What the research is about
Earth's inner core is thought to consist mainly of iron, yet many questions remain about its exact chemical composition and how it behaves. Scientists believe that the inner core contains not only iron but also lighter elements, with hydrogen among the possible candidates. However, which elements are actually present - and in what amounts - is still uncertain.
Because we cannot directly observe Earth's deep interior, seismic waves provide an important window into its properties. The speed of these waves changes depending on factors such as the density and stiffness of the material they pass through. Observations have shown that S-waves, a type of seismic wave, travel through the inner core more slowly than expected and at different speeds depending on direction. Understanding why requires a better picture of both the inner core's composition and the physical state of its materials.
One material studied to explore what might happen if hydrogen were incorporated into iron in the inner core is iron hydride, which consists of iron and hydrogen. Previous theoretical calculations predicted that under the extreme temperatures and pressures expected in the inner core, iron hydride could enter a superionic state. In this unusual state, the iron crystal remains solid while hydrogen moves through the spaces in the crystal almost like particles in a liquid. More generally, a superionic state is one in which some ions become highly mobile within an otherwise solid material. Until now, however, it has been extremely difficult to capture this transition experimentally under such extreme conditions.
Why this matters
A research team led by Professor Kenji Ohta at Institute of Science Tokyo (Science Tokyo) used a diamond-anvil cell at the large synchrotron radiation facility SPring-8 to generate pressures of up to about 1.4 million atmospheres. While heating iron hydride with lasers, the researchers used powerful X-rays to track changes in its crystal structure. The pressure they achieved is comparable to that near the depth where Earth's outer core begins.
As the temperature increased, the researchers observed an unusual change in how the iron crystal expanded. They interpret this change as a sign that the iron hydride may have entered a superionic state. In other words, an effect previously predicted by theory now has experimental evidence pointing in the same direction.
Pressure at the center of Earth reaches about 3.6 million atmospheres, so the experiment did not reproduce conditions at the very center itself. Instead, the researchers extended the relationship between pressure and temperature observed in their experiments to estimate what could happen at greater depths. Their results suggest that iron hydride could enter a superionic state at temperatures above about 2,500°C under conditions corresponding to Earth's center.
Previous theoretical studies have predicted that when hydrogen becomes superionic, iron alloys may become easier to deform under forces that shift one part of the material sideways relative to another. The speed of S-waves depends on how strongly a material resists this kind of deformation. The new findings therefore provide a possible clue to why S-waves travel more slowly than expected through Earth's inner core.
What's next
To understand Earth's deep interior, which we cannot observe directly, researchers must examine candidate materials one by one under extreme conditions and compare their behavior with seismic observations.
Future experiments will need to determine how much the superionic state actually changes the mechanical properties of iron alloys and how those changes affect the way seismic waves travel through them.
Comment from the researcher
High-pressure and high-temperature experiments are cutting-edge tools for exploring the deep interiors of planets and other celestial bodies that we cannot access directly. We hope these findings will provide a clue to better understanding the properties of Earth's inner core.
The creativity and determination of our students were also essential in achieving these results. One of the most exciting parts of exploring the unknown is being able to take on these challenges together with students.
(Kenji Ohta, Professor, Department of Earth and Planetary Sciences, School of Science, Institute of Science Tokyo)
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