A Thermostat 2,000 Kilometers Down ✎

Inside Earth, at depths from 660 to 2,900 kilometers, lies a layer known as the lower mantle. There, pressures reach 1.3 million atmospheres, and temperatures rival the surface of the Sun. Under these conditions, the mineral davemaoite exists as the third most abundant mineral in the lower mantle. It makes up about a quarter of the volume of oceanic crust that slowly sinks into the planet's interior.

The crystal lattice of davemaoite resembles a tiny railway switch. At low temperatures, it is distorted: one axis is longer than the others, as if the switch were bent. When heated, the lattice straightens out and becomes cubic and symmetric. This is not just geometry. When the switch flips, seismic waves passing through the mineral change their path and speed.

Previously, it was thought that this switch occurred at around 600 kelvins. But new experiments show that for pure davemaoite, the transition only happens at 1470–1990 kelvins, depending on pressure. That is 500–1,000 degrees higher than earlier estimates. And if titanium is present, the switch holds even more firmly: the transition temperature rises by 600–700 degrees. In a cold oceanic plate, davemaoite can remain distorted down to depths of more than 2,000 kilometers.

The experiment was conducted in a diamond anvil cell: the sample was compressed between two diamonds and heated with a laser from both sides. Temperature was measured by radiation, and pressure by the compression of gold foil. X-ray diffraction at a synchrotron revealed how the distances between atoms changed. The transition was detected by a sharp change in the width of the diffraction peak.

So the thermostat 2,000 kilometers down flips the switch in davemaoite's crystal lattice. Now geophysicists know at what temperature this happens and can read seismic maps of the mantle more accurately. The puzzle that caused decades of debate between experimentalists and theorists has finally been solved.