Deep below the crust of Earth, past the thick mantle and liquid outer core, lies a 1,220-kilometer (760 mile) ball of solid inner core.
But a new study has suggested that the inner core is not solid at all, instead forming a ‘superionic state’ with hydrogen, oxygen, and carbon, making it unlike either a liquid or a solid.
We can’t exactly drill down the 6,371 km (3,959 miles) to the center of the Earth to check what’s going on, so scientists use Earth’s natural drill – seismic waves from earthquakes – to understand the composition of our planet.
However, even with these measurements, the inner core remains somewhat of a mystery. In the 1930s, indirect evidence suggested it could be solid, and a few decades later it was thought to be a crystalline iron. But this incredibly hot, incredibly dense ball in the middle of our planet is still making us second guess what’s going on in there.
We know from seismic wave data that the inner core is soft, with a low shear-wave velocity, meaning it can’t just be solid iron or iron alloy. Some scientists think there could be a second inner inner core, while others think that due to the lighter density than would be expected by pure iron alone, there could be some light elements as an alloy.
But a new study, led by Yu He from the Chinese Academy of Sciences, has now investigated the potential phase of matter this mix of elements may exist in down, arriving at the suggestion that the ‘solid’ state of the core might really be a superionic state instead.
“We find that hydrogen, oxygen and carbon in hexagonal close-packed iron transform to a superionic state under the inner core conditions, showing high diffusion coefficients like a liquid,” the team writes in their new paper.
“This suggests that the inner core can be in a superionic state rather than a normal solid state.”
Superionic is another state of matter – alongside solid, liquid, and gas – but with distinct differences. In superionic water – which was recently made in a lab – extremely high temperatures and pressures break apart each water molecule, leaving the oxygen ions to form a solid, while the hydrogen ions float around more like a liquid.
In the hot inner core of Earth, the team used computer simulations of how seismic waves would travel through different combinations of elements, and discovered that alloys of iron with carbon, hydrogen, and oxygen could work the same way as superionic water.
The iron atoms were ‘solid’ in the crystalline lattice structure, whereas the carbon, hydrogen, and oxygen molecules would diffuse through the medium, creating the liquid-like element.
“It is quite abnormal,” said He. “The solidification of iron at the inner core boundary does not change the mobility of these light elements, and the convection of light elements is continuous in the inner core.”
This work is unlikely to be the last word on the subject. The paper’s conclusions give a good model for this softer and less dense pure iron, but it doesn’t answer another question about the inner core – why it is seemingly uneven throughout.
For that, we’ll just have to keep digging.
The research has been published in Nature.