短寿命放射性核素和孔隙度对星子早期热-力学演化的影响
The effects of short-lived radionuclides and porosity on the early thermo-mechanical evolution of planetesimals
摘要：
在混沌生长的巨大撞击阶段之前聚集的类地星子的热历史和内部结构可能对类地行星的最终组成和演化产生重要影响。这些参数主要取决于加热与冷却的内部平衡，这主要取决于铝-26和铁-60等短寿命放射性核素（SLR）的存在以及材料的导热性。SLR的加热作用取决于它们的初始丰度、星子的形成时间和大小。有人认为，冷却历史是由颗粒材料的孔隙率，经历了巨大的变化，通过压实过程，并往往随着时间的推移而减少。在这项研究中，我们评估这些参数的影响，年轻的星子的热机械演化与二维和三维模拟。使用代码系列I2 ELVIS/I3 ELVIS，我们已经运行了许多二维和三维数值有限差分流体动力学模型，具有不同的星子半径，形成时间和初始孔隙度。我们的研究结果表明，粉末状材料降低了在星子熔化和对流的阈值，这取决于SLR存在的量。微行星的一个子集保留了一个粉末状的表面层，降低了导热性，阻碍了冷却。初始孔隙度的影响是小的，但是，相比的星子的大小和形成时间，占主导地位的热机械演化和熔融和分化的发病的主要因素。我们评论这项工作的影响，这些星子的结构和演化，以及他们的行为可能的积木的类地行星。
英文摘要：
The thermal history and internal structure of chondritic planetesimals, assembled before the giant impact phase of chaotic growth, potentially yield important implications for the final composition and evolution of terrestrial planets. These parameters critically depend on the internal balance of heating versus cooling, which is mostly determined by the presence of short-lived radionuclides (SLRs), such as aluminum-26 and iron-60, as well as the heat conductivity of the material. The heating by SLRs depends on their initial abundances, the formation time of the planetesimal and its size. It has been argued that the cooling history is determined by the porosity of the granular material, which undergoes dramatic changes via compaction processes and tends to decrease with time. In this study we assess the influence of these parameters on the thermo-mechanical evolution of young planetesimals with both 2D and 3D simulations. Using the code family I2ELVIS/I3ELVIS we have run numerous 2D and 3D numerical finite-difference fluid dynamic models with varying planetesimal radius, formation time and initial porosity. Our results indicate that powdery materials lowered the threshold for melting and convection in planetesimals, depending on the amount of SLRs present. A subset of planetesimals retained a powdery surface layer which lowered the thermal conductivity and hindered cooling. The effect of initial porosity was small, however, compared to those of planetesimal size and formation time, which dominated the thermo-mechanical evolution and were the primary factors for the onset of melting and differentiation. We comment on the implications of this work concerning the structure and evolution of these planetesimals, as well as their behavior as possible building blocks of terrestrial planets.
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