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

地球的核心是一个主要由铁和金属组成的球体。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，随着地球的冷却，液态铁冻结，使内核生长，并向外核提供能量，以产生地球磁场。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，这是一种称为各向异性的特征。沿南北路径(与地球自转轴近乎平行)穿过内核的地震波比沿东西路径(在赤道平面)传播的地震波传播得更快。内核地震各向异性携带了有关铁冻结时的条件的信息。它被归因于铁晶体在特定方向上的排列；但导致这种排列的过程尚不清楚。由于涉及的复杂过程以及核心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，研究人员对铁样品在核心条件下的行为进行了实验测试。他们分析了在钻石顶压室中进行的实验数据，在那里，极端条件是在两个相对的钻石尖端产生的。他们使用计算模型来表征实验过程中的铁晶体排列，并计算出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图。与地球动力学建模相结合，这种多学科方法可以模拟岩心内部的生长，并揭示导致其当今各向异性的过程。该项目支持一位早期的职业科学家。它促进多学科背景下的研究生和本科生的培训，特别是地学中代表性不足的群体的培训。它促进了对当地学校和社区大学的接触。项目成果将广泛和自由地分发给社区。了解内核结晶是理解地球发电机的核心。内核地震各向异性归因于本征各向异性铁晶体的排列。在这里，研究人员调查了这种各向异性的原因和控制因素，以及内部核心过程如何影响外部核心过程。结合地震分析、矿物物理实验和地球动力学模型，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约束铁在核心条件下的行为，从而限制了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，并确定了流动的模式和强度，并迫使其影响晶体取向。他们测试了取向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测试内核可能的增长模型提供了观测约束。该团队解决的关键问题是：全球各向异性是如何在核心内部增长过程中产生的？是什么导致了今天的各向异性呢？增长模型能否与一系列长度尺度上的空间地震结构相协调？结晶织构对最终的各向异性有什么影响？热和电各向异性对地球的其余部分，特别是对发电机的影响是什么？这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.