The Earth is akin to a rocky, molten-in-the-middle nesting doll – or perhaps a rotting, soft-centered onion, though the latter analogy affords our majestic planet much less dignity.
Although only several handfuls (39 fingers' worth, to be exact) of people have alighted on the ocean's deepest depths or upon the surface of the Moon, there's one relatively nearby alien environment that no human will ever personally explore: the Earth's core.
Approaching it, each Earthly layer differs in composition and increases in pressure and temperature, displaying unique material properties that become murkier the deeper we delve.
So in a recent study, researchers probed the most mind-bending of these materials: superionic hydrogen, an otherworldly version of hydrogen that flows like a liquid, conducts electricity, and is found only in the most extreme conditions.
What they found is that this superionic hydrogen could actually be abundant in our planet's core, even though it's found nowhere else on Earth.
To better understand this esoteric elemental form, including the thermodynamics of its existence and hydrogen's overall distribution in the core, the researchers modeled three types of it.
This encompassed two types of superionic iron-hydrogen alloys, which involve hydrogen flowing through crystalline iron lattices arranged in different ways.
The first is the hexagonal close-packed (HCP) arrangement:
The second superionic iron-hydrogen format is known as the body-centered cubic (BCC) arrangement:
But which form predominates in the infernally hot, crushing confines of our chaotic core? Perhaps, neither do, and the core comprises a messy, molten mix in which both iron and hydrogen flow freely.
The researchers calculated that the BCC phase of superionic hydrogen has a higher free energy, or more available energy to perform the physical definition of work, and is therefore less stable and more reactive than the HCP phase.
The HCP phase of superionic hydrogen, in contrast, appears to exhibit greater thermodynamic stability.
The modeling suggests a stability shift occurs as conditions become more extreme. Hypothetically, the BCC phase may become more stable than the HCP phase when the temperature surpasses 6,400 Kelvin (6,100 degrees Celsius or 11,000 degrees Fahrenheit) and the hydrogen content exceeds 20% at 3.6 million atmospheres of pressure – extreme indeed.
Too extreme perhaps, because the predominance of BCC may not be practically possible, as the intense conditions could just melt it into an igneous iron slushy.
As a result, the researchers find that the HCP structure is more likely to permeate Earth's inner core.
"Our calculations show that hydrogen can stabilize a superionic BCC phase at sufficiently high temperature and hydrogen content," the researchers say.
"However, this stability field is superseded by melting, so only the superionic HCP phase coexists with the liquid in the Fe–H system."
In terms of total hydrogen, a radial concentration gradient seems to exist in the inner core, with the hydrogen composition "dropping sharply" while crossing the boundary from Earth's outer core and traveling toward the center of the inner core.
This radial hydrogen gradient may drive superionic hydrogen toward the inner core boundary, where it loses its superionic status and re-partitions into a liquid mix to enrich the outer core.
This result suggests that the hydrogen exchange between the inner and outer core is not limited to the direct partitioning that occurs as the inner core crystallizes.
The exchange also occurs via a continuous redistribution of superionic hydrogen, with the inner core continuing to grow at a rate of 1 millimeter per year.
"Both processes may contribute to chemical buoyancy, which is believed to be an important energy source driving the geodynamo," the researchers state.
Further illuminating the enigmatic density fluctuations and material differences throughout the inner core and its boundary will require accounting for other light elements, such as the "strongly liquid-preferring" oxygen and carbon.
Related: Earth's Core Seems to Be Wrapped in Layers Like an Onion, Study Finds
But, altogether, it seems that the partitioning between the different types of hydrogen in the core is controlled primarily by temperature, rather than pressure.
"Our results, therefore, demonstrate that the nonuniform distribution of superionic hydrogen in the [inner core] is a direct consequence of equilibrium thermodynamics," the researchers conclude.
"This mechanism may also apply to the distribution of other light elements, influencing [the] core's composition, dynamics, and evolution."
This research was published in Proceedings of the National Academy of Sciences.
This article was fact-checked by Fiona MacDonald and edited by Fiona MacDonald. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.