Changes

====Damage theory====

====Damage theory====

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. Additionally, plates in viscoelastic-plastic models have no deformation memory, i.e. as soon as the stress on a lithospheric plate drops below its yield stress it returns to its pre-deformation strength. This stands in contrast to Earth-based observations, which show that plates preferentially break along preexisting areas of deformation.<ref name=Foley_etal_2012>Foley, B. J., D. Bercovici, and W. Landuyt, 2012, The conditions for plate tectonics on super-Earths: Inferences from convection models with damage, Earth and Planetary Science Letters 331-332, pp. 281–290.=</ref>

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. Additionally, plates in viscoelastic-plastic models have no [[deformation]] memory, i.e. as soon as the stress on a lithospheric plate drops below its yield stress it returns to its pre-deformation strength. This stands in contrast to Earth-based observations, which show that plates preferentially break along preexisting areas of deformation.<ref name=Foley_etal_2012>Foley, B. J., D. Bercovici, and W. Landuyt, 2012, The conditions for plate tectonics on super-Earths: Inferences from convection models with damage, Earth and Planetary Science Letters 331-332, pp. 281–290.=</ref>

Damage theory attempts to address this model flaw by simulating voids created in areas of strain, representing the mechanical pulverization of coarse grains of rock into finer grains. In such models, damage is balanced by “healing”, or the temperature and pressure-driven dynamic recrystallization of smaller grains into larger ones. 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.<ref name=Landuyt_etal_2008> Landuyt, W., D. Bercovici and Y. Ricard, 2008, Plate generation and two-phase damage theory in a model of mantle convection, Geophysical Journal International, vol. 174, issue 3, pp. 1065–1080</ref> Conversely, a high surface temperature will have more efficient lithospheric healing, which is another potential explanation for why Venus has a stagnant lid and Earth does not.<ref name=Foley_etal_2012 />

Damage theory attempts to address this model flaw by simulating voids created in areas of strain, representing the mechanical pulverization of coarse grains of rock into finer grains. In such models, damage is balanced by “healing”, or the temperature and pressure-driven dynamic recrystallization of smaller grains into larger ones. 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.<ref name=Landuyt_etal_2008> Landuyt, W., D. Bercovici and Y. Ricard, 2008, Plate generation and two-phase damage theory in a model of mantle convection, Geophysical Journal International, vol. 174, issue 3, pp. 1065–1080</ref> Conversely, a high surface temperature will have more efficient lithospheric healing, which is another potential explanation for why Venus has a stagnant lid and Earth does not.<ref name=Foley_etal_2012 />

External sources of planetary heat (namely, radiation from a planet’s host star) can have drastic effects on geodynamic regime. With all other variables held constant, an Earth-sized exoplanet with a surface temperature of 273 K will evolve over its geological lifetime from a plate tectonic regime, to episodic periods of plate tectonics interspersed with stagnant lid geodynamics, to a terminal stagnant lid phase as interior heat is exhausted. Meanwhile, a “hot” planet (759 K surface temperature) under the same initial conditions will have an amorphous surface (due to lithospheric yield stress being constantly exceeded) to a stagnant lid as interior heat is exhausted, with no plate tectonics observed.<ref name=VanSummeren_etal_2011 />

External sources of planetary heat (namely, radiation from a planet’s host star) can have drastic effects on geodynamic regime. With all other variables held constant, an Earth-sized exoplanet with a surface temperature of 273 K will evolve over its geological lifetime from a plate tectonic regime, to episodic periods of plate tectonics interspersed with stagnant lid geodynamics, to a terminal stagnant lid phase as interior heat is exhausted. Meanwhile, a “hot” planet (759 K surface temperature) under the same initial conditions will have an amorphous surface (due to lithospheric yield stress being constantly exceeded) to a stagnant lid as interior heat is exhausted, with no plate tectonics observed.<ref name=VanSummeren_etal_2011 />

Planets closer than 0.5 [[astronomical units]] from their star are likely to be tidally locked; these planets are expected to have drastically different temperature regimes on their “day” and “night” sides. When this scenario is modeled, the day side displays mobile lid convection with diffuse surface deformation flowing toward the night side, while the night side has a plate tectonic regime of downwelling plates and a deep mantle return flow in the direction of the night side. A temperature contrast of 400 K between day and night sides is required to create such a stable system.<ref name=VanSummeren_etal_2011 />

Planets closer than 0.5 [[astronomical units]] from their star are likely to be tidally locked; these planets are expected to have drastically different temperature regimes on their “day” and “night” sides. When this scenario is modeled, the day side displays mobile lid convection with diffuse surface [[deformation]] flowing toward the night side, while the night side has a plate tectonic regime of downwelling plates and a deep mantle return flow in the direction of the night side. A temperature contrast of 400 K between day and night sides is required to create such a stable system.<ref name=VanSummeren_etal_2011 />

===Presence of surface water===

===Presence of surface water===