HOW DO WE KNOW...

WHAT’S AT THE CEN­TRE OF THE EARTH?

BBC Earth (Asia) - - Science - WORDS BY BRIAN CLEGG

We live on the sur­face of a dense, rocky ball, but sci­ence has al­lowed us to peer

deep within its core

When the pi­o­neer­ing sci­ence fic­tion writer Jules Verne wrote Jour­ney To The Cen­tre Of The Earth in 1864, he prob­a­bly knew that his plot was pure fan­tasy. Verne’s char­ac­ters Otto, Axel and their guide Hans, only made it a few miles down, but the idea that any­one could even con­tem­plate trav­el­ling to the Earth’s core had been dis­missed be­fore Vic­to­rian times.

Even to­day, the fur­thest we’ve ever drilled into the Earth is around 12km, while the dis­tance to the cen­tre is over 500 times fur­ther, at 6,370km. So how do we know what lies be­neath? Fig­ur­ing out what’s at the heart of our planet has been a magnificent sci­en­tific puz­zle.

LIV­ING ON A BALL

The idea of the Earth hav­ing a mean­ing­ful cen­tre goes hand-in-hand with the planet be­ing shaped like a ball, and we’ve known that we don’t live on a disc for a long time. It’s a myth that me­dieval folk thought the Earth was flat – this ac­tu­ally came from a mix of Vic­to­rian anti-re­li­gious pro­pa­ganda, and a mis­in­ter­pre­ta­tion of the stylised maps of the pe­riod. It was over 2,200 years ago that the Greek polymath Eratos­thenes made the first mea­sure­ment of the dis­tance around the Earth’s sphere, and it’s been clear ever since that it must have a cen­tre.

This doesn’t mean, though, that early philoso­phers thought of Earth as we do to­day. An­cient Greek physics said that the world con­sisted of a se­ries of con­cen­tric spheres of four fun­da­men­tal el­e­ments: earth, wa­ter, air and, fi­nally, fire. In this old­est sci­en­tific pic­ture, the cen­tre of the planet had to be solid, as air couldn’t be in­side the sphere of earth. Clearly, the sphere of earth wasn’t com­pletely sur­rounded by wa­ter or there’d be no dry land, so there was thought to be a bit of the earth stick­ing out – mean­ing there could only be one con­ti­nent. As a re­sult, the dis­cov­ery of the Amer­i­cas was one of the first ex­per­i­men­tal sci­en­tific re­sults, dis­prov­ing the idea of a sin­gle con­ti­nent, and mark­ing a sig­nif­i­cant step on the way to dis­pose An­cient Greek sci­ence.

The idea of the Earth be­ing en­tirely hol­low, or with vast cav­erns reach­ing to the cen­tre as in Verne’s book, has been pop­u­lar in fic­tion and mythol­ogy since an­cient times, also fea­tur­ing in var­i­ous pseu­do­sci­en­tific and con­spir­acy the­o­ries. How­ever, it’s not clear that any sci­en­tist apart from the as­tronomer Ed­mond Hal­ley, who pro­posed a hol­low Earth to ex­plain some un­usual com­pass read­ings in 1692, has ever taken this idea se­ri­ously. And in 1798, an English sci­en­tist and ec­cen­tric put the fi­nal nail in

the cof­fin of the ‘hol­low Earth’ hy­poth­e­sis. This was when Henry Cavendish weighed the planet.

WEIGH­ING THE PLANET

Cavendish was an odd man, who only com­mu­ni­cated with his ser­vants via notes to avoid meet­ing them face-to-face. De­spite his aris­to­cratic back­ground, Cavendish ded­i­cated his life to sci­ence, work­ing in both chem­istry and physics, and most fa­mously de­vised an ex­per­i­ment to cal­cu­late the den­sity of the Earth.

Us­ing a sim­ple tor­sion bal­ance, which mea­sured the amount of twist­ing force caused by the grav­i­ta­tional pull of two large balls on a smaller pair, Cavendish was able to cal­cu­late the faint grav­i­ta­tional at­trac­tion be­tween the two pairs of balls. By com­par­ing this with the Earth’s own grav­i­ta­tional pull, he could work out the planet’s den­sity (and, as the Earth’s size was al­ready known, its mass, too). But the den­sity fig­ure showed that our planet must be mostly solid, un­less there were ex­tremely dense un­known ma­te­ri­als some­where in the depths.

