Dreidimensionale thermische Evolutionsmodelle für das Innere von Mars und Merkur

Abstract

The interior structure and evolution of the terrestrial planets are closely related to convective activity in their interior. These endogenic processes are responsible for many geological and tectonic structures on the surface and for the production of the magnetic field by a dynamo process. Thermal evolution models try to understand the different evolution of the planets, especially in contrast to the Earth, and the related processes in the interior. This thesis presents mantle convection models for the thermal evolution of Mars and Mercury.On Mars, young volcanism is concentrated in only two regions: in the Elysium region and, much more prominently, in the Tharsis region. Strong anomalies in the gravity field in the volcanic regions support the assumption that thermal convection in the martian mantle differs from that in the Earth's mantle and that the convection pattern is dominated by two strong plumes. A possible reason for this strong reduction to a few plumes could be the endothermic phase transition in the mineral structure from γ -spinel to perovskite and magnesiowüstite. In the Earth this transition is located at a depth of 660 km but in Mars it may occur close to the core-mantle boundary (CMB). The magnetic field of Mars also differs from that of the Earth and is today dominated by a strong crust magnetization. The crust magnetization - especially in the old southern hemisphere - suggests that Mars had an active dynamo in the first 0.5 Ga of its evolution but which subsequently died out.In extended three -dimensional models, which take the cooling of the planet and possible phase transitions into account, it is possible to reach a convection pattern with only two upwellings, which could explain the concentrated volcanism on Mars. In addition, the models show that only in the early stages of evolution is the heat flux at the CMB sufficiently high as to allow a thermally driven dynamo. A chemically driven dynamo is not possible because the martian core stays completely liquid during the evolution and does not freeze out a solid inner core. This can explain the lack of a global martian magnetic field today.In the second part of the thesis, evolution models for Mercury are shown. The internal structure of Mercury is not well-known at present. The extremely high density of the planet suggests a big iron core. The available data does not provide insight as to whether this big iron core is completely liquid or solid or at least partially solid. Mercury has a weak global magnetic field and the mechanism which can produce this field is not yet well constrained. Thermal evolution models for Mercury which can simulate the effects of freezing out a solid inner core can provide important information about the interior structure and give some hints for the magnetic field.Axisymmetric and first fully 3-D results show that mantle convection in the mercurian mantle is restricted to a thin layer and that the mantle is dominated by a very thick lithosphere. The size of the solid inner core strongly depends on the exact melting conditions of iron with a small sulfur content, which are not yet well known. The assumed convection strength and the initial temperature conditions additionally influence the evolution and size of the inner core. Freezing out of an inner core allows for chemical convection, which could explain the present-day magnetic field. A thermally driven dynamo can be ruled out in all simulations because the heat flux out of the core which is transported by the mantle is too low to allow for thermal convection in the core.