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Sunday, October 14, 2007

Super-Earths plate tectonics

Super-Earths plate tectonics

Super-Earths will have plate tectonics
Super-Earths" - rocky planets up to 10 times the mass of Earth that orbit other stars - probably have similar structures to our world, with a solid inner core surrounded by a liquid mantle and then a crust. They may even have plate tectonics, which some argue is necessary for life to evolve.

Dimitar Sasselov of the Harvard-Smithsonian Center for Astrophysics and colleagues came to this conclusion after modelling geological processes on planets of various sizes. They found that as planetary mass increases, more heat is trapped and convection increases. As a result the shear stress within the crust increases too and plate thickness decreases. That means the plates are weaker and plate tectonics becomes "inevitable" ( Our own planet seems to lie at the threshold. If it were any less massive, it would probably not be geologically active.

Plate tectonics may boost biodiversity by recycling chemicals and minerals through the crust. "When it comes to habitability, super-Earths are our best destination," says Sasselov.

"The idea is right," says Jack Lissauer of NASA's Ames Research Center in Moffett Field, California. "Plate tectonics is more likely on more massive planets."

The recent discovery of super-Earths (masses less or equal to 10 earth-masses) has initiated a discussion about conditions for habitable worlds. Among these is the mode of convection, which influences a planet's thermal evolution and surface conditions. On Earth, plate tectonics has been proposed as a necessary condition for life. Here we show, that super-Earths will also have plate tectonics. We demonstrate that as planetary mass increases, the shear stress available to overcome resistance to plate motion increases while the plate thickness decreases, thereby enhancing plate weakness. These effects contribute favorably to the subduction of the lithosphere, an essential component of plate tectonics. Moreover, uncertainties in achieving plate tectonics in the one earth-mass regime disappear as mass increases: super-Earths, even if dry, will exhibit plate tectonic behaviour.

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