called American Miscellaneous Society hatched a plan to fetch mantle samples. .
Numerous challenges had to be met – everything from finding funding to inventing the technology to keep a drilling ship stationary on the high seas. They couldn’t borrow ideas from offshore oil companies – they weren’t drilling in deep water at the time – so the Mohole team developed a technology called dynamic positioning, where cleverly placed propellers and thrusters keep a ship stable and in place. The first core was drilled to 183 metres off the coast of Guadalupe Island in the Pacific in April 1961. It was also the last.
Soon after the expedition returned, the leading scientists were side-lined, management changed hands, costs spiralled, and a certain young politician called Donald Rumsfeld stuck his nose in. In 1966, Project Mohole folded after the US Congress voted to drop its funding.
Despite this, drilling into oceanic crust did continue. Still, we have never got further than about a third of the way to the mantle. The closest a drill has got is a . It’s not the deepest hole ever – – but the crust there is estimated to be less than 5.5 kilometres thick. Some boreholes on land extend much further from the surface, but since continental crust is far thicker, their deepest points are tens of kilometres from the mantle.
As far as the geologists behind the 2012 Mohole to Mantle project are concerned, there is a clear scientific rationale to firing up the drill once more. After all, while the mantle makes up 68 per cent of the Earth’s mass, we actually know very little about it. “There are currently no pristine mantle samples, so we just have hints of what’s going on,” says Damon Teagle at the UK’s National Oceanography Centre in Southampton, who is part of the international team working on the Japanese-led project.
Some samples have reached the surface, but they are all contaminated. For example, rare rocks called mantle nodules have erupted in volcanoes, showing the mantle is made of magnesium-rich, silicon-poor minerals like olivine and pyroxene. And in some parts of the ocean floor, rocks that were once part of the mantle lie exposed, but contact with seawater has changed their composition dramatically. Think of these samples as the difference between Martian meteorites and actual rocks picked up from the Red Planet. Without fresh samples, geologists struggle to confirm even simple facts about our planet, including what exactly the mantle is made of, how it formed and how it works.
Precious stones
Instead, they have had to piece together their theories about the mantle using indirect evidence. Its broad layering structure is inferred by tracking the speed of seismic waves, as Mohorovicic did. Further clues to its composition have come from meteorites, which were forged from the same cosmic debris as our rocky planet, or more recently via exotic methods such as looking at the neutrinos produced during the radioactive decay of certain elements.
Many questions remain unanswered, however. Getting our hands on tracers of mantle convection, such as noble gases and isotopes, would reveal how and when our planet differentiated into the core, mantle and crust, and when plate tectonics started. Identifying the chemicals and isotopes that make up the upper mantle would show how water, carbon dioxide and energy are transferred to the crust, and how they influence global geochemical cycles. And finding out how heterogenous the mantle is would reveal how magma wells up and then erupts onto the sea floor at mid-ocean ridges.
Perhaps the most extraordinary thing we might find in the mantle is life. While any creatures won’t quite live up to the prehistoric monsters envisioned by Jules Verne in A Journey to the Centre of the Earth, they would still be significant. Recent discoveries suggest such extremophiles might be possible.
Last year, Tullis Onstott at Princeton University uncovered microscopic roundworms, known as nematodes, living an incredible 4 km down in a gold mine in South Africa. Considering their size, Onstott likened the discovery to finding Moby Dick in Lake Ontario (). He has also found single-celled microbes at even greater depths – up to 5 km down.
Under the sea floor, microbes have turned up 1.6 km down off the east coast of Canada (). The researchers who found them speculate they might be hundreds of millions of years old. “We showed that the bacteria might be dividing as slowly as, say, once in 100,000 years,” says John Parkes of Cardiff University, UK.
Pressure does not seem to be a problem for many extremophiles. In the lab, microbes can tolerate up to 1000 atmospheres, and there are bacteria living happily under 11 km of water in the Mariana Trench in the western Pacific. In fact, pressure is crucial for survival in searing hot conditions, because it stops water boiling – steam can be a killer.
