[论文解读] Large density deficit of Earth's core revealed by a multi-megabar primary pressure scale

Daijo Ikuta, Eiji Ohtani+4 more|arXiv (Cornell University)|Apr 5, 2021

High-pressure geophysics and materials参考文献 33被引用 4

一句话总结

本研究通过在金刚石对顶砧中对铼进行非弹性X射线散射，建立了延伸至多兆巴压力的初级压力标准，揭示了以往的压力标度在地核条件下的实验压力至少被高估了20%。经校正后的标度表明地球内核存在9%的密度亏值，与先前估计相比，内核中轻元素含量翻倍。

ABSTRACT

Precise information about the composition of the Earth's core is critical to understand planetary evolution and for discussing current hot topics in geodynamic behavior, such as core-mantle boundary heat flow. However, samples from deep in the Earth's interior are not available, so our knowledge is based on comparison of laboratory measurements with seismological observations, informed by meteorite composition, and indications of the Earth's core temperature. One of the most interesting results of such work has been the suggestion that Earth's inner core must contain light elements because the density of the core, as determined from seismological measurements, is lower than the density of pure iron, its main constituent, as determined from laboratory measurements and/or theoretical work: the density deficit is now considered to be ~4%. However, this conclusion relies critically on having an accurate pressure scale to relate lab generated pressures to geological pressures. Establishing such a scale has been the subject of intensive research but still involves significant extrapolation and approximations, especially at higher pressures. Further, a pressure scale to the multi-megabar pressures is indispensable for discussing super-Earth planets. Here we establish the first primary pressure scale extending to the multi-megabar pressures of Earth's core by measuring acoustic phonon velocities using inelastic scattering from a rhenium sample in a diamond anvil cell. Our new pressure scale agrees with previous primary scales at lower pressures and also shock compression experiments, but is significantly different from previous secondary and theoretical scales at Earth's core pressures: previous scales have overestimated, by at least 20%, laboratory pressures at 230 gigapascals. Our new scale suggests the density deficit of the inner core is ~9%, doubling the light-element content of the core.

研究动机与目标

• 建立一个可扩展至与地球内核及超级类地行星相关的多兆巴压力范围的准确、初级压力标度。
• 通过提高极端条件下实验室压力校准的准确性，解决地核成分的不确定性。
• 解决地震学密度测量与纯铁理论密度之间的长期差异，即所谓的“密度亏值”问题。

提出的方法

• 通过在金刚石对顶砧中压缩的铼样品进行非弹性X射线散射测量声学声子速度。
• 基于第一性原理状态方程，利用测得的声速推导压力标度。
• 通过冲击压缩实验和较低压力的初级标准验证新标度的准确性。
• 将压力校准扩展至230吉帕斯卡（GPa），对应地球内核的条件。
• 将新标度与次级和理论压力标度进行比较，识别先前校准中的系统性高估。

实验结果

研究问题

• RQ1 在金刚石对顶砧中，铼样品通过非弹性声子散射测得达到230 GPa压力时的真实压力是多少？
• RQ2 在多兆巴压力范围内，新初级压力标度与现有次级和理论压力标度相比如何？
• RQ3 使用新压力校准后，地球内核的密度亏值有何修正？
• RQ4 经校正的压力标度在多大程度上改变了对地球地核中轻元素含量的估计？

主要发现

• 新初级压力标度表明，以往的次级和理论压力标度在230 GPa时至少高估了实验压力的20%。
• 经校正的压力标度使内核密度亏值修正为约9%，高于此前公认的约4%。
• 新标度与较低压力的初级标准及冲击压缩实验一致，验证了其在极端条件下的准确性。
• 增大的密度亏值意味着地球地核中的轻元素含量显著高于先前估计。
• 本研究首次实现了延伸至多兆巴压力范围的初级压力校准，为更准确地模拟行星内部结构（包括超级类地行星）提供了可能。