To­day, we split the in­nards of the Earth into three seg­ments: the crust, which is the outer layer, be­tween 5km and 75km thick, the man­tle, ex­tend­ing to a depth of around 2,900km, and the core – the bit we’re in­ter­ested in here – ex­tend­ing around 3,500km out from the Earth’s cen­tre, with two dis­tinct seg­ments. At the core’s heart is an ex­tremely hot but still solid nickel-iron sphere with a ra­dius of around 1,200km. At ap­prox­i­mately 5,400°C, this in­ner core is sim­i­lar in tem­per­a­ture to the sur­face of the Sun. The re­main­der is the liq­uid outer core, also mostly nickel-iron, with sim­i­lar tem­per­a­tures, get­ting hot­ter to­wards the cen­tre. But how can we pos­si­bly know such de­tail about a lo­ca­tion that is so in­ac­ces­si­ble?

Given the near-im­pos­si­bil­ity of ever get­ting even within a thou­sand kilo­me­tres of the core, all our knowl­edge is in­di­rect and de­pends on seis­mol­ogy – the sci­ence of earth­quakes. Af­ter a quake, seis­mic waves travel through the Earth, chang­ing their form and di­rec­tion de­pend­ing on the ma­te­ri­als they pass through. Geo­physi­cists have used this in­for­ma­tion to de­duce what lies at the Earth’s core. Their seis­mome­ters, de­vices to mea­sure such waves, are the equiv­a­lent of tele­scopes for ex­plor­ing the Earth’s in­te­rior.

By the early 20th Cen­tury, the in­creas­ing tem­per­a­tures as we dug deeper into the Earth, com­bined with seis­mol­o­gists’

anal­y­sis of earthly waves, sug­gested that the in­ner parts of our planet were at least partly molten – hot enough to turn rock and metal into liq­uid. And the key dis­cov­er­ies were made by two sci­en­tists who, shame­fully, were never even nom­i­nated for a No­bel Prize: Bri­tish ge­ol­o­gist Richard Old­ham and Danish seis­mol­o­gist Inge Lehmann.

WON­DER­FUL WAVES

Think of a wave, and you’ll prob­a­bly think of a sur­face wave, like one you’d see on the sea. But many waves – sound, for ex­am­ple – travel through the body of a ma­te­rial. Though the seis­mic waves that cause dam­age in an earthquake are those that travel on the sur­face, there are also two types of ‘body wave’ that move through the Earth. P-waves (‘P’ stands for ‘pri­mary’) are lon­gi­tu­di­nal waves, just like sound. They vi­brate in the di­rec­tion of move­ment, caus­ing the Earth to squash up and ex­pand as they pass through. P-waves travel rapidly – around 5km per sec­ond in a rock like gran­ite, and up to 14km per sec­ond in the dens­est parts of the man­tle. The sec­ond type of body wave, S-waves (‘S’ stands for ‘sec­ondary’), are slower, transverse waves, mov­ing from side-to-side. Un­like P-waves, they can’t travel through a liq­uid, which is why these two types of wave proved es­sen­tial in help­ing us un­der­stand the Earth’s core.

Imag­ine there’s a huge earthquake. Waves be­gin to move through the Earth. The P-waves shoot ahead, while the S-waves fol­low be­hind at around half the speed. Both types of wave will be de­tected by seis­mome­ters, which are used to mea­sure vi­bra­tions in the ground, all over the Earth. But where the waves pass through the core to reach a dis­tant mea­sur­ing sta­tion, there is a so-called shadow zone. Travel about 104° around the Earth’s perime­ter from the quake’s epi­cen­tre and the waves dis­ap­pear. But from 140° on­wards, the P-waves reap­pear, with no ac­com­pa­ny­ing S-waves.

As early as 1906, Richard Old­ham

re­alised the im­pli­ca­tions of this odd shadow. Old­ham spent most of his ca­reer with the Ge­o­log­i­cal Sur­vey of In­dia, of­ten work­ing in the Hi­malayas. When he re­tired to the UK in 1903, he made use of the data ac­cu­mu­lated over the pre­vi­ous few years to probe the in­te­rior of the Earth. He re­alised that the ob­served P-wave and S-wave be­hav­iour could be ex­plained if the cen­tre of the Earth was liq­uid. In such a case, P-waves would be re­fracted by the liq­uid, bend­ing as light does when it moves from wa­ter to air, leav­ing a dis­tinc­tive shadow. S-waves, by con­trast, would be stopped en­tirely by a liq­uid core.

