岩浆海洋的诞生和衰落。第二部分：早期吸积星子的摆动热历史
Birth and decline of magma oceans. Part 2: wobbling thermal history of early accreted planetesimals
摘要：
一个理论模型，描述了一个悬浮液的演变，其中晶体可以沉积形成一个密集的累积或可能产生一个轻的浮选地壳已在一个配套文件中推导出来。我们用这个模型来研究在太阳系形成的非常早期阶段的早期吸积行星体的热历史。我们研究所需的条件，形成和保存浮选结壳和基底堆，以及星子的热演化的影响。计算了由$\rm{^{26}Al}$和$\rm{^{60}Fe}$衰变产生内部加热的早期吸积星子的温度演化。对于半径为$R>30\, \rm{km}$的星子，部分熔融达到40%，星子经历流变转变并形成岩浆海洋，即：一种悬浮液，晶体可以从中分离并形成漂浮的外壳和/或致密的堆积物。由于绝热浮壳的形成，这一岩浆洋幕的特征是相对较长的时间寿命，缓慢的冷却速率，和弱的表面热通量。该模型还预测了一个周期性的演变，地壳增厚的情节与熔融引起的地壳变薄的情节交替。这些循环反过来又产生了振荡的热历史，并防止了星子的热加热失控。在岩浆洋幕末期，当晶体含量大于60%时，星子地幔发生向固态对流的转变。该阶段的特征在于高粘性剪切应力，其倾向于侵蚀先前形成的晶体沉积物。然而，这一侵蚀过程的时间尺度大于星子的寿命。因此，固态演化可以描述为一个良好的混合地幔嵌入一个亚稳定地壳和保存在其底部的堆晶。
英文摘要：
A theoretical model that describes the evolution of a suspension in which crystals can sediment to form a dense cumulate or may produce a light flotation crust has been derived in a companion paper. We use this model to study the thermal history of early accreted planetary bodies accreted during the very early stages of the formation of the solar system. We study the conditions required to form and preserve flotation crusts and basal cumulates, and the implications for the thermal evolution of planetesimals. We calculate the temperature evolution in an early accreted planetesimals internally heated by the decay of $\rm{^{26}Al}$ and $\rm{^{60}Fe}$. For planetesimal with radius $R>30\, \rm{km}$, partial melting reaches 40%, planetesimals undergo a rheological transition and form a magma ocean, i.e.: a suspension from which crystals can segregate and form a floating crust and/or a dense cumulate. Because of the formation of an insulation floating crust, this magma ocean episode is characterized by a relatively long time life, a slow cooling rate, and a weak surface heat flux. The model further predicts a cyclic evolution where episodes of crustal thickening alternate with episodes of melting-induced crustal thinning. These cycles produce in turn an oscillating thermal history and prevent runaway thermal heating of the planetesimals. At the end of the magma ocean episode, when the fraction of crystals becomes larger than 60\%, a transition occurs to solid-state convection in the planetesimal's mantle. This stage is characterized by high viscous shear stress that tends to erode previously formed crystal deposits. However, the time scale of this erosion process is larger that the lifetime of the planetesimal. Hence solid-state evolution can be described by a well mixed mantle embedded by a metastable crust and a preserved cumulate at its base.
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