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Mantle Convection in Terrestrial Planets  

Elvira Mulyukova and David Bercovici

All the rocky planets in our solar system, including the Earth, initially formed much hotter than their surroundings and have since been cooling to space for billions of years. The resulting heat released from planetary interiors powers convective flow in the mantle. The mantle is often the most voluminous and/or stiffest part of a planet and therefore acts as the bottleneck for heat transport, thus dictating the rate at which a planet cools. Mantle flow drives geological activity that modifies planetary surfaces through processes such as volcanism, orogenesis, and rifting. On Earth, the major convective currents in the mantle are identified as hot upwellings such as mantle plumes, cold sinking slabs, and the motion of tectonic plates at the surface. On other terrestrial planets in our solar system, mantle flow is mostly concealed beneath a rocky surface that remains stagnant for relatively long periods. Even though such planetary surfaces do not participate in convective circulation, they deform in response to the underlying mantle currents, forming geological features such as coronae, volcanic lava flows, and wrinkle ridges. Moreover, the exchange of material between the interior and surface, for example through melting and volcanism, is a consequence of mantle circulation and continuously modifies the composition of the mantle and the overlying crust. Mantle convection governs the geological activity and the thermal and chemical evolution of terrestrial planets and understanding the physical processes of convection helps us reconstruct histories of planets over billions of years after their formation.


Terrestrial Planets: Interior Structure, Dynamics, and Evolution  

Doris Breuer and Tilman Spohn

The three terrestrial planets Mercury, Venus, and Mars (ordered by their distance from the sun) share the same first-order internal structure with the Earth. There is an iron-rich core at the center, overlain by a silicate mantle and a crust that is generated by partial melting of the mantle. But while Mars and Venus have a core with a radius of about half the planetary radius, just as the Earth, the core of Mercury extends to about 80% of the planet’s radius. The interiors of the terrestrial planets are heated by the decay of radioactive elements and cool by removing internal energy. In addition to radiogenic heat, internal energy was deposited during planet formation and early differentiation. Heat transport is dominated by mantle and core convection and volcanic heat transfer although conduction through the lithosphere on top of the mantle matters. The convection powers the planetary heat engine which converts thermal energy into gravitational energy, mechanical (tectonic) work, and magnetic field energy. None of the terrestrial planets has plate tectonics such as the Earth although surface renewal and some form of lithosphere subduction is debated for Venus. The tectonics of Mars and Mercury is best described as stagnant-lid tectonics, with a thick rigid lid overlying the convecting mantle. Both planets show early volcanism, with Mars in particular being locally volcanically active even until a few million years ago. Because of Mercury’s large core, the mantle is comparatively thin, and convection may be sluggish or may even have ceased. Magnetism is another property that the terrestrial planets share with the Earth although it is still not confirmed by data that Venus ever had a magnetic field. A dynamo process driven by buoyancy released through the growth of a solid inner core is producing the present-day magnetic fields of Earth and Mercury, but Mars’ dynamo has likely ceased to be active. Crust units with remanent magnetization testify to the early dynamo. The terrestrial planets have been explored to differing degrees by spacecraft missions which allow a deeper physical understanding of the interiors and their dynamics and evolution.