In a recent publication in Science Advances, a group of researchers has revolutionized our comprehension of the Earth’s core composition by establishing a novel pressure scale. They accomplished this breakthrough using highly potent X-rays from a specialized spectrometer at RIKEN’s SPring-8 Center, avoiding the substantial approximations made in previous studies. Surprisingly, their findings revealed that the preceding pressure scale had overestimated pressures by more than 20%, particularly at 230 gigapascals (equivalent to 2.3 million atmospheres), a pressure magnitude akin to the conditions within the Earth’s core.
To put this in perspective, the previous scenario was akin to a marathon runner who believed they had completed a 42-kilometer race but had, in reality, only run 34 kilometers. While a 20% correction might seem modest, its implications are profound.
Accurate pressure calibration is of paramount importance for understanding Earth’s composition. The composition of the Earth’s core, in particular, has been a subject of intense debate because it holds significance for our understanding of the present state of our planet and the evolution of our solar system in the distant past. Although it is generally agreed that the core consists mainly of iron, evidence from seismic wave propagation suggests the presence of lighter materials in the core.
When the newly established pressure scale was applied to interpret seismic models, the research team made a remarkable discovery: the amount of lighter material in the inner core is approximately double what was previously believed. Furthermore, the total mass of this lighter material in the entire core may exceed five times that of the Earth’s crust, the outermost layer we inhabit.
In this groundbreaking study, led by Alfred Q.R. Baron of the RIKEN SPring-8 Center, along with Daijo Ikuta and Eiji Ohtani from Tohoku University, the researchers employed Inelastic X-ray Scattering (IXS) to measure the sound velocity of a rhenium sample under extreme pressure conditions. They achieved this by subjecting a minuscule rhenium sample (less than 0.000000001 grams, or 1 nanogram) to immense pressure through compression between two diamond crystals within a Diamond Anvil Cell (DAC).
The DAC containing the sample was then positioned within the high-powered IXS spectrometer at BL43LXU. The researchers meticulously recorded minute shifts in the X-ray energy scattered by the rhenium, enabling them to calculate the sound velocity of the material. They measured both compressional (vp) and shear (vs) sound velocities, in addition to determining the density of rhenium. This comprehensive data allowed them to precisely quantify the pressure to which the rhenium was subjected.
This study establishes a direct relationship between rhenium density and pressure, with potential implications for broader scientific investigations. As Baron explains, measuring rhenium’s density at high pressures is a relatively straightforward and efficient process, and numerous facilities worldwide can perform such measurements. However, determining sound velocity under these extreme conditions is a formidable challenge and, at these pressures, is likely only feasible using RIKEN’s specialized spectrometer at BL43LXU within SPring-8. The research team’s efforts have paved the way for other scientists to employ the more accessible method of measuring density to infer pressure.
In the words of Ikuta, Ohtani, and Baron, “When we applied our newly established scale to analyze the behavior of metallic iron under high pressure and compared it to the seismic model of the Earth, we uncovered that the presence of light material in the inner core is probably approximately twice as much as previously estimated. Similar substantial alterations, possibly even greater in magnitude, may be anticipated when evaluating the properties of other planets’ structures. Our work also implies a reevaluation of the pressure dependence of nearly all material properties measured at pressures equivalent to or greater than those in the Earth’s core.”
This groundbreaking research opens up new horizons in our understanding of Earth’s composition, shedding light on the mysterious depths of our planet and potentially revolutionizing our knowledge of planetary structures beyond Earth.
For more information, please refer to the publication:
Daijo Ikuta et al, “Density deficit of Earth’s core revealed by a multimegabar primary pressure scale,” Science Advances (2023). DOI: 10.1126/sciadv.adh8706.