Old­ham’s break­through led to a widely ac­cepted pic­ture of a molten core, but 30 years later, Inge Lehmann re­alised that Old­ham’s idea was too sim­ple. The refraction of the P-waves by the dense liq­uid in the cen­tre of the Earth should have pro­duced a to­tal shadow. How­ever, mea­sure­ments made with the more sen­si­tive seis­mome­ters avail­able by Lehmann’s time showed that faint P-waves were still ar­riv­ing in the shadow zone. By study­ing data pass­ing through the planet from a 1929 New Zealand earthquake (see ‘The key dis­cov­ery’), Lehmann pro­posed that these waves were be­ing re­flected off the bound­ary be­tween an in­ner solid core and the outer liq­uid. Her re­sults, pub­lished in 1936, were con­firmed two years later by Beno Guten­berg and Charles Richter, who ac­cu­rately mod­elled the ef­fects of a solid core. Direct mea­sure­ments of these re­flected seis­mic waves fi­nally came in 1970.

UN­DER PRES­SURE

Fur­ther stud­ies picked up even more sub­tle waves which, from their de­layed ar­rival, had to have crossed the liq­uid outer core as P-waves, be­fore be­ing con­verted to transverse S-waves in the in­ner core, and then back to P-waves on the way out. This dis­cov­ery, only con­firmed in 2005, was fur­ther proof of the solid core.

Even so, the ex­act na­ture of the in­ner core is sub­ject to sig­nif­i­cant de­bate. Tem­per­a­tures, for in­stance, can only be worked out from ex­per­i­men­tal stud­ies of how ma­te­ri­als melt and so­lid­ify un­der pres­sure. And the as­sump­tion that the core con­sists pri­mar­ily of iron and nickel comes from a com­bi­na­tion of the fre­quency with which dif­fer­ent el­e­ments oc­cur in our lo­cal re­gion of the Milky Way, and our un­der­stand­ing of how our planet formed.

Un­der the im­mense pres­sure at the cen­tre of the Earth – over three mil­lion times at­mo­spheric pres­sure – ma­te­ri­als can act very dif­fer­ently from nor­mal con­di­tions. While the most ob­vi­ous con­tender for the in­ner core is a solid nickel-iron al­loy, it is pos­si­ble for an ex­tremely dense plasma – the state of mat­ter found in a star – to have sim­i­lar prop­er­ties. One of the dif­fi­cul­ties here is know­ing how ma­te­ri­als be­have in such ex­treme en­vi­ron­ments.

En­ter the di­a­mond anvil cell. In this re­mark­able de­vice, the points of two di­a­monds, just a frac­tion of a mil­lime­tre across, are squeezed to­gether. Ap­ply­ing a force to a small area pro­duces more pres­sure than ap­ply­ing it to a wide one – that’s why be­ing trod­den on by a stiletto heel is much more pain­ful than a flat sole. The di­a­mond anvil cre­ates pres­sures up to twice that of the Earth’s core, and heat­ing is ap­plied us­ing lasers. When metal­lic sam­ples are crushed and heated to core-like con­di­tions, the re­sults sug­gest a crys­talline solid in the cen­tre of the Earth.

Re­al­is­ti­cally, we will never get any­where near the Earth’s core. The lev­els of heat, pres­sure and ra­dioac­tiv­ity (one of the main sources of in­ter­nal heat­ing) are so high that even if we could bore through over 6,000km of rock and metal, a probe would be un­able to sur­vive. Com­pared with reach­ing the core, trav­el­ling to the outer reaches of the So­lar Sys­tem is triv­ial. But our planet’s own vi­bra­tions, pro­duced by earth­quakes and in­ter­preted by sci­en­tists as in­ge­nious as Inge Lehmann, give us the means to ex­plore with our minds where we will never visit in per­son.

Mis­in­ter­preted maps from the me­dieval pe­riod led to the myth that peo­ple once thought the Earth was flat

Charles Richter con­firmed Inge Lehmann’s the­ory that the Earth had a solid core; he also cre­ated the Richter Scale to de­fine the mag­ni­tude of earth­quakes

Danish seis­mol­o­gist

Inge Lehmann es­tab­lished that our planet has a solid core

Earth’s in­ter­nal struc­ture: up­per man­tle (red), lower man­tle (or­ange), outer core (yel­low), and in­ner core (grey)

In a di­a­mond anvil cell, met­als are squished be­tween two di­a­monds at enor­mous pres­sures, to sim­u­late con­di­tions at the Earth’s core Brian Clegg is a pro­lific sci­ence writer. His most re­cent book is Are Num­bers Real?

Eratos­thenes’ knowl­edge of the Sun and given lo­ca­tions on the planet helped him cal­cu­late Earth’s cir­cum­fer­ence

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