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

地球的固体铁内核每年增长约1毫米，因为它冻结了液态的外核。大部分液态的外核是一种对流的、导电的金属流体，它的搅动运动维持着地球的磁场。液态外核的对流运动和内核的冻结，都是由地球冷却地核驱动的。地幔。然而，正如地震探测所见证的那样，内核的复杂性是非常令人惊讶的，因为原始内核的生长速度极慢，几乎是理想的条件。这种复杂性包括内核边界的波纹，强小散射体的存在，弹性性能的径向和半球形差异，以及具有排列织物和/或晶体结构的区域。我们的项目旨在了解内核生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。动力机制包括由于在液态外核底部形成树突或浆状“雪”而形成的糊状层（液体散布在固体颗粒之间），以及随后固体铁沉积物在自身重量下的压实。压实的糊状物本身可能变得不稳定，从而放大化学、物理性质和内核边界粗糙度的空间变化。我们还计划了解内核边界区域的结构如何对地核冷却速率敏感，而地核冷却速率又与地球深部地幔板块构造循环的深度、范围和速率有关。该项目结合了地震波与固化内芯边界相互作用的分析和固化过程的数学/数值模拟，以确定内芯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不相容元素在冻结时释放的程度非常重要，这有助于在液态外核中驱动对流，这也被认为对水星等行星很重要。在这个凝固过程中需要回答的基本问题包括：观察到的内核结构的半球差异是如何产生和维持的，这些空间差异最终是如何与地球深处的冷却联系在一起的，以及它们如何与表明内核存在的观测结果相协调？月球的自转相对于上面的固体地球可能不同或波动。该项目要求的跨学科和计算工作将有助于指导研究生，并为他们在材料和信息科学的广泛领域的工作做准备。这项研究的结果对于了解地球的化学成分、磁场随时间维持的条件（这对地球表面的生命很重要）、帮助量化地球的能量预算、帮助理解金属凝固的自然尺寸（这反过来又对工业上的重要过程很重要）非常重要。地球核心的冻结过程和其他行星核心的冻结过程之间也有重要的联系，在这个和其他太阳系的类地天体中产生行星磁场，以及行星发展适合生命居住的表面的能力。

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.