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Plate Tectonics

What makes Super-Earths favorable for life? One surprising ingredient might be the unusual phenomenon of planetary plate tectonics.

Plate tectonics is a process from the family of tectonics – processes that shape a planet's surface by stretching, compression, or other forces acting on the planet's rigid outer layer (called lithosphere). Very quickly after formation, rocky planets like Earth are covered by a crust – a hard and brittle layer that is about 10 miles thick on average and forms part of the lithosphere. The mostly molten and very viscous interior of the planet – called mantle, churns over constantly by convective currents. The flow of heat and melt in the upper mantle is the source driving plate tectonics.

Earth's lithosphere is broken up into a dozen or so plates – large and small, that gradually move in different directions (see the figure below). To understand this motion, it is important to realize two important facts. First, the Earth's crust consists of ocean plates (or seafloor crust) that are denser and "younger", and continental plates that are less dense and thicker. Basalt rocks are most common for ocean plates, while granite is the main ingredient of continents. Second, the Earth's ocean plates are seen to emerge at mid-ocean ridges, where mantle lava erupts onto the seafloor and pushes apart the plates on each side. On a global scale, dense ocean plates form and spread apart, eventually colliding with less dense continental plates. This collision usually leads to subduction – the dense ocean plate slides under the continental plate, down into the hot mantle, where it gets molten, mixed and recycled.

Gases replenishing the atmosphere, subduction of crustal plates and the recycling of minerals inside the planet's mantle - all make the geochemistry of a planet active, but also stable. Under such conditions geochemical cycles are often dominating the exchange of gases and compounds and establish long-term stability and surface conditions, including climate.

For example, on Earth this exchange occurs through the global CO2 cycle, also known as the "carbonate-silicate cycle". Schematically the cycle is illustrated in Figure 2, showing the essential role played by plate tectonics in bringing the rocks back into the mantle, where they get molten, mixed, and the gas is recycled back into the atmosphere. The CO2 cycle does more than just recycling gas and minerals – it acts as a global "thermostat" for the planet's surface and makes the climate and surface temperature stable over millions and even billions of years. We described the workings of this "thermostat" in a previous unit entitled "Why Super-Earths?".

Why is all this beneficial to life? The answer, in a nutshell, is stability and chemical concentration. Over billions of years the Earth has kept its surface temperature stable; the oceans, for example, have generally always been liquid. We know this from geological evidence. We also know that our Sun has brightened up by 30 percent since the Earth formed. The solution to this seeming mystery appears to be a global geochemical cycle. The Earth is a large ball that is very hot and boiling inside; on the surface this energy moves the set of rocky continental and ocean plates around.

The essential thing about planetary plate tectonics is exchange. The molten, mixed interior can exchange chemical elements with the surface and atmosphere and vice versa. The elements are not simply recycled in this process, but their chemical transformations and concentrations exchange energy in a rich dynamic equilibrium. The alternative is a much poorer steady-state equilibrium that will set in on the inside and at the surface with no local energy sources. Thus plate tectonics makes a planet dynamic, renewing, and vital. As Ward and Brownlee put it in their book "Rare Earth", plate tectonics promotes environmental complexity. The exchange occurs through global cycles.

Is the process of plate tectonics common among Earth-like exoplanets? We do not know, but we expect so. In any case, the details are less important as long as the process promotes tectonic activity and exchange between the mantle and the surface. Rocky Super-Earths are expected to be this kind of planets and life, we hope, will find many of them habitable.

Солнечная система и ее тайны