Mars Once Had Bizarre 'Shell' Tectonics

Towering above their surroundings, Mars' massive volcanoes are symbols that the quiescent planet was once fiery and dynamic.

Now new research suggests that before the planet cooled off, it played host to a bizarre form of plate tectonics unlike any the solar system has seen.

Plate tectonics is key to sustaining life on Earth. Wherever crust is born in the deep sea, or destroyed as it dives back into the the mantle in an endless loop of recycling, volcanoes erupt gases and nutrients that regulate climate and continuously renew life.

One-third Earth's size, Mars is too diminutive to hold enough heat energy to sustain tectonics for very long. But billions of years ago its crust slid around like a rigid skin on a molten interior, according to Shijie Zhong of the University of Colorado in Boulder.

"Earth is the only planet with plates that move around," Zhong said. "But you can have the entire shell of crust rotating with respect to the interior."

In a paper published this week in the journal Nature Geoscience, Zhong used a computer simulation of the planet to argue this "single plate" form of tectonics was fueled by uneven convection in the mantle. In the northern hemisphere, cool material sank down toward the planet's core, while hot buoyant material rose toward the surface in the southern hemisphere.

Zhong's theory explains one of the biggest mysteries of Mars -- why the southern half of the planet is so much higher than the north. The hot, rising material pushed up the crust and made it thicker. Where it was hottest, a plume of magma erupted on to the surface as a volcano.

Today, three of those volcanoes form a straight line across the Tharsis Rise highlands. As the planet's shell of crust rolled to the south between 4.1 and 3.7 billion years ago, Zhong thinks a stationary plume of magma punched holes in the crust, erupting volcanoes in a row one after another, much like the Hawaiian Islands on Earth.Testing this theory will be difficult, though. New surveys of the red planet could look for fracture patterns in the crust that indicate the entire planet's surface once moved as one. The rocks' magnetic signature could also hint at ancient motion.

The finding also has implications for other planets.

"There may be a lot of other places in the solar system where this is important," Frances Nimmo of the University of California, Santa Cruz said. "Mercury, Venus, even some moons -- almost everywhere you look outside of Earth you have single-plate bodies."