RUI: A Laboratory Model for the Solidification of the Earth's Core

BergmanEAR-0229670这笔赠款是用于研究地核凝固的实验室模型。地球的核心正在从熔融的铁合金外核凝固。最近的地震研究推断出内核的一些有趣的性质：它是弹性各向异性的，靠近自旋轴的方向是快的，它可能在这种弹性各向异性中既表现出深度变化，又表现出横向变化，它可能具有异常高的衰减和衰减各向异性，以及可能存在相对于地幔的进动旋转的证据。这些问题的物理起源尚不清楚，但大多数关于弹性各向异性的假设都来自于内核中个别各向异性的铁晶体的织构。已经提出了许多织构方法，大致分为涉及凝固的方法和涉及凝固后变形的方法。在任何一种情况下，旋转对外核对流的重要性似乎至少会起到间接的作用。首席研究员将使用离心力加速度来提供垂直于旋转轴的重力分量，就像在自引力行星中一样。通过在半球壳的中心冷却，温度梯度和有效重力的方向，至少在下半球是相反的。旋转还提供了科里奥利力。首席调查员将固化盐水(氯化钠)水，因为盐水形成一个共晶系统，该体系捕获了岩心相图可能的基本特征，因为咸水是透明的，而且由于冰具有六方最紧密堆积(HCP)晶体结构，这可能是核心条件下铁的晶体结构。首席调查员的一个目标是评估科里奥利力和旋转约束对对流换热的控制程度，从而评估凝固速度的纬度相关性。这一速率的横向变化已被引用为一种可能的纹理机制。第二个相关的目标是评估树枝状、柱状晶体的柱状对称性随旋转和凝固速度的变化趋势。这些目标将通过对冰整体和单个颗粒的温度测量和固化后的光学和纹理分析来实现。此外，已经观察到，在HCP海冰固化过程中的流体流动可以导致横跨生长方向的纹理。首席调查员也发现了HCP金属合金中类似的流动效应。如果这些效应也发生在HCP铁合金中，那么内芯各向异性的复杂性可能反映了凝固过程中外芯底部的不同流动。因此，该项目的第三个目标是通过中性浮力、反射颗粒的长时间曝光照片，将记录在固体中的凝固纹理与熔体中的流动图像进行比较。这些实验将在一定的自转和凝固速度范围内进行，这样就有可能外推到地核。未来的工作可能还包括研究凝固过程中的变形。由于安瑞的资助，这个项目的一个重要组成部分是本科生将进行大部分实验的设计、建造和运行，以及执行数据分析和解释。涉及的许多理论和技术对本科理科专业的学生来说都是易懂的，首席调查员之前的研究涉及本科生，他们中的许多人已经进入研究生教育，并在科学和工程领域就业。

BergmanEAR-0229670This grant is for a study of a laboratory model for the solidification of the Earth's core. The Earth's inner core is solidifying from the molten iron alloy outer core. Recent seismic studies have inferred some interesting properties of the inner core: it is elastically anisotropic, with the direction close to the spin axis being fast, it may exhibit both depth and lateral variations in this elastic anisotropy, it may have an anomalously high attenuation as well as an attenuation anisotropy, and there may be evidence for prograde rotation relative to the mantle. The physical origins of these remain uncertain, but most hypotheses for the elastic anisotropy arise from texturing of the individually anisotropic iron crystals in the inner core. Many means of texturing have been suggested, falling broadly into those involving solidification and those involving post-solidification deformation. In either case the importance of rotation on outer core convection seems likely to play at least an indirect role.The Principal Investigator will use the centrifugal acceleration to provide a component of gravity that is perpendicular to the rotation axis, as in a self-gravitating planet. By cooling at the center of a hemispherical shell, both the temperature gradient and the direction of effective gravity, at least in the lower hemisphere, are reversed. The rotation also provides for the Coriolis force. The Principal Investigator will solidify salt (NaCl) water, because salt water forms a eutectic system, which captures the likely essential features of the core phase diagram, because salt water is transparent, and because ice has a hexagonal closest packed (hcp) crystal structure, the likely crystal structure of iron under inner core conditions.One goal of the Principal Investigator is to assess the extent to which the Coriolis force and rotational constraints control convective heat transfer, and hence the latitudinal dependence of the solidification rate. Lateral variations in this rate have been invoked as a possible texturing mechanism. A second, related goal is to assess the tendency towards cylindrical symmetry of the dendritic, columnar crystals as a function of the rotation and solidification rates. These goals will be accomplished by means of temperature measurements and post-solidification optical and texture analysis of the ice as a whole, and the individual grains.In addition, it has been observed that fluid flow during solidification of hcp sea ice can cause a texture transverse to the growth direction. The Principal Investigator has also found similar flow effects in hcp metallic alloys. If these effects also occur in hcp iron alloys, then it is possible that the complexity of the inner core anisotropy reflects the varied flow at the base of the outer core during solidification. Thus, a third goal of this project is to compare the solidification texture recorded in the solid with the flow imaged in the melt by means of long time exposure photographs of neutrally buoyant, reflective particles. The experiments will be carried out for a range of rotation and solidification rates, so that it will be possible to extrapolate to the Earth's core. Future work may also involve studies of deformation during solidification.As an RUI grant a significant component of this project is that undergraduates will carry out much of the design, building, and running of the experiments, as well as perform the data analysis and interpretation. Much of the theory and techniques involved are accessible to undergraduate science majors, and the Principal Investigator's prior research has involved undergraduates, many of whom have gone on to graduate education and careers in science and engineering.