- 學位論文
行星胚的鐵核形成
Numerical modeling of iron core formation in planetary embryos
摘要
岩石行星核心的鐵形成是太陽系早期重要的事件之一。鐵核形成的過程也是目前當紅的研究主題，但是目前仍未達到共識。本論文以數值模式研究一種可行的鐵核形成機制：在這個機制中，鐵核心為分布全球的熔融鐵層往下移動、穿過未經鐵與矽酸鹽分化的固態物質而形成。本研究中的行星體，其起始的內部構造來自冷加積的過程、擁有三層結構，從内到外為：未經分化的原始核、全球分布的鐵層、最外覆蓋著富含矽酸鹽質的岩層。這樣的構造在重力上是非常不穩定的，最後終將導致密度較高的鐵進入行星中心，而原含有矽酸鹽的物質則被推擠在外、圍繞在鐵核周圍形成地幔。 本論文在序論中，提供目前關於行星形成、與鐵核形成的看法。第二章則簡述在模擬二維行星體內部、因重力不穩定引起物質重新分布時，所使用的數值方法。在第三章，首先呈現的是，校準數值模型的相關測試結果。再來，以簡單的模型來瞭解鐵核形成的過程，亦即：假設模擬中的各種物質為牛頓流體，有其各自固定的黏滯度、即忽略重力位能釋放時，對黏滯度帶來的影響；實驗結果發現，此行星體的原始核，在核形成的過程中，會短暫地暴露在行星表面；並且鐵核形成所需的時間，與原始核的強度有關。 第四章的實驗則考慮物質為非牛頓流體，即物質的黏滯度會受到溫度、壓力、與變形率的影響，同時將重力位能轉換出的熱能計算進去。實驗結果可以區分出三種核形成的形態：暴露型、破裂型與過渡型。暴露型的核形成，如同第三章考慮牛頓流體的結果，原始核會短暫地使整個行星體偏離球形、並且原始核也會短暫地露出地表。而破裂型的核形成，則未在第三章的實驗中觀察到。屬於此型態的原始核會受到破壞，破壞可能是藉著變形集中在原始核內產生行星尺度、可以切割原始核的大型破裂帶，或者是下沉的大型鐵滴或片狀物切穿原始核而成。模式計入的能量釋放，並未改變核形成的型態，但卻影響行星內部的溫度分布。溫度變化集中在移動中的原始核周圍、或者在切割原始核的大型破裂帶內，溫度可升高幾百度。若原始核較大或有較高的黏滯度，則大部分的熱量將會集中在形成的鐵核之中。
並列摘要
The core formation of rocky planets is one of the most important events during the early history of these planets. The process of core formation is a topic of active research, and so far no consensus was reached. This dissertation presents a numerical investigation of a possible process of core formation, namely the descent of metal diapirs from a global ponded iron layer through an undifferentiated solid interior, leading to the formation of an iron core. The initial structure assumed in this study derived from cold accretion scenario and consists in three layers: a central undifferentiated protocore, a global iron shell, and an outer silicate-rich mantle. This structure is gravitationally unstable and leads to a differentiation in a dense, iron core in the center surrounded by a silicate rich mantle. After an introductory chapter that discuses recent ideas in planetary formation and core formation, Chapter 2 describes the numerical methods used to model the gravitational redistribution process in a 2D planetary body. In Chapter 3, accuracy tests are first conducted, and core formation process is explored with a simplified model that assumes a constant viscosity for each material and neglects the rheological effects of gravitational energy dissipation. Results indicate a transient exposure of the protocore to the planetary surface, and predict that the time for core formation depends on the strength of the solid protocore. Experiments in Chapter 4, include a non-Newtonian, temperature-, pressure-, and strain rate-dependent viscoplastic rheology, and take into account the thermal contribution from gravitational energy dissipation. Three different core formation regimes are observed, the exposure mode, the fragmentation mode, and the transition mode. Like models with Newtonian rheology in chapter 3, the core experiences large deviations from the spherical shape and may temporarily be exposed at the surface (exposure mode). By contrast to the Newtonian models, however, the destruction of the protocores observed in the fragmentation modes is driven by (i) the spontaneous strain localization along planetary-scale shear zones forming inside the protocore, and/or (ii) descending localized iron diapirs or sheets penetrating the protocore. Feedback from energy dissipation influences planetary temperature distribution although it does not significantly affect core formation regimes. However, it causes a temperature increase up to several hundred K (i) around the moving and deforming protocore, and (ii) along planetary scale rupture zones that form inside the protocore. If the protocore is large and has a high viscosity, a large fraction of the dissipated heat is partitioned to increase the temperature of iron.