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。通过与其他较小的行星体的多次碰撞，每一次碰撞都为成长中的行星提供了能量和金属。高能量的吸积撞击导致早期地球大规模熔融，岩浆海洋形成，促进富铁金属从硅酸盐中分离，形成地核和地幔。关于地球历史早期的主要问题仍然没有答案。例如，在吸积过程中是否有一个单一的长期的深岩浆海洋（正如人们经常假设的那样），如果是这样，它的深度是如何随着时间的推移而演变的？或者，是否存在一系列短暂的深岩浆海洋，如果是的话，它们的深度是如何随着时间的推移而演变的？这些问题取决于岩浆海洋冷却/结晶的时间尺度，而这反过来又严重依赖于绝缘大气的存在与否。虽然假设在形成月球的巨大撞击之后存在绝缘的大气层，但也表明吸积撞击会导致大气层损失。岩浆海洋的演变以及它是如何耦合到绝缘大气将在这个项目中使用一种新的方法进行调查。我们以前集成了一个多阶段的核心形成模型与行星吸积的数值模拟，是基于高度简化的假设岩浆海洋深度。采用这种综合的方法，我们建议实际计算的熔化深度在每一个成千上万的吸积影响的基础上的动能和影响角度。熔融的深度是至关重要的，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，这反过来又决定了地幔和地核的化学演化。此外，将确定由此产生的全球岩浆海洋的冷却/结晶时间尺度，并将其纳入不同绝缘大气情景的模型中。合并的吸积/分化模型还将使地球的建筑材料的组成，以确定其在原行星盘的起源日心距离的函数。

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.