Iron, one of the most abundant elements globally and a crucial constituent of the Earth’s core, exhibits fascinating behavior under extreme conditions, holding significant implications for geology and the Earth’s evolutionary processes.
Led by Lawrence Livermore National Laboratory, a team of researchers employed a combination of lasers and X-ray diffraction methods to investigate the relationship between various crystal structures of iron and its behavior when subjected to ultrahigh pressures and temperatures. Their study, published in the journal Physical Review B, sheds light on the complex dynamics of iron under extreme conditions.
Utilizing the Dynamic Compression Sector beamline at Argonne National Laboratory, the researchers subjected iron to nanosecond laser shock compression, reaching pressures exceeding 275 gigapascals (GPa) — equivalent to more than 2 million times atmospheric pressure. They employed in situ picosecond X-ray diffraction to analyze the iron’s structure under these extreme conditions. This novel approach provided valuable insights into materials science and the internal dynamics of Earth and other terrestrial exoplanets.
Lead author Saransh Soderlind emphasized the collaborative effort involved in conducting these experiments, highlighting the remarkable coordination required to unravel profound questions regarding Earth’s formation.
The Earth’s core, primarily composed of solid iron with minor elemental impurities, undergoes recrystallization at the liquid-solid boundary between the outer and inner core as it cools. This process releases heat, contributing to the convective currents in the outer fluid core, which are integral to the Earth’s geodynamo and magnetic field generation.
Contrary to previous assumptions, the study suggests that the heat released from recrystallization may play a smaller role in powering the geodynamo than previously believed. Instead, alternative sources such as lower-mantle radiogenic heat generation and the buoyancy of light elements excluded during recrystallization may drive outer core convection currents more prominently.
The team’s experiments revealed crucial insights into iron’s behavior at extreme pressures and temperatures, indicating a transition from its usual body-centered cubic (bcc) structure to a hexagonal close-packed (hcp) crystal structure, followed by complete melting into a liquid as pressure increases. This phase transition is gradual, with the coexistence of both structures over a range of pressures.
Characterizing iron’s liquid structure provides valuable data for comparison with theoretical models, aiding in understanding phase transitions and mechanical properties. Although the exact mechanism behind the phase transition remains undetermined, the study significantly advances our understanding of iron’s behavior under extreme conditions.
Future research may involve studying iron-nickel alloys to better comprehend the composition of the Earth’s core, with ongoing experiments aimed at refining our understanding of core solidification and energy contributions.
The collaborative effort between scientists from various institutions underscores the interdisciplinary nature of research aimed at unraveling the mysteries of Earth’s composition and evolution.
