Improved Model of Outer Core 1D Structure

We have published a new model of 1D structure of the outer core, parameterized using two different equations of state: Irving_Cottaar_Lekic_2018

Brief highlights:

  • We infer the seismic properties and equation of state of the outer core using Earth’s oscillations
  • We address a longstanding discrepancy between high and low frequency investigations of the outer core
  • Our work is analogous to conducting a mineral physics experiment using the Earth to assess the material properties of the outer core
  • Our results could be used to model the properties of exoplanets around other stars

The outer core is a liquid iron-nickel alloy with an uncertain mixture of other, light elements. Its vigorous flow creates the magnetic field, which protects life on Earth from solar radiation. Convection in the outer core is driven thermally, by the gradual cooling of the Earth since its formation, and compositionally, by light elements being released as the inner core solidifies. Understanding the composition of the outer core is crucial for understanding these driving forces over the history of the magnetic field.  

Physical properties (the seismic velocity and density) of the outer core can be determined using normal modes. Normal modes are Earth’s standing waves which can be measured after large earthquakes (we could say that the Earth ‘rings like a bell’ at characteristic frequencies). We tackled the question of the average structure of the outer core using new published measurements of normal modes including higher precision data. Unlike previous studies, we tie together velocity and density variations with a physical framework -- called an Equation of State -- used to represent the behavior of materials at high pressure in mineral physics experiments. The advantage of this framework is that we can more directly infer the physical nature of the iron alloy, and the parameters we constrain can be extrapolated to different pressures to simulate cores of super-Earth exoplanets. Additionally it means the outer core’s seismic structure can be described with only three parameters that can be readily compared against laboratory results. The model we have produced, EPOC (‘Elastic Parameters of the Outer Core’), also shows that the outer core’s alloy is more compressible and potentially contains more light elements than previously thought.

Seismic body waves, higher frequency waves which propagate in all directions from an earthquake, offer different constraints on the structure of the outer core. Over the past decades, multiple studies have shown that their behavior has not been well captured by models constructed using normal mode data. This led seismologists to propose that a seismically slow layer of up to several hundreds of kilometers in thickness exists at the top of the outer core, which would have important implications for outer core convection and Earth’s magnetic field. EPOC fits body waves to such a degree that this anomalous layer may not be required (although a thinner layer might escape detection).