CSEDI Collaborative Research: Combined Geodynamical and Seismological Modeling of the Inner Core Boundary Region

地球的固体铁内核每年都会增长约1毫米，因为它冻结在液体外核之外。大多数液态外核是一种对流的、导电的金属流体，其搅拌运动负责维持地球的磁场。流体外核中的对流运动和内核的冻结都是由地球-S地幔对内核的冷却驱动的。然而，地震学探测证明了内核的复杂性，考虑到生长原始内核的超慢和近乎理想的条件，这是非常令人惊讶的。这种复杂性包括内部核心边界的波纹，存在强大的小散射体，弹性性质的径向和半球差异，以及具有对齐的组构和/或晶体结构的区域。我们的项目旨在了解在内核增长过程中可能导致内核边界区域如此巨大复杂性的动力学条件，并将动力学机制产生的地震预测与实际数据进行比较。动力学机制包括由于在液体外核底部形成树枝晶或浆体“雪”而形成糊状层(其中液体散布在固体颗粒之间)，以及随后固体铁沉淀物在其自身重量下的压实。压实的混合体本身可能会变得不稳定，从而放大内部核心边界在化学、物理性质和粗糙度方面的空间差异。我们还计划研究内核边界区域的结构对地核冷却速率的敏感程度，而冷却速率又与地球深部地幔板块构造环流的深度、程度和速率有关。该项目将地震波与凝固内核边界相互作用的分析与凝固过程的数学/数值模拟相结合，以确定内核边界附近流体和固体的化学成分和性质。凝固过程的性质对于确定冻结时释放的不相容元素在多大程度上可以帮助驱动液体外核的对流很重要，这在水星等行星上也被认为是重要的。在这个固化过程中需要回答的基本问题包括：如何产生和维持观测到的内核结构中的半球差异，这些空间差异最终如何与地球深处的冷却联系在一起，以及它们如何与暗示地球内核的观测结果相一致？S的自转可能与上面的固体地球不同或波动。该项目所需的跨学科和计算工作将有助于指导研究生，帮助他们为材料和信息科学的广泛领域的工作做准备。这项研究的结果将有助于了解地球的化学成分，磁场在一段时间内保持的条件(这对地球表面的生命很重要)，有助于量化地球的能量平衡，并有助于了解金属凝固的自然尺寸，这反过来又有利于工业上重要的过程。地球核心的冻结过程与其他行星核心的冻结过程、在这个太阳系和其他太阳系的地体中产生行星磁场，以及行星形成适合生命居住的表面的能力之间也有重要的联系。

Earth's solid iron inner core grows about 1 mm every year as it freezes out of the liquid outer core. Most of the liquid outer core is a convecting, electrically conducting, metal fluid, whose churning motions are responsible for maintaining Earth's magnetic field. Both the convective motions in the liquid outer core, and freezing of the inner core, are driven by cooling of the core by Earth?s mantle. Yet the complexity of the inner core, as witnessed by seismological probing, is very surprising given the ultra-slow and nearly ideal conditions for growing a pristine inner core. Such complexity includes corrugation of the inner core boundary, the presence of strong small scatterers, radial and hemispherical differences in elastic properties, and regions with aligned fabric and/or crystalline structure. Our project aims to understand the dynamical conditions during inner core growth that could give rise to such enormous complexity in the inner core boundary region, and to compare the seismic predictions generated by dynamical mechanisms with real data. Dynamical mechanisms include formation of a mushy layer (in which liquid is interspersed between solid particles) owing to formation of dendrites or slurry "snow" at the base of the liquid outer core, and subsequent compaction of the solid iron sediment under its own weight. A compacting mush may itself become unstable in a manner that would amplify spatial variations in chemistry, physical properties, and roughness of the inner core boundary. We also plan to see how the structure of the inner core boundary region is sensitive to the rate of cooling of Earth's core, which is in turn related to the the depth extent and rate of plate tectonic circulation in the Earth's deep mantle.This project combines analysis of seismic waves interacting with the solidifying inner core boundary and mathematical/numerical modeling of the solidification process to determine the chemical composition and the nature of fluids and solids near the inner core boundary. The nature of the solidification process is important to determining the extent to which release of incompatible elements upon freezing can help drive convection currents in the liquid outer core, which is also thought to be important in planets such as Mercury. Among the fundamental questions to be answered in this solidification process are how observed hemispherical differences in inner core structure may be created and sustained, how are these spatial differences ultimately linked to cooling of the deep Earth, and how might they be reconciled with observations that suggest the inner core?s rotation can differ or fluctuate relative to the solid Earth above.Interdisciplinary and computational work required by the project will assist in the mentoring of graduate students and their preparation for jobs in broad areas of materials and information science. Results from this study will be important to understanding the chemical composition of the Earth, the conditions for maintenance of the magnetic field through time (which is important for life on Earth's surface), help quantify the energy budget of the Earth, and help understand the natural dimensions of metal solidification which is in turn liked to industrially important processes. There are also important connections between freezing processes in Earth's core and those in the cores of other planets, the generation of planetary magnetic fields in terrestrial bodies in this and other solar systems, and the ability for a planet to develop a habitable surface suitable for hosting life.