| Summary: | 博士 === 國立臺灣大學 === 海洋研究所 === 99 === 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.
|
|---|
