Journey to the core of the Earth
Giulia Pacchioni
The composition of the Earth’s core is not exactly known. Its precise determination is hindered by the
complexity of the experiments that could probe it. New measurements of the properties of liquid iron
under temperatures and pressures close to those at the Earth’s core — 116 GPa and 4350 K — reported
by Yasuhiro Kuwayama, Yoichi Nakajima, Kei Hirose and colleagues in Physical Review Letters shed new
light on this matter.
Iron is the main element in the Earth’s core, but we know it’s not the only one. “The determination of
the composition of the Earth’s core is one of the most important topics in Earth science since Francis
Birch proposed that it may be lighter than pure iron in 1952,” notes Kuwayama. The properties of the
core that can be directly probed are the density and longitudinal sound velocity, which are derived from
seismic data. Comparing these values with those for pure iron under similar conditions is key to
determining the core’s composition and behaviour.
Credit: Kei Hirose, the University of Tokyo
However, the determination of the density of liquid iron at extreme temperatures and pressures is
challenging. First, it requires the maintenance of high pressure and high temperature simultaneously, for
sufficient time for the measurements to be taken. Second, determining the density of a disordered
material under extreme conditions is far from trivial as the existing analytical methods have high
uncertainty. Third, the measurements require a very brilliant and focused X-ray beam.
“To measure liquid samples, which produce a weaker signal than solid samples, over the past two
decades we have developed a very brilliant and highly focused X-ray source at the SPring-8 synchrotron
light source in Japan, optimized for high-pressure experiments,” explains Kuwayama. The researchers
used static compression in a laser-heated diamond anvil cell for the experiments and introduced a new
method to analyse diffuse X-ray scattering in a liquid to extract the density of the iron from the data. This
allowed them to determine that the Earth’s outer core is ~8% less dense than pure iron.
“Our results also show that oxygen concentration in the outer core is less than 3.8 wt%,” adds
Kuwayama. “It has been repeatedly suggested that oxygen is a main core light element, but these results
indicate that the core includes other impurity elements.” The amount and species of impurities
determine the physical properties of the core, such as viscosity and thermal and electrical conductivity,
and can provide information on the origin and evolution of the Earth.
over the past two decades we have developed a very brilliant and highly focused X-ray source at the
SPring-8 synchrotron light source in Japan, optimized for high-pressure experiments
This new analytic procedure to extract the density of a liquid from its X-ray scattering signal can be used
for any material, opening up new research directions. “The technical and analytical techniques we
developed for liquid materials are applicable also to melted rocks, magmas,” concludes Kuwayama. “We
are now planning to apply these techniques to the study of melted rocks to understand the nature of
magmas in the deep Earth.”
References
Original article
Kuwayama, Y. et al. Equation of state of liquid iron under extreme conditions. Phys. Rev. Lett. 124,
165701 (2020)
part 6
Is the earth's core a liquid?
Earth's core consists of a liquid outer core (yellow) surrounding an inner core (brighter yellow sphere).
New computer simulations suggest that, instead of being a normal solid, the inner core may be
superionic, a state of matter that has properties of both a solid and liquid.Feb 9, 2022
