Wednesday, April 23, 2014

Venus' Crust Heals too Fast for Plate Tectonics

The emergence of plate tectonics is arguably Earth's defining moment, the authors of a new Nature paper write. Out of all the planets we’ve looked at carefully, Earth is the only one that has a hard outer crust with distinct pieces that shift and move. Our home is unique in its continents and quakes.

Some scientists think that plate tectonics are essential for life—so much so that if they could figure out a way to spot tectonic action on exoplanets, they think it would be a good indication that there might be life there, too. Tectonic activity recirculates minerals and recycles carbon. As one plate slides under another (a process called subduction), it pushes carbon down into the mantle with it.

Without plate tectonics, carbon would build up in the atmosphere. Venus, which does not have tectonics, shows the results: an atmosphere that is 96 percent carbon dioxide. It's toxic. Yet Venus is about the same size and composition as our planet, so why doesn't it have plate tectonics?

Some researchers made a model to explore how Earth initiated plate movements, and these same researchers made one model of its neighbor for comparison. A 1.5-billion-year-old Earth and a similarly aged Venus were modeled as a hot, mushy material made of tiny particles of rock. The model uses physics at the one-millimeter rock grain scale to explain how the whole planet behaves. According to David Bercovici, a geophysicist at Yale who was an author on the paper, the model also shows how plate tectonics emerged on Earth but not on her twin.

The mantle moved material around beneath the material, causing it to sink or drip down in some places. This sinking of the mushy pre-plate material is called proto-subduction to contrast it with modern subduction, where one plate meets and slides under another.

At this point, you reach the core of the paper: the concept of damage mechanics. This describes what happens once the proto-subduction begins. If a nascent crust starts to bend, does it repair itself or snap into distinct plates?

Dripping causes the grains of rock to get smaller. With heat, these clusters rebuild or heal the crust. The researchers calculated dimensionless damage parameters based on the way the material responded to stress. Earth had a damage parameter of 100. On the other hand, Venus had a mere 10. Similarly, they input two healing numbers, determined from earlier experimental work: Venus has 10, and 10-4; Earth 1 and 10-1.

In the Earth-crust simulation, the damage accumulated faster than the grains could rebuild into clusters and heal it. By contrast, the Venus model, which was a couple hundred Kelvin hotter, saw damage that tended to heal faster than it accumulated. The tiny grains of minerals were able to accumulate and stick together faster than the mantle caused them to drip and pull apart.

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