“If we want to understand how the Earth was formed, then one of the things you need to know is what planet is made out of,” explains Frost. Lots of geologists assume that the Earth was made from the same stuff as meteorites from the asteroid belt. The problem is that most meteorites that fall to Earth have a higher proportion of silicon than we find in the Earth’s crust. So where did it go? One option is that it’s stuck in the lower mantle.
To answer this kind of question, Frost turns to two kinds of presses. The first uses a powerful piston to squeeze tiny samples of crystals at up to 280,000 times atmospheric pressure, as they are simultaneously cooked by a furnace. That recreates the conditions in the top layers of the lower mantle at around 800 or 900km (500 to 562 miles) below the Earth’s surface, causing the atoms of the crystal to rearrange into denser structures.
A second anvil then crushes the newly formed minerals so that they resemble those found in even deeper reaches of the Earth. It is made from two tiny gem-quality diamonds that slowly squeeze the crystals. “It’s like having a very pointy pair of stiletto heels,” he says. The result is 1.3 million times that of atmospheric pressure. While the sample is still in this device, he then measures the way sound waves travel through the resulting crystal. By comparing this data to readings of seismic waves traveling through the Earth’s interior, he can work out whether his sample is close to the composition of the mantle.
His findings have been somewhat surprising: the mantle does not seem to hold a high enough proportion of silicon to match the composition of meteorites. Perhaps it has sunk even deeper, to the core, says Frost. Another possibility is that the Earth initially had a much bigger crust, full of silicon that was then blasted away through impacts with meteorites. Alternatively, we may need to rethink the raw materials that the Earth was first made from in the first place.
The process of intense pressure has also created the mineral ringwoodite, a deep-blue magnesium iron silicate that seems to hold water. The results suggest that the mantle may be hiding “oceans” deep in the Earth.
The experiments may even, counter-intuitively, tell us about the air we breathe – and it is here that Frost’s diamonds come in. He suspects that a series of geological processes could pull CO2 out of the oceans, into rocks and then down into the mantle, where it is converted into diamonds. These gemstones are less volatile than other forms of carbon, says Frost, meaning that it is less likely to be released back into the atmosphere. A diamond-studded mantle could have therefore have slowed down the warming of the Earth, potentially helping the evolution of life.
The key ingredient for this to happen, he thinks, is iron. The high pressures of the mantle force carbon dioxide from the rocks into the iron-rich minerals, which strip away oxygen, leaving the carbon to form a diamond. And that is exactly what Frost found when he recreated the process using his presses – essentially forging a diamond from thin air.
Frost is hardly likely to make a fortune from his harvest; the diamonds take an agonizingly long time to grow. “If we wanted a two-or-three-millimeter diamond, we would need to leave it for weeks,” he says. That hasn’t stopped him experimenting with other sources for his diamond maker, however; at the behest of a German TV station, he attempted to create some diamonds from carbon-rich peanut butter. “A lot of hydrogen was released that destroyed the experiment,” he says, “but only after it had been converted to diamond.”
On a more serious note, his institute is looking at whether they can make artificial diamonds with different properties; doping the diamonds with boron might make better semiconductors for electronics that don’t heat up with use, for instance– one of the biggest wastes of energy in electronics at the moment.
Using other structures of carbon as raw material – in the form of tiny “nanotubes”, for instance – might even make a new type of super-strong diamond, tougher than any other known material.
For the most part, however, Frost is interested in the further secrets his work will tell us about the Earth’s history – and, potentially, life elsewhere. “We’re interested in how the Earth’s interior has interacted with the surface; over the age of the Earth, that’s been very significant,” he says. “And if we are looking for other habitable planets, we will have to consider many of these processes.” Vital work that surely calls for the sacrifice of a little peanut butter – and the odd explosion.
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