The American Astronomical Society is holding its annual meeting in Austin, so you might notice that Thinking Meat is taking on a definitely star-struck feel this week. A story that made the news today is about plate tectonics on Earth-like planets.
The processes of plate tectonics are believed to be responsible for some crucial features that make Earth habitable, including the cycling of carbon dioxide between rock and atmosphere and back again (essential for temperature control, which in turn is essential for maintaining liquid water on the surface). Not to mention the creation of the continents and the maintenance of the magnetic field that helps keep our planet from being pasteurized by cosmic radiation—plate tectonics are good for living things. (Peter Ward and Donald Brownlee spend a whole chapter on plate tectonics in their book Rare Earth: Why Complex Life Is Uncommon in the Universe.)
But the news from Austin is that Earth appears to be at the small end of the range for planets with suitable conditions for plate tectonics. Simulations of rocky planets much larger than Earth showed that the movements of continental plates become easier on bigger planets, where the plates are thinner. Based on the simulations, scientists would expect to find plate tectonics operating on rocky planets three times as massive as the Earth and larger (with an upper limit of about ten times the Earth’s mass, because any bigger than that planets start to turn into gas giants). Perhaps the presence of water on Earth adds enough subterranean pressure to get the plates moving even though our planet is so small. Note that Mars and Venus, both smaller than Earth, do not have plate tectonics, and as far as we know have no life, certainly no complex life. It looks like rocky “super-Earth” planets might be relatively common in the cosmos, which makes this good news for anyone who hopes to someday find evidence for extraterrestrial life.
However, it’s a little disconcerting to realize that Earth is somewhat marginal in the plate tectonics department, especially in light of another news story about how the process might occasionally quit working. Geochemical evidence suggests that about a billion years ago, the volcanism associated with plate tectonics stopped. Around the same time, an ocean basin closed and the subduction zones surrounding it (where rock on the ocean floor collides into continental rock and gets pushed beneath it) were shut down, evidently bringing plate tectonics to a halt. Today subduction, an important driver for volcanism, earthquakes, and associated disturbances, is mostly happening in the Pacific basin, which is predicted to close in about 350 million years, when North and South America run into Eurasia. When this happens, plate tectonics might again grind to a halt (I know that phrase is a cliche, but it seems appropriate for something that involves the collision of continents). So it may be that our planet is not just borderline geologically active, but only intermittently geologically active as well.
