Impact-induced melting, magma ocean evolution and core-mantle differentiation during accretion of the Earth

在太阳系的早期历史中，地球通过一个与行星吸积强烈耦合的多阶段过程分化为金属核心和硅酸盐地幔。吸积是通过与其他较小的行星发生多次碰撞而发生的，每个较小的行星都向这颗不断增长的行星提供了能量和金属。吸积碰撞所涉及的高能导致早期地球的大规模熔融和岩浆海洋的形成，这有利于富铁金属从硅酸盐中分离出来，形成核和地幔。关于地球早期历史的主要问题仍然没有答案。例如，在吸积过程中是否存在单一的长期存在的深部岩浆海洋(如通常假设的那样)？如果存在，其深度是如何随时间演变的？或者，是否存在一系列短暂的深部岩浆海洋？如果是的话，它们的深度是如何随时间演变的？这些问题取决于岩浆海洋冷却/结晶的时间尺度，而后者又严重依赖于绝缘大气的存在与否。尽管在月球形成的巨型撞击之后存在隔热大气层，但也有证据表明，吸积碰撞会导致大气损失。在这个项目中，将使用一种新的方法来研究岩浆海洋的演化以及它是如何与隔热大气耦合的。我们之前已经将多阶段核心形成模型与行星吸积的数值模拟相结合，该模型基于关于岩浆海洋深度的高度简化的假设。利用这一综合方法，我们建议根据动能和撞击角，实际计算数千次吸积撞击中每一次的熔化深度。熔融深度至关重要，因为它决定了液态金属和液态硅酸盐之间发生化学平衡的压力-温度条件，而压力-温度条件又决定了地幔和地核的化学演化。此外，还将确定由此产生的全球岩浆海洋的冷却/结晶时间表，并将其纳入不同隔热大气情景的模型中。结合的吸积/分异模型还将使地球建筑材料的组成能够被确定为它们在原行星盘中起源的日心距离的函数。

During the early history of the Solar System, the Earth differentiated into its metallic core and silicate mantle by a multistage process that was strongly coupled to accretion of the planet. Accretion occurred through numerous collisions with other smaller planetary bodies, each of which delivered both energy and metal to the growing planet. The high energies involved in accretional impacts caused large scale melting of the early Earth and magma ocean formation which facilitated the segregation of iron-rich metal from silicate to produce the core and mantle. Major questions about this early period of Earth's history remain unanswered. For example, was there a single long-lasting deep magma ocean during accretion (as is often assumed) and, if so, how did its depth evolve with time? Alternatively was there a series of short-lived deep magma oceans and, if so, how did their depths evolve with time? Such questions depend on magma ocean cooling/crystallization timescales which, in turn, depend critically on the presence or absence of an insulating atmosphere. Although an insulating atmosphere has been postulated to be present after the Moon-forming giant impact, it has also been shown that accretional impacts cause atmospheric loss. Magma ocean evolution and how it is coupled to insulating atmospheres will be investigated in this project using a novel approach. We have previously integrated a multistage core formation model with numerical simulations of planetary accretion that is based on highly simplified assumptions about magma ocean depths. Employing this integrated approach we propose to actually calculate the depth of melting during each of thousands of accretional impacts based on kinetic energies and impact angles. The depth of melting is of critical importance because it determines the pressure-temperature conditions at which chemical equilibration between liquid metal and liquid silicate occurs, which, in turn, determines the chemical evolution of Earth's mantle and core. Furthermore the cooling/crystallization timescale of resulting global magma oceans will be determined and included in the model for different insulating atmosphere scenarios. The combined accretion/differentiation model will also enable the compositions of Earth's building materials to be determined as a function of their heliocentric distances of origin in the proto-planetary disk.