Plate tectonics on a planet far, far away
When thinking about what makes life on Earth possible, plate tectonics probably isn’t high on most people’s lists. But plate tectonics plays a vital role in regulating the planet’s surface temperatures by helping to recycle greenhouse gases like carbon dioxide. Now scientists are trying to determine whether Earth-like planets outside our solar system have plate tectonics, which might increase the likelihood that such remote worlds harbor life. But as two recent studies demonstrate, researchers are far from reaching a consensus.
Most known planets orbiting other stars are massive bodies larger than Jupiter and are probably composed mainly of gas: not hospitable places for life to thrive. Over the past few years, however, astronomers have started to detect smaller planets — only one to 10 times more massive than Earth — that are more likely to be rocky, and therefore, a better bet for hosting life.
Discussions of the existence of life on these “super-Earths” often center on whether the planet has liquid water, but Diana Valencia, a graduate student at Harvard University in Cambridge, Mass., thinks plate tectonics is also important. “Plate tectonics allows a planet to have a built-in cycle to regulate temperature over a geological time scale,” she says. On Earth, the only planet in the solar system with active plate tectonics, the process plays an important role in the carbon cycle. After carbon dioxide is released into the atmosphere via volcanic eruptions, chemical reactions remove some of the atmospheric carbon, which gets locked into sediments and eventually returned to the mantle through the action of plate tectonics. In the long run, this limits the buildup of greenhouse gases in the atmosphere, preventing Earth from reaching blistering hot temperatures.
Because Valencia could not observe these remote planets directly to assess the possibility of plate tectonics, she and her colleagues modeled scaled-up versions of Earth to see how an increase in mass affects convection in the mantle, which on Earth, is responsible for generating the forces that deform the planet’s plates and cause them to subduct below one another — necessary components for plate tectonics. “The model basically extrapolates what we know happens on Earth to planets around other stars,” Valencia says.
Their results showed that plate tectonics is inevitable on super-Earths, the team wrote in The Astrophysical Journal Letters on Nov. 20. That’s because as the mass of a planet increased, its mantle churned more vigorously to create stronger forces that act to deform the plates, Valencia says. This also led to thinner and, therefore, weaker plates. The combination of stronger forces and weaker plates strongly favors plate tectonics, she says.
But not everyone is ready to conclude that all rocky super-Earths have plate tectonics. Vicki Hansen, a geologist at the University of Minnesota at Duluth, was not convinced by Valencia’s model: “It seemed extremely simplistic,” she says, in part because it didn’t address how a planet’s plates form. “The concept of making huge, brittle, dense tracts of crust that will recycle to the mantle is not trivial,” Hansen says. The model doesn’t consider the composition of super-Earths, which Hansen says is important because not all rocks behave the same way. For example, although Earth’s crust is made of brittle rocks that break or fracture in response to the mantle’s forces, Hansen suggests that a planet could be covered in a material that instead bends or flows, which would give it a Silly-Putty®-like surface, making plate tectonics impossible.
Adrian Lenardic, a geodynamicist at Rice University in Houston, Texas, who recently modeled plate tectonics on super-Earths with Craig O’Neill of Macquarie University in Sydney, Australia, is also skeptical. “It’s not so clear cut that an increase in size will really guarantee, or strongly favor, plate tectonics,” he says. “There are other factors that come into play. One specific factor that we looked at with increasing planet size was the increase in gravity.”
In their numerical model, Lenardic and O’Neill, whose results were published in October’s Geophysical Research Letters, found that the forces generated in the mantle do increase as the mass of a planet increases, just as Valencia’s team found. “But the gravity will also increase,” Lenardic says, “which in turn increases the resistance of the plates.” So, even though the forces acting to break apart the plates is getting stronger, so too are the forces acting to keep them together.
Thus, the larger planets in Lenardic and O’Neill’s model had “stagnant lids,” or “surfaces that don’t really move,” O’Neill says, “because the rocks are too strong relative to the engine driving the planet.” But this could be due to the age of the planet, Valencia says. As planets age, they lose their radioactive material that helps heat the planet and fuels the mantle’s convection. Someday even Earth will no longer have active plate tectonics after all of its radioactive material has decayed. But because Lenardic and O’Neill did not control for age in their model, all of the larger planets were also older planets, which could explain their results, Valencia says.
Rather than provide definitive answers, these models show just how complicated plate tectonics really is, Lenardics says. Running these simulations will ultimately help scientists better understand the range of circumstances under which plate tectonics can form, including how it began on Earth, Lenardic and Valencia both say.