There’s a mystery brewing at the centre of the Earth. Scientists can only see it when they study seismic waves – subterranean tremors generated by earthquakes – passing through the planet’s solid-iron inner core. For some reason, waves move through the core significantly faster when they’re travelling between the North and South Poles than when they’re travelling across the equator.
Researchers have known about this discrepancy, known as seismic anisotropy, for decades, but have been unable to come up with an explanation that’s consistent with the available data. Now, using computer simulations of the core’s growth over the last billion years, a recent study offers a solution that finally seems to fit: every year, little by little, Earth’s inner core is growing in a ‘lopsided’ pattern, with new iron crystals forming faster on the east side of the core than on the west side.