EU-funded study finds planet's inner core moving much slower than previously believed
New EU-funded research offers the world an accurate estimate of the rotating speed of the Earth's core, suggesting it is a lot slower than was previously believed. However, the core continues to turn at faster rates than the rest of the planet. Presented in the journal Nature Geoscience, the findings are an outcome of the EARTH CORE STRUCTURE ('Thermal and compositional state of the Earth's inner core from seismic free oscillations') project, which was funded with a European Research Council (ERC) Starting Grant worth EUR 1.2 million under the Seventh Framework Programme (FP7). This information will allow the correction of the values usually used in models, which were previously considered to be higher rates of rotation movements of the core. If the models were based on wrong assumptions, all the previous calculations would need to be corrected on the basis of this new knowledge and data.
Researchers from the University of Cambridge, UK say earlier estimates that the core is rotating one degree a year faster than the rest of the planet were really off the mark. Based on their data, the core is actually moving at around one degree faster every million years. The team calculated the rotation rate from the shift of the core boundaries and the growth rate of the inner core.
Research leader Lauren Waszek, a PhD student from the Department of Earth Sciences at Cambridge, said: 'The faster rotation rates are incompatible with the observed hemispheres in the inner core because it would not allow enough time for the differences to freeze into the structure. This has previously been a major problem, as the two properties cannot co-exist. However, we derived the rotation rates from the evolution of the hemispherical structure, and thus our study is the first in which the hemispheres and rotation are inherently compatible.'
Scientists know that the inner core grows as material from the fluid outer core solidifies onto its surface. When this happens, say the researchers, an east-west hemispherical difference in velocity is frozen into the inner core's structure.
Using seismic body waves that pass through the inner core, the team evaluated the difference between the time it took those waves to travel with waves that reflect from the planet's inner core surface. This helped them determine the velocity structure of the uppermost 90 kilometres of the inner core. They used this information to establish the velocity for the inner core's east and west hemispheres.
'The Earth's solid core was first discovered by the observation of PKiKP, a seismic wave that travels through the mantle and outer core before reflecting from the sharp inner core boundary,' the authors write. Composed mostly of iron, the inner core grows thanks to the solidification of outer core material onto the inner core boundary surface as the planet cools. The outcome is an older deeper structure.
'Although the thermal history of the inner core is debated, its uppermost structure results from processes occurring in the recent past, of which we have greatest understanding,' the researchers write. 'This resulting time depth variation of the upper inner core is key to investigating any changing environment at the inner core boundary region associated with inner core super-rotation.'
Positioned at 5 200 kilometres beneath the surface of the Earth, the inner core plays a huge role in the surface of the planet. Heat released during the solidification — as the inner core grows — fuels convection in the outer core's fluid. This convection is responsible for producing the planet's geomagnetic field, which in turn keeps us safe from solar radiation and ultimately ensures life on Earth.
'This result is the first observation of such a slow inner core rotation rate,' said Ms Waszek says. 'It therefore provides a confirmed value which can now be used in simulations to model the convection of Earth's fluid outer core, giving us additional insight into the evolution of our magnetic field.'