Earth's solid inner core may contain significant amounts of a rare substance dubbed 'extreme hydrogen,' which behaves in ways unlike anything found on the planet's surface, according to a new study published in the journal PNAS.
This material, known as super-ionic hydrogen, exhibits properties of both a liquid and a solid under the extreme conditions found approximately 3,200 miles (5,100 km) beneath the surface. At these depths, intense pressure and temperatures nearing those of the sun's surface cause hydrogen to flow like a liquid through a solid iron lattice and conduct electricity.
Scientists have long suspected the presence of hydrogen within the inner core, but its distribution and behavior have remained challenging to determine. The current study employed quantum-mechanical simulations to model how hydrogen might behave under the inner core's immense pressures and temperatures. These simulations suggest that super-ionic hydrogen is not uniformly distributed but is most concentrated at the boundary between the solid inner core and the liquid outer core.
At the inner core boundary, where temperatures reach approximately 5,226°C (5,500 K), hydrogen is estimated to make up about 16 percent of the atoms. This concentration decreases to around nine percent closer to the Earth's center.
The inner core, a sphere of iron alloy weighing 102 quintillion tons, is one of the most extreme environments in the solar system. Subjected to pressures exceeding 3.3 million atmospheres, it maintains a solid state despite its high temperature. However, its seismic properties, such as the slowing of earthquake shockwaves and a malleability closer to butter than steel, suggest the presence of lighter elements mixed with iron. Hydrogen is a prime candidate due to its abundance when Earth formed and its ability to dissolve into iron under certain conditions.
The simulations indicated that at extreme heat and pressure, hydrogen atoms flow freely through the iron crystal lattice. This behavior aligns with the observed seismic characteristics of the inner core, which has baffled scientists for decades. The researchers also found that the solid iron likely forms a 'hexagonal close-packed' crystal structure, rather than a previously considered body-centered cubic (BCC) phase, which would become unstable and melt under the inferred temperatures.
This atomic movement of super-ionic hydrogen between the solid inner core and the liquid outer core could play a crucial role in shaping Earth's magnetic field. Previous studies have suggested that the migration of hydrogen creates buoyancy in the outer core, driving the convection of liquid metal that generates the geodynamo—the process responsible for Earth's protective magnetic shield. Without this magnetic field, the planet's surface would be exposed to harmful cosmic radiation, potentially preventing life from developing.
As the inner core grows and crystallizes, the super-ionic hydrogen is believed to migrate outward into the liquid outer core. This process is thought to be a vital part of the energy sources that sustain the planet's life-preserving magnetic field.