Candy wrapper core

coreThat’s the title of a paper published in Scientific Reports this last week. The lead author, Maurizo Mattesini, a geophysicist from Madrid, proposes a new model for the make-up of Earth’s inner core. The paper title is captivating, but its explanation for the core’s structure is complicated.

The inner core is known to be formed of crystalline iron, but the exact atomic arrangement of the iron is uncertain, and remains an enigmatic puzzle in the most inaccessible part of our planet.

Data from seismic waves, passing through Earth’s inner core, have suggested differences between the eastern and western hemispheres of the inner core, which sits within the molten liquid metal outer core. The inner core first began to crystallise from within the outer core more than a billion years ago, as Earth cooled. Since that time it has grown to its present size, around 1,220 km radius, and it continues to solidify and grow at a rate of about 0.5 mm per year.

While seismic waves tell us about the inner cores density and elasticity, they do not distinguish the precise arrangement of atoms. The conditions of extreme pressure and temperature of the inner core – more than 6000 K and well over three million atmospheres – make it very difficult to replicate in the laboratory, and quantum mechanical calculations of the physical properties of iron under those conditions remain the best approach. But various calculations differ.

Mattesini and colleagues suggest that the differences seen in more than 1000 seismic events with waves passing through different parts of the core are due to differences in the atomic structure of iron in the various parts. It builds a picture of a heterogeneous inner core, with variations in structure and mixing of material on one side, more than the other.

The reasons for the differences in the east and west sides of the core are themselves uncertain, but a prevailing explanation is that following the last major asteroid impact event the inner core suffered an impulse as the Earth rattled, so that still the inner core has an eastward drift with the front side continually melting as the trailing side gradually crystallises, so that dynamically the sphere remains central.

The core certainly appears intriguing, with potential stories yet to tell. The approach adopted in this week’s report is promising, because it links mineral physics directly to seismology, and provides a way of reconciling disparate results and building a consistent picture of this, the deepest depths of the Earth. Via EarthStory

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