Geodynamics of terrestrial exoplanets

One of the generally agreed[1] requirements for a life-sustaining planet is a mobile, fractured lithosphere cyclically recycled into a vigorously convecting mantle, in a process commonly known as plate tectonics.

Planets with episodic tectonic regimes will have immobile surface lids for geologically long spans of time, until a shift in equilibrium conditions is precipitated by either weakening lithosphere or increasing mantle driving forces.

Exoplanets have been directly observed and remotely sensed,[10] but due to their great distance and proximity to obscuring energy sources (the stars they orbit), there is little concrete knowledge of their composition and geodynamic regime.

[12] These stagnant-lid periods were not necessarily planet-wide; when supercontinents such as Gondwanaland existed, their presence may have shut off plate motion over large expanses of the Earth's surface until mantle heat buildup underneath the superplate was sufficient to break them apart.

[13] Indirect and direct observation methods such as radial velocity and coronagraphs can give envelope estimates of exoplanet parameters such as mass, planetary radius, and orbital radius/eccentricity.

For example, an exoplanet close enough to its host star to be tidally locked may have drastically different "dark" and "light" side temperatures and correspondingly bipolar geodynamic regimes (see insolation section below).

In such models, different planetary physical parameters are manipulated (i.e. mantle viscosity, core-mantle boundary temperature, insolation, “wetness” or hydration of subducting lithosphere) and the resultant impact on the geodynamic regime is reported.

[9] Therefore, changes to the lithospheric temperature, whether from external sources (insolation) or internal (mantle heating) will increase or decrease the likelihood of plate tectonics in viscoelastic-plastic models.

Later studies such as that of Noack and Breuer (2014)[1] show that this assumption may have important implications, resulting in a gradual increase of the temperature differential between the core and mantle.

A flaw of viscoelastic-plastic models of exoplanet geodynamics is in order for plate tectonics to be initiated, unrealistically low yield stress values are required.

If the reduction of grain size (damage) is intensely localized in a stagnant lid, an incipient crack in the mantle can turn into a full-blown rift, initiating plate tectonics.

[15] For rocky exoplanets larger than Earth, the initial interior temperature after planetary convalescence may be an important controlling factor of surface motion.

The outgassing of mantle-derived carbon and sulfur that occurs along plate margins is critical for producing and maintaining an atmosphere, which insulates a planet from solar radiation and wind.