Oct. 10, 2013 — The iron in the Earth's inner core weakens dramatically before it melts, explaining the unusual properties that exist in the moon-sized solid centre of our planet that have, up until now, been difficult to understand.
Scientists use seismic waves -- pulses of energy generated during earthquakes -- to measure what is happening in the Earth's inner core, which at 6000 km beneath our feet is completely inaccessible.
Problematically for researchers, the results of seismic measurements consistently show that these waves move through the Earth's solid inner core at much slower speeds than predicted by experiments and simulations.
Specifically, a type of seismic wave called a 'shear wave' moves particularly slowly through the Earth's core relative to the speed expected for the material -- mainly iron -- from which the core is made. Shear waves move through the body of the object in a transverse motion -- like waves in a rope, as opposed to waves moving through a slinky spring.
Now, in a paper published in Science, scientists from UCL have proposed a possible explanation. They suggest that the iron in the Earth's core may weaken dramatically just before melting, becoming much less stiff. The team used quantum mechanical calculations to evaluate the wave velocities of solid iron at inner-core pressure up to melting.
They calculated that at temperatures up to 95% of what is needed to melt iron in the Earth's inner core, the speed of the seismic waves moving through the inner core decreases linearly but, after 95%, it drops dramatically.
At about 99% of the melting temperature of iron, the team's calculated velocities agree with seismic data for the Earth's inner core. Since independent geophysical results suggest that the inner core is likely to be at 99-100% of its melting temperature, the results presented in this paper give a compelling explanation as to why the seismic wave velocities are lower than those predicted previously.
Professor Lidunka Vo
Scientists use seismic waves -- pulses of energy generated during earthquakes -- to measure what is happening in the Earth's inner core, which at 6000 km beneath our feet is completely inaccessible.
Problematically for researchers, the results of seismic measurements consistently show that these waves move through the Earth's solid inner core at much slower speeds than predicted by experiments and simulations.
Specifically, a type of seismic wave called a 'shear wave' moves particularly slowly through the Earth's core relative to the speed expected for the material -- mainly iron -- from which the core is made. Shear waves move through the body of the object in a transverse motion -- like waves in a rope, as opposed to waves moving through a slinky spring.
Now, in a paper published in Science, scientists from UCL have proposed a possible explanation. They suggest that the iron in the Earth's core may weaken dramatically just before melting, becoming much less stiff. The team used quantum mechanical calculations to evaluate the wave velocities of solid iron at inner-core pressure up to melting.
They calculated that at temperatures up to 95% of what is needed to melt iron in the Earth's inner core, the speed of the seismic waves moving through the inner core decreases linearly but, after 95%, it drops dramatically.
At about 99% of the melting temperature of iron, the team's calculated velocities agree with seismic data for the Earth's inner core. Since independent geophysical results suggest that the inner core is likely to be at 99-100% of its melting temperature, the results presented in this paper give a compelling explanation as to why the seismic wave velocities are lower than those predicted previously.
Professor Lidunka Vo