Institute of Planetary Research
Berlin-Adlershof

Department of
Planetary Physics

Mantle Convection in Planets and Moons

Thermal and chemical convection provides a framework to reconcile observations of planetary magnetic and gravity fields, surface heat flow, distribution of volcanoes and tectonic structures, geo- & cosmochemistry, and mineral physics.

Solid but liquid
The mantles of terrestrial planets consist of rock, which can be partially molten in small regions, but the vast majority of the mantle material is solid. In response to short-term (~ 1s) force, e.g. seismic waves, the mantle behaves like an elastic solid body. However, in geological time scales (~100 Ma) the rock behaves like a viscous fluid and allows global-scale circulation. These interior movements can leave behind some fingerprints on the surface, for instance in the form of tectonic structures likes faults and mountains, horizontal plate movements or volcanoes.

The engine behind convection
Thermal convection in terrestrial bodies is driven by internal heat sources. This heat stems from accretion, the decay of radioactive elements, tidal dissipation, and the gravitational energy, which is released during the differentiation into core, mantle, and crust. The relative importance of each of these heat sources is a function of time and can vary from one terrestrial body to another. It should be noted that, although solar irradiation can be orders of magnitude bigger than internal heat, it has no effects on deep internal dynamics. Solar irradiation indeed penetrates less than a few hundred meters and only drives processes close to the surface.

Putting a planet in the computer
To understand thermal convection in terrestrial bodies, numerical modelling is our method of choice. Thermal convection can be described in terms of the conservation equations for energy, momentum, and mass, equation(s) of state, initial and boundary conditions. By coupling conservation equations for chemical species to that system, the interplay between convection and differentiation can be investigated. The solutions of these models depend on many parameters, which are not well known. A sucessful model should reproduce the observations e.g. provided by space missions.
We distinguish among several types of models. In parameterised models the efficiency of heat transport is described by a scaling law. In this case, only a global energy budget is considered, which is computationally inexpensive. It allows the investigation of a wide parameter space and is useful for a global overview. If one is interested in further details, e.g. convection structure and temperature fields, the full system of equations must be solved. Usually this is done in 2-d or 3-d, cartesian or spherical geometry. Two-dimensional cartesian models generally consume less resources than others; 3-d spherical models are more realistic. The choice of the models depends on the specific problem considered. The computations of our group are done on PCs, clusters or on one of the most powerful supercomputers in Europe.