Subterranean secrets and subatomic ghosts

Lyme Regis beach sandcastles
It’s tough work digging tunnels on Lyme Regis beach. Where would we emerge if we kept going?

What lurks beneath the streets of Lyme Regis?

Science Feature by Professor Glenn Patrick

AS a young boy I was fascinated by the adventure novels of Jules Verne, especially Journey to the Centre of the Earth.

This interest was resurrected on a recent trip to Iceland where I was reminded that Snaefellsjökull – or snow fell glacier – was the location for the volcanic tube that was supposedly the entrance to a subterranean world going all the way to the centre of the Earth!

This ice-capped volcano in west Iceland just adds to the mystery of discovering a secret world lurking beneath our feet complete with caverns full of dinosaurs.

This boyish interest was piqued again when I read about attempts to discover coal beneath Lyme Regis.

In 1901, against the advice of geologists, local landowners arranged for a drilling rig to bore into the ground to search for coal. This failed to find any coal at the anticipated depth of 600 feet so boring was continued down to 1,200 feet – again without success.

With funds getting short, a Mr A.C. Pass of Wootton Fitzpaine stepped in and paid for further boring to try to achieve a definitive result. The boring eventually reached 1,300 feet – but still no coal!

As these borings passed through the various strata of the Jurassic Coast, their commercial prospects diminished, but their interest to geologists increased.

They had the last laugh as the core samples from this speculative drilling were saved and sent to various museums for their scientific interest – something that otherwise would have been unaffordable.

This story set me wondering about what would happen if the Lyme Regis drilling had continued right through the Earth and whereabouts it would emerge.

This is the so-called antipode – the place that you would come out on the Earth’s surface if you tunnelled in a straight line from Lyme Regis through the centre of the Earth.

It turns out that our antipode is in the Indian Ocean just south of New Zealand. The closest settlement is Portobello with a population of about 1,100, but even this is 721 km away from the antipode. Perhaps, another twinning opportunity for the town?

The deepest vertical hole ever bored is only 7.5 miles (or 12.2 km) deep. I say “only” because the Earth has a diameter of about 7,900 miles and there is a long way to go to achieve the tunnel of my boyhood dreams.

This is the Kola Superdeep Borehole located deep within the Arctic Circle near the Russian border with Norway. Starting in 1970, it took the Russians almost 20 years to reach this depth.

The hole is only nine inches in diameter and was part of a race by the superpowers to drill as deep as possible through the Earth’s crust in the hope of obtaining samples of the mantle – that layer of silicate rock between the thin crust and the molten outer core of our planet.

Further drilling beyond the 7.5 miles was stopped because the temperatures reached 180 degrees Celsius – almost twice as high as predicted – deforming and melting the drill bits. As a result, the hole only penetrated a third of the way through the 25-mile-thick crust of the Earth.

Today, the Kola borehole is capped and lies abandoned and derelict in the Arctic wilderness. Superstitions are rife with locals swearing that they can hear the screams of tortured souls emerging and newspapers even dubbing it the “well to Hell”.

Ghostly neutrinos can easily make the journey through the Earth revealing some of its subterranean secrets

Despite this, our fascination with the deep Earth continues and fact is often stranger than fiction. Rather than screams emerging from the subterranean world, there are, in fact, billions upon billions of ghostly subatomic particles which we call neutrinos!

The name neutrino was coined by Enrico Fermi from the Italian for “little neutral one”. It is extremely difficult to detect neutrinos because they interact only via the weak nuclear force (responsible for radioactivity) meaning that they can travel vast distances across the Universe.

They can easily pass straight through the Earth – hence their nickname of “ghost particle”.

Given their diffidence, it may surprise you to learn that the neutrino is the second most abundant particle (after the photon) in the Universe.

It is estimated that there are 339 relic neutrinos left over from the Big Bang in each bit of space the size of a sugar cube!

As well as this soup of primordial particles, there are other neutrino sources. One hundred million neutrinos from the Sun also hit each square metre of the Earth every second.

In fact, we would not be alive were it not for these neutrinos as they are products of the fusion reactions that take place deep inside the Sun providing us with light and warmth.

Nuclear reactors also produce vast numbers of neutrinos from the decays of fission fragments inside their cores. We even emit around 4,400 neutrinos every second from the decay of radioactive potassium inside our own bodies, but due to their properties they do us no harm.

High energy neutrinos also bombard us from sources across the Universe such as supernovae and even black holes beyond our galaxy.

There is a detector – with the appropriate name of IceCube – which sits at the South Pole detecting these cosmic neutrinos and working out where they come from.

Using hot water hoses, holes were drilled down to a depth of 2.5 km in a cubic kilometre of pure Antarctic ice so that strings of instruments could be lowered to build up this “neutrino telescope” of 5,000 detectors.

As well as looking at neutrinos from the cosmos, telescopes like IceCube can also use neutrinos to study processes deep within our own Earth, which we know surprisingly little about.

Geologists, for example, have used 30,000 boreholes (those again) around the Earth to estimate that some 44 trillion watts of heat continually flow from the Earth’s interior. But where does it come from?

A principal candidate for the source of this heat is the decay of radioactive isotopes such as Uranium 238 and Thorium 232 present inside the Earth.

As we have seen, the deepest borehole is only 7.5 miles deep, but it would be great to be able to probe into the deep Earth to test this idea.

Well, it turns out that neutrinos emitted during these radioactive decays – so called “geoneutrinos” – make the reverse journey.

The Earth actually shines with geoneutrinos and by capturing some of them in giant detectors like IceCube we can work out what is going on deep within the Earth!

There are many challenges, but it has already been shown that at least 50 per cent of the Earth’s heat is generated from radioactive decay.

We really do have messengers that can journey from the centre of the Earth and survive to tell us a story of what secrets they have found.

Next time you are digging holes in the sand on Lyme Regis beach, think about the effort it took to dig the Kola Superdeep Borehole and how ghostly neutrinos make light of slicing through the Earth.

Glenn Patrick is a particle physicist and science communicator who explores the quantum world of sub-atomic particles (including at the Large Hadron Collider) and now lives in Lyme Regis.

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