The focus of this research topic will be the interior dynamics of planets and their effects on the exchange of volatiles between the interior and the atmo- and hydrospheres. We will further look at the generation of magnetic fields. Volatile exchange rates are strongly dependent on the tectonic mode of a planet (i.e., plate tectonics, stagnant lid tectonics or episodic plate tectonics) which again depends on the style of mantle convection as dicuseed above. Although we have a basic understanding of these modes, it is not well understood why and when planets act in which regime (compare Fig. 1.1). The stability of a tectonic mode and key rates (e.g., outgassing, subduction) mainly depend on (1) the size and internal structure of a planet and (2) on the rheology of the crust and the mantle. The latter depends (3) on the concentration of volatiles, (4) the generation of partial melt, and (5) the surface boundary conditions.
Planetary formation models in general predict the existence of massive terrestrial planets in the between 0.5 -10 Earth masses (Ida and Lin, 2004). The interior structure of a generic terrestrial planet depends on the composition of the reservoir from which it forms. It may be completely rocky if it forms close to the central star or may have accumulated a substantial amount of water. A hydrosphere may between a few meters depth (possible early Mars) and hundreds of kilometres as is the case for the large icy giant planet satellites in our solar system. Large planets are more likely to have plate tectonics than small ones although simple scaling laws are not available and rheology matters. In addition, phase transitions in the ice or rock mantles can be significant for dynamics and heat transport. The spinelperovskite transition, for instance, may result in layered convection in a rock mantle. Eventually, after cooling, the layering may overturn catastrophically and transform to single layer convection. An outburst in volcanic activity and volatile loss may be the consequence. It is also possible that transitions of the tectonic mode may be triggered by such an overturn but this has not been studied in any detail. The depth to a phase transition boundary is determined by pressure and temperature and therefore varies between planets of different sizes.
Several studies of the conditions under which tectonic regimes prevail have been published (see e.g., van Thienen et al., 2007 for a review), and it has been suggested that the balance between viscous and/or brittle strength of the lithosphere and convective stresses in the mantle eventually will determine the tectonic style. But these numerical models either assumed rotational symmetry or Cartesian geometry. It is likely that more realistic 3D spherical geometry models will yield significantly different results. Furthermore, current models assume that ductile weakening and stress localisation mechanisms in the midlithosphere are the most important parameters for generating weak plate boundaries. In these models, a mobile lid regime is initiated by e.g., introducing a critical ductile yield strength which may depend on the volatile content. The elasticity of the crust and the upper lithosphere, however, is neglected in these approaches. The latter has been included in so called local models of subduction zones where the driving stresses are introduced by ad-hoc boundary conditions. These models have shown that the presence of water may promote the concentration of stresses and the formation of a shear zone cutting through the entire lithosphere (Regenauer-Lieb et al., 2001).
The presence of water is known to reduce the solidus and liquidus temperatures of mantle rock by up to several hundred K depending on its concentration (Hirth and Kohlstedt, 1996). As a result, a water rich mantle undergoes more chemical differentiation through partial melting than a dry one. The enhanced differentiation also results in enhanced rates of volatile loss and the interior dries up with time unless – as in the case of plate tectonics - volatiles are efficiently recycled back into the mantle. First models of the evolution of a terrestrial planet taking volatiles into account used parameterized convection to calculate transfer rates (McGovern and Schubert, 1989; Franck and Boumana, 1995; Franck et al., 1999). Unfortunately, there exists no satisfying parameterization of transfer rates that account for the volatile dependence of rheology and that allow for transitions in tectonic mode. The generation of melt and its segregation is a complex process that potentially has a strong influence on the thermo-chemical evolution of a planet and is little understood.
Isotopic studies on rock samples is one important tool to constrain the internal differentiation of a planet and its volatile loss as a function of time. They are essential to constrain numerical modelling. These studies have shown that separate chemical reservoirs developed on Earth during the first few hundred million years and kept differentiating continuously through time, thus providing a continuous stream of volatiles to the Earth’s surface. The currently very limited data set for radiogenic isotopes for Martian samples has provided some evidence for the development of mantle reservoirs during the first ca. 200 million years by magmatic differentiation (e.g. Kleine et al., 2004). However, very little is known about differentiation and mass transfer from the mantle to the crust after the initially very vigorous geologic activity. Some ages of Martian meteorites plus the geology (topic 2.4) of the Isotopic studies on rock samples is one important tool to constrain the internal differentiation of a planet and its volatile loss as a function of time. They are essential to constrain numerical modelling. These studies have shown that separate chemical reservoirs developedon Earth during the first few hundred million years and kept differentiating continuously through time, thus providing a continuous stream of volatiles to the Earth’s surface. The currently very limited data set for radiogenic isotopes for Martian samples has provided some evidence for the development of mantle reservoirs during the first ca. 200 million years by magmatic differentiation (e.g. Kleine et al., 2004). However, very little is known about differentiation and mass transfer from the mantle to the crust after the initially very vigorous geologic activity. Some ages of Martian meteorites plus the geology (topic 2.4) of the Martian surface strongly suggest a long lived magmatic activity on this planet. Combined isotope and trace element data obtained for available Martian meteorites will yield firm constraints on the times of silicate melt production and volatile depletion of the Martian mantle and their transport to the surface.
The surface temperature (that may not only vary with time but also laterally on a global scale e.g. caused by a catastrophic impact) and the presence of water, provide the upper thermal boundary condition for a solid-surface planet interior. A first study of the influence of the surface temperatures on the interior has been performed by Phillips (2001) with simple coupled evolution models for Venus. They find that a hot atmosphere will delay the cooling of the mantle thereby affecting its magmatic/volcanic evolution. The study suggests a complex interplay between Venus’s convective evolution, volcanic activity and atmospheric evolution. More elaborate models are required to obtain a more detailed understanding of the interactions also in case of global events like large impacts that increase the temperature locally at the surface and down to a depth of few hundred kilometres.
Defining the characteristics of a planet that generates a magnetic field is of utmost importance to determine the likelihood of planetary habitability. A first-order criterion for the existence of a dynamo is that the fluid core needs to convect, either due to the growth of an inner core or due to core cooling. The latter possibility, a purely thermal dynamo has been suggested for the early Earth and Mars. However, a dynamo generated in an entirely fluid core most likely can be sustained only for a few hundred million years, particularly if the planet is in a stagnant lid regime such as Mars (Breuer and Spohn, 2003). Although the energy to power such a dynamo may be available, it is not understood a dynamo in an entirely fluid core works in detail. Dynamo models further suggest that the existence and characteristics of the magnetic field depend strongly on the strength and pattern of core heat loss through mantle convection and the planetary rotation rate (Christensen and Wicht, 2007).