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March 2023 ESRFnews

GEOPHYSICS

H

UMANS have climbed Mount Everest,

landed on the Moon and probed the farthest

reaches of the solar system – even, in the

case of NASA’s Voyager spacecraft, sent signals from

interstellar space. Yet despite being a paltry 3,000

kilometres under our feet, the core of the Earth

remains something of a mystery. We know that it is

predominantly comprised of iron and nickel, but as for

its precise composition – no-one knows.

Ilya Kupenko intends to change that With custom

built instrumentation and a starting grant from the

European Research Council the ESRF mineral physicist

is planning to recreate the conditions of the Earths core

at several ESRF beamlines in order to determine once

and for all what it is really made of The answer could help

explain why some elements are curiously missing from the

Earths interior and improve our understanding of how

the Earths magnetic field is generated

Much of our knowledge of the Earths core comes

from seismology By measuring the time it takes for

seismic waves to travel from one side of the planet to

the other and knowing their speed of travel in different

materials at core-like pressures and temperatures, it

is possible to narrow down which materials can form

part of the composition. Seismic waves travel too slowly,

and the core density is too low, for the core to be a pure

iron–nickel alloy: in the region of 3–7% of it must

consist of one or more lighter elements, with hydrogen,

carbon, oxygen, silicon and sulphur being the most

likely candidates. Moreover, seismic waves take less time

to travel north to south than east to west. Whatever the

core is made of then it must transmit waves anisotrop

ically that is faster in one direction than the other

Novel instrumentation

Recreating extreme conditions requires specialist appar

atus diamond anvil cells DACs for high pressures

and powerful lasers for high temperatures The

trouble is that laserheating systems are sizeable pieces of

equipment and at synchrotrons tend to be permanently

located on specialised beamlines typically diffraction

that are illsuited for the type of elasticity measure

ments required to determine the speed of sound

For that reason Kupenko has designed his own

No-one knows

precisely what the

centre of the Earth

is made of. One ERC

grantee at the ESRF

is trying to find out.

The core

question

S H U T T E R S T O C K/ R O S T 9