Super-Earths are not a good place for plate tectonics

JUMBO-SIZE PLANET Big rocky super-Earths such as Kepler-62f, shown in this illustration, probably don’t have active plate tectonics, new research suggests.

Plate tectonics doesn’t rumple the surfaces of Earth’s supersized cousins, new research suggests.

Simulating the extreme pressures inside giant exoplanets called super-Earths, researchers discovered that these planets probably have thick, stagnant outer shells and sluggish internal circulation. Those properties make the existence of fragmented jigsaw puzzles of sliding and shifting surface sections unlikely, the researchers report in a paper to be published in the Journal of Geophysical Research: Planets.

On Earth, plate tectonics drives the carbon cycle that helps regulate the planet’s temperature and allows life to flourish, notes study coauthor Takehiro Miyagoshi, an earth and planetary scientist at the Japan Agency for Marine-Earth Science and Technology in Yokohama.

“We think super-Earths are boring,” he says. “This point should be kept in mind in our search of habitable planets.”

Earth’s tectonic plates are driven by a conveyor belt of sinking and rising rock. Previous studies predicted that the extra heat insulated inside super-Earths would easily power similar convection.

Those studies, however, repurposed simulations of Earth’s internal movements without considering the changes that come with a bigger planet, Miyagoshi says. Larger planets put more pressure on their interiors, boosting temperatures at lower depths.

Miyagoshi and colleagues simulated a planet 10 times Earth’s mass. As cold rock blobs descended into the simulated super-Earth’s interior, rising pressures heated the rock and stalled its fall. Similarly, climbing magma plumes decompressed as they rose toward the surface, lowering their temperature and buoyancy. This lethargic movement created a stagnant shell around the planet roughly 1,800 kilometers thick, about the radius of the moon.

The new work isn’t the final word on plate tectonics on super-Earths, says Brad Foley, a geodynamicist at the Carnegie Institution for Science in Washington, D.C. Scientists don’t fully understand why Earth has plate tectonics while other planets such as Venus don’t, he says. “Until we know that well, we’ll always be all over the place when we try to predict what will happen on super-Earths.”
T. Miyagoshi, M. Kameyama and M. Ogawa. Thermal convection and the convective regime diagram in super-Earths. Journal of Geophysical Research: Planets,
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