Thermal evolution and differentiation of planetesimals and planetary embryos
Thermal evolution and differentiation of planetesimals and planetary embryos
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星子和行星胚胎的热演化和分化
DOI：
10.1016/j.icarus.2011.11.021
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发表时间：
2012
期刊：
影响因子：
3.2
通讯作者：
S. Labrosse
中科院分区：
文献类型：
--
作者：
O. Šrámek;L. Milelli;Y. Ricard;S. Labrosse
在太阳系早期，在行星形成的失控生长阶段，行星体的分布逐渐从大量的星子演化为数量较少、少数占主导地位的胚胎的天体。在这里，我们在一系列复杂性不断增加的模型中研究这些星子和行星胚胎可能的热和成分演化。我们表明，通过现已灭绝的同位素（特别是 26 Al）的放射性衰变和冲击加热来加热星子的加热阶段可以分两个阶段或同时发生。根据吸积速率，熔化从中心向外、在浅外壳中向内进行、或者在两个位置发生。我们讨论了这些情况的状态域，并表明控制星子的行星增长率 R˙∝Rβ 的指数 β 起着至关重要的作用。对于给定的终端半径和吸积持续时间，β的增加使星子保持非常小，直到吸积结束，因此允许放射性热量在吸积大质量之前辐射出去。要在失控生长阶段熔化~500公里小行星的中心，天体物理学家预测β=2的值，它需要在第一个固体凝结后的几百万年内形成。然后，我们开发了一个多相模型，其中在一维球形几何中考虑了压实引起的相变和相分离。我们的模型同时处理固态和液态的金属和硅酸盐。原核的偏析通过将 26 Al 限制在硅酸盐外壳中而降低了放射加热的效率。可以获得部分分化、有时在多个金属硅酸盐层中分化的各种类型的星子。
In early Solar System during the runaway growth stage of planetary formation, the distribution of planetary bodies progressively evolved from a large number of planetesimals to a smaller number of objects with a few dominant embryos. Here, we study the possible thermal and compositional evolution of these planetesimals and planetary embryos in a series of models with increasing complexities. We show that the heating stages of planetesimals by the radioactive decay of now extinct isotopes (in particular26Al) and by impact heating can occur in two stages or simultaneously. Depending on the accretion rate, melting occurs from the center outward, in a shallow outer shell progressing inward, or in the two locations. We discuss the regime domains of these situations and show that the exponent β that controls the planetary growth rate R˙∝Rβof planetesimals plays a crucial role. For a given terminal radius and accretion duration, the increase of β maintains the planetesimals very small until the end of accretion, and therefore allows radioactive heating to be radiated away before a large mass can be accreted. To melt the center of ∼500km planetesimal during its runaway growth stage, with the value β=2 predicted by astrophysicists, it needs to be formed within a couple of million years after condensation of the first solids. We then develop a multiphase model where the phase changes and phase separations by compaction are taken into account in 1-D spherical geometry. Our model handles simultaneously metal and silicates in both solid and liquid states. The segregation of the protocore decreases the efficiency of radiogenic heating by confining the26Al in the outer silicate shell. Various types of planetesimals partly differentiated and sometimes differentiated in multiple metal–silicate layers can be obtained.