Historically, while numerous geodynamical mechanisms have been presented to explain the observed intricate structure of Earth's inner core, the scientific community has been divided due to an absence of definitive data on the core's viscosity. This viscosity plays a crucial role in determining the dominant mechanism of the inner core dynamics, with current theories ranging from equatorial inner core growth to plume convection models.
Central to this conundrum is hexagonal close-packed iron (hcp-iron). The inner core of our Earth is believed to consist primarily of hcp-iron, making it essential to understand its viscosity to get a clearer picture of the dynamics at play.
To demystify this, researchers delved deep into the rheology of hcp-iron by employing high-pressure and high-temperature deformation experiments. The series of uniaxial deformation tests were executed using advanced equipment: the D111-type apparatus found on beamline NE7A at PF-AR, KEK and the deformation-DIA device on beamline BL04B1 at SPring-8.
The team utilized a pre-sintered iron rod as their sample material, conducting experiments in the pressure range of 16.9-22.6 GPa and temperatures spanning 423-873 K. These conditions were chosen as they are the stable range for hcp-iron. Crucially, to accurately determine the stress and strain of the sample during deformation, the researchers made use of a monochromatized synchrotron X-ray which facilitated two-dimensional X-ray diffraction and X-radiography.
The findings from this rigorous study provided enlightening insights. Notably, the dominant deformation mechanism in hcp-iron isn't static; it varies with temperature. Above ~800 K, power-law dislocation creep takes precedence, while below this threshold, low-temperature creep becomes more dominant.
With these new experimental results in hand, the team extrapolated the data to estimate the inner core's viscosity. Their findings put it at an impressive 1019 Pa s. What does this mean in layman's terms? It offers a significant hint towards the dominant geodynamical mechanism in action within Earth's inner core. The data suggests that the equatorial growth or translation mode model stands as the predominant geodynamical mechanism operating within this mysterious region of our planet.
This study, while pivotal, represents just one more piece in the puzzle of our Earth's inner workings. However, it's a sizable piece, paving the way for future research and potentially leading us to a more comprehensive understanding of our planet's heart.
Research Report:Rheology of Hexagonal Close-Packed (hcp) Iron
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