So temperature could be the deciding factor. Just below the Moho, geologists believe it could be as low as 120 oC. “This is tantalisingly close to the known upper limit for life: 122 oC,” says Parkes. An organism living on hot ocean vents was shown to be capable of growing at this temperature in 2008 ().
Still, Matt Schrenk at East Carolina University in Greenville, who studies microbiology in extreme environments, thinks the chances of finding mantle life are slim. Apart from the heat, he says, fluid circulation will be minimal, so the flow of nutrients would be too.
Despite his doubts, Schrenk supports the Mohole to Mantle project as he thinks it could define the physiological limits of life – and even help the study of climate change since the biosphere down there may influence the circulation of the “deep” carbon cycle. Deep life could also prove useful in medicine. “If the organisms are evolutionarily distinct, they could carry out unique activities or possess unique enzymes that could be of use in biotechnology,” he says.
Mantle samples could also help us unravel the role of microbial life in the evolution of our planet. Recent research by geophysicist Norman Sleep at Stanford University in California found that life can be subducted into the crust – and its products, such as ammonium, can be dragged even further down. Essentially, all the nitrogen in the mantle comes from subducted ammonium in organic matter (). This raises the possibility that life on the very early Earth changed the composition of the mantle – and useful samples for studying life in this period might still be down there.
At the National Oceanography Centre, Teagle and colleagues have been helping to assemble all of these scientific reasons for the Mohole to Mantle project. In the labs upstairs, scientists carry out delicate analyses of cores from ocean drill holes. The chances are, this is where many of the precious mantle samples will be scrutinised.
Teagle says it’s not surprising that it has taken decades to pick up where Project Mohole left off. “Technology, time and money were previously the limiting factors to drilling to the mantle,” he says.
First, consider the accuracy required to drill 6 km into the crust beneath the ocean floor. “It will be like lowering a piece of steel string the width of a human hair to the bottom of a 2-metre-deep swimming pool,” says Teagle, “and then drilling 3 metres into the foundations.” That means a new extra-long drill will have to be built for Chikyu, which cannot reach such depths at the moment.
“It’s like lowering a thin hair into a swimming pool and drilling it 3 metres into the foundations”
New materials will also be required. When drilling a 30 centimetre-wide hole in hard igneous rock at a speed of 1 metre an hour, drill bits only last about 50 hours. They can also fail catastrophically and be ground into smooth stumps. The uber-tough materials being developed for the project will need to cope with pressures of 2 kilobars and temperatures of up to 250 °C.
The good news is that an independent review carried out in 2011 by Blade Energy, a deep-water drilling firm, concluded that the project is technically feasible. “It always used to be that an engineer would invent some gadget and then ask scientists whether they could use it in some way. More and more, now, the needs of science are driving technology,” says Teagle.
In fact, whether the plan succeeds relies less on technology and more on political and scientific will. Teagle reckons the operation of the research vessel alone will cost at least $1 billion. Fortunately, the Japanese government is committed to covering a significant portion of these costs. While this is a big investment, it is understandable considering that Chikyu might eventually help with earthquake forecasting. And it’s not only the Japanese who are getting behind the project – others have expressed interest too.
If the Mohole to Mantle team wins an official thumbs up in the next year or so, its members hope to strike mantle gold within a decade. First, a decision needs to be made on which of the three potential drilling sites to choose. They are all in the Pacific – one candidate includes the Project Mohole site – and each one is relatively close to mid-ocean ridges, where new crust forms. Rising magma pushes up the seabed here, making the water shallow enough to reach down with a drill. The rocks at the three sites have also cooled down enough to penetrate safely, and, crucially, the crust formed quickly, so it should be reasonably uniform, which will make drilling easier.
Getting to the mantle is going to be extraordinarily tough, but Teagle sees the project as vital to answering some of the biggest questions challenging geologists today. It will give us a significantly better understanding of how our planet evolved, he says, as well as defining the limits of life. “The project will require a space mission-level of planning, but will cost a fraction of going back to the moon or returning rocks from Mars. Yet a pristine mantle sample would be a geochemical treasure trove, like bringing back the Apollo rocks.”
