CSEDI Collaborative Research: The Origins and Implications of Inner Core Seismic Anisotropy

地核是一个主要由铁金属组成的球。它由一个液体外壳——外核——包裹着固体内核组成。随着时间的推移，地球冷却下来，液态铁冻结，形成内核，并为外核提供能量，产生地球磁场。内核是球形的，但看起来并不均匀。地震波穿过它的速度取决于它们的方向，这种特征被称为各向异性。地震波沿着南北方向（几乎与地球自转轴平行）穿过内核，比沿着东西方向（在赤道平面上）传播的地震波要快。内核地震各向异性携带着铁冻结时条件的信息。这被归因于铁晶体在特定方向上的排列；但导致这种一致性的过程尚不清楚。由于涉及复杂的过程以及核心中普遍存在的极端压力和温度，解释这一特征一直具有挑战性。在这里，研究人员通过实验测试了铁样品在核心条件下的表现。他们分析了在钻石砧室进行的实验数据，在那里，两个对立的钻石的尖端产生了极端条件。他们使用计算模型来表征实验过程中的铁晶体排列，并计算由此产生的地震速度。他们使用地震分析方法来绘制内核的速度结构。结合地球动力学建模，多学科方法可以模拟内核生长并揭示导致其当前各向异性的过程。这个项目支持一位早期的科学家。它促进在多学科背景下对研究生和本科生的培训，特别是来自地球科学领域代表性不足的群体的培训。它促进了与当地学校和社区学院的联系。项目成果将广泛和免费地分发给社区。了解内核的结晶是了解地球动力学的核心。核内地震各向异性是由本征各向异性铁晶体排列引起的。在这里，研究人员探讨了这种各向异性的原因和控制因素，以及内核过程如何影响外核过程。结合地震分析、矿物物理实验和地球动力学建模，他们研究了内核内晶体排列的动力学和矿物学控制。他们使用实验室数据和计算塑性模型来限制铁在内核条件下的行为，限制能够产生地震各向异性的矿物学过程。同时，通过地球动力学模型，他们模拟了内核的生长，并确定了影响晶体取向的流动模式和强度以及作用力。他们测试了排列晶体的各向异性热和电输运性质对内核动态演化的可能反馈。现有的和新的地震测量为检验可能的内核增长模型提供了观测约束。该团队解决的关键问题是：在内核生长过程中，全球各向异性是如何产生的？是什么导致了今天的各向异性取向？生长模型能在一定长度尺度上与空间地震结构相协调吗？结晶织构对最终各向异性有什么影响？热和电的各向异性对地球的其他部分，特别是对地球动力学的影响是什么？该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The Earth's core is a ball of mostly iron metal. It consists of a liquid outer shell - the outer core - enveloping the solid inner core. As the Earth cools down over time, liquid iron freezes, growing the inner core and providing energy to the outer core to generate Earth’s magnetic field. The inner core is spherical in shape but appears not to be uniform. The speed of seismic waves traveling through it depends on their direction, a feature known as anisotropy. Seismic waves traversing the inner core along a north-south path (near-parallel to the Earth’s rotation axis) go faster than those propagating along an east-west path (in the plane of the equator). Inner-core seismic anisotropy carries information about the conditions at the time of iron freezing. It has been attributed to alignment of iron crystals in specific directions; but the processes causing this alignment is unclear. Interpreting this feature has been challenging because of the complex processes involved and the extreme pressures and temperatures prevailing in the core. Here, the researchers test experimentally how samples of iron behave at core conditions. They analyse data from experiments carried out in the diamond anvil cell where extreme conditions are generated at the tips of two opposing diamonds. They use computational models to characterpize iron crystal alignment during the experiments and calculate the resulting seismic velocities. They use seismic analytical methods to map the velocity structure of the inner core. Combined with geodynamic modeling, the multidisciplinary approach allows simulating inner-core growth and unveiling the processes causing its present-day anisotropy. The project supports an early career scientist. It promotes the training in a multidisciplinary context of graduate and undergraduate students, notably from underrepresented groups in geosciences. It fosters outreach towards local schools and community colleges. The project outcomes will be broadly and freely distributed to the community. Understanding inner-core crystallization is central to understanding the geodynamo. Inner-core seismic anisotropy is attributed to alignments of intrinsically anisotropic iron crystals. Here, the researchers investigate the causes and controlling factors of this anisotropy, and how inner-core processes influence outer core processes. Coupling seismic analysis, mineral physics experiments, and geodynamic modeling, they investigate the dynamics and mineralogical control of crystal alignments within the inner core. They use both laboratory data and computational plasticity models to constrain the behavior of iron at inner-core conditions, placing limits on the mineralogical processes able to generate seismic anisotropy. Concurrently, with geodynamic models they simulate inner-core growth and determine the pattern and strength of flows and forcing that impact crystal orientation. They test possible feedback of anisotropic thermal and electrical transport properties of aligned crystals on inner-core dynamic evolution. Existing and new seismic measurements provide observational constraints to test possible growth models for the inner core. Key questions addressed by the team are: how is global anisotropy generated during inner-core growth? What caused the present-day orientation of anisotropy? Can growth models be reconciled with spatial seismic structure across a range of length scales? What effect does crystallization texture have on final anisotropy? What are the implications of thermal and electrical anisotropy for the rest of the Earth, notably for the geodynamo?This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.