Earth's inner core could be filled with a rare form of matter known as "extreme hydrogen," according to a new study published in the journal PNAS. Researchers say this material, called super-ionic hydrogen, behaves unlike anything found on the surface and could play a role in shaping the magnetic field that makes life on Earth possible.
At the extreme pressures found 3,200 miles (5,100 km) beneath the surface, this form of hydrogen flows like a liquid through solid iron and even conducts electricity, the study found. Scientists have long suspected hydrogen exists in the inner core, but working out exactly how it is distributed has proven difficult.
To investigate, the researchers used quantum-mechanical simulations to model how hydrogen would behave under the extreme conditions found at the center of the planet. They found that super-ionic hydrogen is not spread evenly through the inner core, but is instead concentrated at the boundary between the inner and outer core.
At temperatures of 5,226°C (5,500 K), hydrogen makes up 16 per cent of the atoms at the inner-core boundary, the study found, a figure that drops to around 9 per cent near the center of the planet.
A sphere of extreme pressure
The inner core is a 102 quintillion-tonne sphere of iron alloy located more than 3,000 miles beneath the surface, making it one of the most extreme environments in the solar system. There, the core is crushed by more than 3.3 million atmospheres of pressure and heated to temperatures close to those found on the surface of the sun.
Scientists know the inner core is made of superheated iron kept solid by this extreme pressure. Yet in many other ways it behaves as though it were almost molten: shockwaves from earthquakes passing through the inner core are slowed, and the core shows a level of malleability closer to butter than steel.
Researchers say this suggests lighter elements are mixed into the iron, giving the inner core properties of both a solid and a liquid. Hydrogen is considered one of the most likely candidates, since it was common in the universe when Earth formed and can dissolve into iron under the right conditions. At extreme heat and pressure, hydrogen atoms flow freely through a crystal lattice of iron atoms, which researchers say could explain the inner core's unusual seismic behavior.
Modelling a place no one can reach
Because humans cannot travel to the inner core, and its conditions cannot be recreated in a laboratory, the researchers relied on computer simulations. They set out to determine which crystal structures would be most stable inside the inner core and how that would affect the spread of hydrogen.
The simulations showed that the inner core's solid iron most likely takes the form of a "hexagonal close-packed" crystal structure. Scientists had previously thought the iron might be squeezed into a different shape known as a body-centred cubic phase, in which atoms sit at the corners of a cube with one trapped at the center.
However, the researchers found that phase only becomes stable at temperatures so extreme that they would melt the crystals back into a liquid, meaning it would not exist in practice. "Our calculations show that hydrogen can stabilize a superionic BCC phase at sufficiently high temperature and hydrogen content. However, this stability field is superseded by melting," the authors wrote.
Earth's layered structure
The study's supporting material describes Earth as made up of four major layers. The crust, where all life exists, is between 3 and 43 miles thick. The mantle, the largest layer, measures about 1,802 miles thick and makes up 84 per cent of the planet's volume. The outer core is about 1,367 miles thick and consists of liquid nickel and iron heated to 5,500°C (9,932°F). The inner core is a hot, dense ball of iron roughly the size of the moon, with temperatures reaching 5,200°C (9,392°F).
Feeding Earth's magnetic field
The simulations also revealed how hydrogen moves between the solid inner core and the liquid outer core. As the inner core grows and crystallizes, the researchers believe super-ionic hydrogen migrates to its outer boundary before passing into the liquid beyond.
Previous studies have suggested this movement of atoms could create buoyancy in the liquid outer core, driving the churn of molten metal that generates Earth's magnetic field. The field is believed to form as differences in temperature, pressure and composition drive convection currents in the outer core, with the Coriolis force from Earth's spin adding a swirling motion, a self-sustaining loop known as the geodynamo. Without that field, the researchers note, Earth's surface would be bombarded by harmful cosmic radiation, making it impossible for life to develop. That means the movement of extreme hydrogen could be a critical part of the process that keeps the planet's life-preserving shield intact.
