Earth’s inner core has an inner core?

We all know that the Earth’s mostly iron core is divided into a molten outer core and solid inner core. But that may not be the whole story: some just-published seismic data suggests that the Earth’s inner core is divided into distinct inner and outer portions.

In most materials, the speed at which seismic waves move through varies by a few percent depending on which direction they are travelling. The new data measures the seismic anisotropy within the core in much more detail than has been achieved before, and indicates that the orientation of the directions of fastest and slowest travel, and the actual difference in speeds between these two directions, is different in the central ~650 km of the inner core compared to the outermost ~650 km.

The different seismic properties of the ‘innermost inner core’ has been observed previously, but this new study is strongly arguing that it is due to distinct layering – there is a sharp change at a particular depth within the inner core, rather than a more gradual change with depth.

How they got this information is pretty cool – they found seismic stations close to the antipode – the exact other side of the world – from a big earthquake, which is what you need to pick up seismic waves that travel through the core before returning to the surface. But those waves also get reflected back down back through the centre of the Earth close to where they originated, and then reflected back through the earth *again*. Then the same thing happens again… and again… and again. Each time the signal is weaker, but they managed to capture up to 5 of these reverberations from the same earthquake! And because the stations are not exactly antipodal, each time the wave samples a slightly different part of the inner core, which allows a high-resolution mapping of its properties.

If there is indeed an ‘innermost inner core’, it might give us a clue about the deep history of our planet, because it would imply potentially two distinct phases of inner core formation, rather than it simply being a product of gradual cooling over time. That would also have an impact on the evolution of the Earth’s magnetic field (which is generated in the liquid outer core, but the inner core appears to play a role in stabilising it).

I’m not sure I’m a fan of ‘innermost inner core’ though – it’s a little clunky.