The iron-snow regime in Fe-FeS cores: a numerical and experimental approach

在地球上，发电机的作用与地核冻结密切相关。它有一个坚固的内核，内核的生长提供了一个浮力流来驱动发电机。在这种情况下，浮力来自固体内核和流体外核之间组成的差异。然而，在比地球小的行星体中，这种地核分化过程可能会有所不同——铁可能在地核-地幔边界（CMB）沉淀，而不是在中心，并可能以铁雪的形式降落，并在更深的深度开始融化。随着时间的推移，一个化学稳定的沉积带形成，包括整个地核，这时一个固体的内核开始生长。这个系统的动力学尚不清楚，也不清楚它是否能产生磁场。木星的卫星木卫三（Ganymede）显示出当今的磁偶极子场，是这种情况被提出的候选者。我们计划用数值和实验方法来研究这种铁雪状态。在数值研究中，我们使用二维/三维热化学对流模型，该模型考虑了铁晶体的结晶和下沉以及液芯相的动力学（对于三维情况，进一步研究了铁雪过程旋转的影响）。数值计算将由两个系列的实验来补充：(1)通过x射线检查对金属合金的研究，(2)通过光学技术对透明类似物的测量。这些实验将检验铁雪状态的典型特征。一方面，它们将作为验证数值方法的工具，另一方面，它们将对铁雪制度的子过程产生重要的见解，这些子过程由于其复杂性而无法在数值方法中访问。

In the Earth, the dynamo action is strongly linked to core freezing. There is a solid inner core, the growth of which provides a buoyancy flux that drives the dynamo. The buoyancy in this case derives from a difference in composition between the solid inner core and the fluid outer core. In planetary bodies smaller than the Earth, however, this core differentiation process may differ – Fe may precipitate at the core-mantle boundary (CMB) rather than in the center and may fall as iron snow and initially remelt with greater depth. A chemical stable sedimentation zone develops that comprises with time the entire core - at that time a solid inner core starts to grow. The dynamics of this system is not well understood and also whether it can generate a magnetic field or not.The Jovian moon Ganymede, which shows a present-day magnetic dipole field, is a candidate for which such a scenario has been suggested. We plan to study this Fe-snow regime with both a numerical and experimental approach. In the numerical study, we use a 2D/3D thermo-chemical convection model that considers crystallization and sinking of iron crystals together with the dynamics of the liquid core phase (for the 3D case the influence of the rotation of the Fe snow process is further studied).The numerical calculations will be complemented by two series of experiments: (1) investigations in metal alloys by means of X-ray radioscopy, and (2) measurements in transparent analogues by optical techniques. The experiments will examine typical features of the iron snow regime. On the one hand they will serve as a tool to validate the numerical approach and on the other hand they will yield important insight into sub-processes of the iron snow regime, which cannot be accessed within the numerical approach due to their complexity.