Inward solidification of planetary cores as a mechanism for dynamo generation

尽管过去似乎在不同的行星体中产生了磁场，但目前已知太阳系中只有三个岩石天体(地球、水星和木卫三)产生了活跃的磁场。目前的发电机理论无法解释这些原本相似的岩石天体的发散磁场演化，以及它们长期热化学演化的含义。地球目前的磁场是由从不断增长的内核中排出的轻元素提供动力的。然而，围绕核心的年龄以及地球发电机在凝固之前是如何驱动的，仍然存在重大争议。岩石物体中的磁场是由发电机提供动力的：其导电金属核心熔融部分的对流。对至少部分熔融的核心的要求意味着发电机与行星的热历史有关。岩石行星的核心主要由铁和镍组成，另外还有一些较轻的元素，如硫、氧和硅。例如，对地核的地震研究表明，它含有5%-10%的这种较轻的元素。这种对流可以由三种来源共同提供动力：温度引起的密度差(热对流)、结晶引起的密度差(成分对流)和机械强迫(例如潮汐搅拌)。热对流需要穿过核幔边界(CMB)的高热流密度和低导热系数，而成分对流则受核温度和成分梯度的影响。这意味着磁场的产生与核心的组成以及行星体如何随时间冷却有着内在的联系。我们不能直接观察行星的内部，但磁场的存在/不存在是了解地球、其他行星和卫星及其热史上难以接近的内部的一个强大而独特的窗口。当仅靠热对流不足以产生发电机时，成分对流可以提供额外的能量来源，这两种机制结合在一起形成热化学对流。当铁芯结晶和轻元素从凝固的铁中排出时，就会产生成分对流，导致密度差异，从而驱动对流。核心的结晶是从CMB开始还是从核心中心开始取决于核心温度(绝热温度)和结晶温度(液线)如何随深度变化。当核心从中心(自下而上)结晶时，从固体中排出的轻元素密度低于周围的核心，因此浮力上升到CMB。这就是今天产生地球发电机的过程。当核心从CMB向内(自上而下)结晶时产生的发电机知之甚少，但对于太阳系中众多较小天体中的发电机来说意义重大，因为CMB处于较低的压力。这包括小行星、卫星和潜在的水星，水星的核心很大，地幔很薄。有几种可能的结晶机制，包括千米大小的铁块下沉或铁雪。特别是，铁雪是对流的一个重要机制，因为它产生持续的发电机活动。当固体铁颗粒(10-2 mm)在CMB结晶时，就会出现铁雪。晶体落在结晶区(雪区)，直到它们到达底部，在那里它们重新融化。这在堆芯液态部分密度较低的铁合金顶部形成了一层更致密的纯铁层，由于重力不稳定，因此下沉并驱动对流。

Although magnetic fields appear to have been generated in a diverse range of planetary bodies in the past, only three rocky bodies in our solar system (Earth, Mercury and Ganymede) are known to produce active fields today. Current dynamo theory cannot explain the divergent magnetic field evolution of these otherwise similar rocky bodies, and the implications for their long-term thermochemical evolutions. The current magnetic field of the Earth is powered by light elements being expelled from the growing inner core. However, there is still major debate surrounding both the age of the inner core, and how the geodynamo was driven prior to solidification.Magnetic fields in rocky bodies are powered by a dynamo: convection in the molten part of their electrically conductive metallic cores. The requirement for an at least partially molten core means dynamos are linked to a planet's thermal history. Cores of rocky planets consist primarily of iron and nickel, with some additional lighter elements, such as sulfur, oxygen and silicon. For example, seismic studies of the Earth's core suggest it contains 5 - 10% of these lighter elements. This convection can be powered by a combination of three sources: temperature induced density differences (thermal convection), crystallisation induced density differences (compositional convection), and mechanical forcing (e.g. tidal stirring). Thermal convection requires a high heat flux across the core-mantle boundary (CMB) and the core to have a low thermal conductivity, whilst compositional convection is affected by the core temperature and composition gradients. This means that magnetic field generation is intrinsicallylinked to core composition and how planetary bodies cool with time. We cannot observe planetary interiors directly, but the absence/presence of magnetic fields is a powerful and unique window into the properties of the inaccessible interiors of Earth, other planets and moons and their thermal histories.When thermal convection alone is not strong enough to generate a dynamo, compositional convection can provide an additional power source with the two mechanisms combining in thermo-chemical convection. Compositional convection arises when the core crystallises and light elements are expelled from the solidifying iron, resulting in density differences which can drive convection. Whether crystallisation of the core starts at the CMB or the centre of the core depends on how the core temperature (adiabat) and crystallisation temperature (liquidus) vary with depth. When the core crystallises from the centre (bottom-up), light elements which are rejected from the solid are less dense than the surrounding core so rise buoyantly to the CMB. This is the process which generates the Earth's dynamo today. Dynamos generated when the core crystallises from the CMB inwards (top-down) are poorly understood, but are significant for dynamos in the numerous smaller bodies in the Solar System, where the CMB is at lower pressure. This includes asteroids, moons and potentially Mercury, which has a large core and thin mantle. There are several possible crystallisation mechanisms, including sinking of km size iron chunks or iron snow. In particular, iron snow is an important mechanism for convection because it produces sustained dynamo activity. Iron snow occurs when solid iron particles ( 10-2mm) crystallise at the CMB. The crystals fall through the crystallised region (the snow zone) until they reach the base where they remelt. This forms a more dense pure iron layer on top of the less dense iron alloy in the liquid part of the core, which is gravitationally unstable so sinks and drives convection.