17 October 2007

The plate tectonics of alien worlds

Cosmos Online
Extrasolar planets that are superficially similar to our own and sometimes termed 'super-sized Earths' may in fact have stagnant outer surfaces rendering them inhospitable to life.
The plate tectonics of alien worlds

Less than hospitable: An illustration of what it might look like from the surface of a super-sized Earth, based on the study. The atmosphere is assumed to be similar to Venus and the planet is close to its star. Credit: Craig O'Neill

SYDNEY: Extrasolar planets that are superficially similar to our own and sometimes termed ‘super-sized Earths’ may in fact have stagnant outer surfaces rendering them inhospitable to life.

These rocky bodies are probably not much like Earth at all, according to a new study by Australian and U.S. planetary scientists. Instead of possessing active plate tectonics, those that are five to ten times the size of Earth most likely have a geologically dead surface more akin to the hellish surface of Venus, they said.

“A super-sized Earth would look more like Venus than Earth,” said author Craig O’Neill, a planetary scientist at Macquarie University in Sydney. “The surface conditions of such bodies are a strong function of the volcanic and tectonic history of the planet. There’s an assumption that super-Earths have the same tectonics as Earth, but that’s not the case.”

Gravity and stress

The new findings, published in the Geophysical Review Letters and recently presented at the 7th Australian Space Science Conference in Sydney, might put a dampener on the hopes of those looking for life on such terrestrial planets.

To make the find, O’Neill and colleague Adrian Lenardic of Rice University in Houston, Texas, modelled how gravity and stress forces would affect a planet if it were a scaled-up version of Earth.

They found that a while the increased forces generated by convection currents in the mantle would act on a super-sized Earth, increased gravity would act to strengthen the rocky crust so the surface would no longer resemble the cracked eggshell-like configuration of Earth’s tectonic plates.

In other words, the surface of the planet would be too strong to allow the driving forces of magma to break the crust into large, active plates where rock is created or destroyed. The research suggests that the size of the planet and its atmosphere would be useful indicators of a planet’s life-sustaining capability.

“At the moment, the only way to tell habitability is how far away a planet is from its star,” O’Neill said. “But we know that that’s not a reliable indicator. The main problem is that it doesn’t explain our Solar System.” If the distance between a planet and the star was key, O’Neill said, then Mercury (which is closer to the Sun) should be hotter than Venus – but that isn’t the case.

Experts believe a lack of active plate tectonics partly accounts for the inhospitable conditions on Venus. “Mantle convection drives the evolution of atmosphere,” O’Neill said. “It’s the one fundamental process on Earth.”

Atmosphere regulating

Tectonic processes releases molecules such as carbon dioxide and water into the atmosphere when magma breaks through the surface, but they also recycles those compounds at subduction zones. These are where the crust at the edge of a plate gets pushed down into the mantle underneath another plate.

Plate tectonics acts to “regulate the atmosphere and buffer the Earth from extremes,” O’Neill said. “On other planets with no plate tectonics the atmosphere would be hellish like Venus’.”

With no regular outlet for heat generated from the core, such a planet would have unpredictable and widespread volcanic activity, he argues. It would also suffer from a runaway greenhouse effect, caused by the build up of carbon dioxide that has no way of getting recycled. Moreover, any water on the planet’s surface would likely be evaporated away by its star.

Planetary scientist Graziella Caprarelli of the University of Technology, Sydney, agrees that the research confirms super-sized Earths could not sustain plate tectonics. “However, we know that planet dynamic regimes are not static; Earth itself evolved from an early magma ocean regime to plate tectonics, and models of other planets in the Solar System suggest that they also may have undergone geodynamic changes.” Additional studies would help clarify how a planet’s tectonic activity might evolve, she said.

Caprarelli added that the research could also help focus the search for life in space. “[This] type of planetary modelling… has broad repercussions in how we go about looking for life outside of the Solar System. It also helps us understand better how other planets in our Solar System have evolved, which in turns makes us understand more about Earth.”

An independent study published in the Astrophysical Journal Letters by scientists at the Harvard-Smithsonian Centre for Astrophysics in Massachusetts conversely argues that super-sized Earths would have plate tectonics. However, O’Neill refutes that research, arguing that the study failed to take into account the effect that increased gravity, and therefore pressure, would have in strengthening the crust.


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