The discovery of extrasolar Earth-sized planets has encouraged research into their potential for habitability. One of the generally agreed 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. Plate tectonics provide a means of geochemical regulation of atmospheric particulates, as well as removal of carbon from the atmosphere. This prevents a “runaway greenhouse” effect that can result in inhospitable surface temperatures and vaporization of liquid surface water. Planetary scientists have not reached a consensus on whether Earth-like exoplanets have plate tectonics, but it is widely thought that the likelihood of plate tectonics on an Earth-like exoplanet is a function of planetary radius, initial temperature upon coalescence, insolation, and presence or absence of liquid-phase surface water.
Potential exoplanet geodynamic regimes In order to characterize the geodynamic regime of an Earth-like exoplanet, the basic assumption is made that such a planet is Earth-like or “rocky”. This implies a three-layer stratigraphy of (from center to surface) a partially molten iron core, a silicate mantle that convects over geologic timescales, and a relatively cold, brittle silicate lithosphere. Within these parameters, the geodynamic regime at a given time point in the planet's history is likely to fall within one of three categories:
Plate tectonics The mantle of a planet with plate tectonics has driving forces that exceed the yield strength of the brittle lithosphere, causing the lithosphere to fracture into plates that move relative to each other. A critical element of the plate tectonic system is these lithospheric plates become negatively buoyant at some point in their evolution, sinking into the mantle. The surface mass deficit is balanced by new plate being formed elsewhere through upwelling mantle plumes. Plate tectonics is an efficient method of heat transfer from the interior of the planet to the surface. Earth is the only planet plate tectonics is known to occur on, although evidence has been presented for Jupiter's moon Europa undergoing a form of plate tectonics analogous to Earth's.
Stagnant lid A stagnant lid regime occurs when mantle driving forces do not exceed the lithospheric yield strength, resulting in a single, continuous rigid plate overlying the mantle. Stagnant lids only develop when the viscosity contrast between the surface and planetary interior exceeds about four orders of magnitude.
Episodic tectonics Episodic tectonics is a general term for a geodynamic regime that possesses aspects of both plate tectonics and stagnant lid dynamics. 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. When this occurs, the shift to plate tectonics is usually catastrophic in nature and can involve resurfacing of the entire planet. After such a resurfacing event (or period of resurfacing events), stagnant lid equilibrium conditions are regained, resulting in a quiescent, immobile lid.
Methods of predicting exoplanet geodynamic regimes Exoplanets have been directly observed and remotely sensed, 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. Therefore, the majority of information and conjectures made about them come from alternative sources.
Solar System analogues All the rocky planets in the Solar System except Earth are generally believed to be in the stagnant lid geodynamic regime. Mars and particularly Venus have evidence of prior resurfacing events, but appear to be tectonically quiescent today. Geodynamic inferences about Solar System planets have been extrapolated to exoplanets in order to constrain what kind of geodynamic regimes can be expected given a set of physical criterion such as planetary radius, presence of surface water, and insolation. In particular, the planet Venus has been intensely studied due to its general physical similarities to Earth yet completely different geodynamic regime. Proposed explanations include a lack of surface water, the lack of a magnetic geodynamo, or large-scale evacuation of interior heat shortly after planetary coalescence. Another source of insight within the Solar System is the history of the planet Earth, which may have had several episodes of stagnant lid geodynamics during its history. 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.
Observation of exoplanets
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. Since distance from the host star and planetary size are generally believed to influence exoplanet geodynamic regime, inferences can be drawn from such information. 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). Spectroscopy has been used to characterize extrasolar gas giants, but has not yet been used on rocky exoplanets. However, numerical modeling has demonstrated that spectroscopy could detect atmospheric sulfur dioxide levels as low as 1 ppm; presence of sulfur dioxide at this concentration may be indicative of a planet without surface water and with volcanism 1500–80000 times higher than Earth.
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