Our planet is in constant flux. Tectonic plates—the large slabs of rock that divide Earth’s crust so that it looks like a cracked eggshell—jostle about in fits and starts that continuously reshape our planet—and possibly foster life.
These plates ram into one another, building mountains. They slide apart, giving birth to new oceans that can grow for hundreds of millions of years. They skim past one another, triggering earth-shattering quakes. And they slip under one another in a process called subduction, sliding deep into the planet’s innards and producing volcanoes that spew gases into the atmosphere. And not only is Earth alive, it is a vessel for life. Because it is the only known planet to host both plate tectonics—that ongoing shuffling of tectonic plates—and life, many scientists think the two might be related. In fact, some researchers argue that shifting plates, which have the ability to help regulate a planet’s temperature over billions of years, are a crucial ingredient for life.
This connection raises the tantalizing possibility that if scientists could find exoplanets that quake and rumble, they might be able to find life beyond our Pale Blue Dot. So, Cayman Unterborn, an astronomer at Arizona State University, set out to determine the likelihood that distant exoplanets undergo plate tectonics. In a paper posted July 3 to the preprint server arXiv and currently undergoing peer review, he and his colleagues found that the majority of exoplanets are probably unable to sustain plate tectonics over long periods of time. Their results are still uncertain because scientists do not fully understand how plate tectonics began on Earth (let alone on how they would other planets), but they do suggest that even if the process does begin, it may not last. That means Earth is not only the sole planet known to host moving plates in the solar system (although some recent evidence suggests Mercury might as well), it might also be one of a low number of such planets across the Milky Way. “If you do need plate tectonics [for life], this paper sounds like bad news,” says John Armstrong, an astronomer at Weber State University who was not involved in the study. Still, astronomers suspect that as many as 40 billion potentially habitable Earth-size planets dot the galaxy. Even if only a third of these planets can sustain plate tectonics (as Unterborn’s study suggests), those roughly 13 billion planets, Armstrong says, are “still a lot of possible habitable worlds!”
But just how essential is plate tectonics for life? Hints can be found from our own planet’s history. Around 2.5 billion years ago the sun was so cold that Earth’s liquid oceans should have been frozen in a snowball-like state—only they were not. Scientists think plate tectonics, which acts as a global thermostat, might have been our savior by creating volcanoes that spewed carbon dioxide into the atmosphere, helping it to retain more heat. Then, as the sun grew brighter and hotter, rainfall scrubbed the carbon dioxide from the atmosphere and plate tectonics later subducted it into the Earth’s mantle (the layer of hot rock above the core), locking it away. It is this cycle, which acts on million-year timescales, that helps keep Earth’s temperature stable enough to support life.
Yet Earth’s example does not prove plate tectonics is a requirement for life. Planets can, after all, be geologically active without plate tectonics. Just take a look at Mars, which boasts the largest volcano in the solar system. Still, that volcano no longer rumbles to life. In fact, most solar system planets (and even dwarf planets and moons) that were once geologically active are now quiet. Without plate tectonics, volcanism declines rapidly (with some notable nontectonic exceptions such as Jupiter’s Io and Saturn’s Enceladus). As such, Mars’s numerous but extinct volcanoes do not have the ability to belch carbon dioxide into the atmosphere, leaving the Red Planet quite chilly today. Such examples suggest plate tectonics—particularly long-lasting plate tectonics—is the best method of regulating a planet’s temperature and is therefore a useful ingredient in the cocktail of life.
The latest study seems to contradict some previous investigations of whether or not exoplanets might shake like Earth. In 2007 planetary scientist Diana Valencia, then at Harvard University, concluded that super-Earths (rocky planets larger than ours) are so likely to host plate tectonics, it is practically inevitable. Because planets more massive than Earth would retain significantly more internal heat from their initial formation, and because heat drives plate tectonics (via the conveyor belt of sinking and rising rock within the mantle), plate activity should be prolonged on such planets. The trouble is that Valencia’s study (and many studies that came later) analyzed only one parameter: a planet’s size. Unterborn’s study is among the first to address plate tectonics based on a planet’s composition.
To carry out this analysis, Unterborn and his colleagues needed to determine what an exoplanet’s chemical composition might look like. Although astronomers can currently decipher the elements within an exoplanet’s atmosphere, there is no way to peer deep into an exoplanet’s rocky interior—yet. So Unterborn and his team turned toward the planets’ host stars. Because the stars and their planets are built from the same swirling disk of dust and gas, they tend to be made of the same stuff. The researchers looked at nearly 1,500 stars (including 123 stars observed with the Kepler space telescope that astronomers know have orbiting exoplanets) and then used computer models to discover how rocks of these varying compositions would react to the high interior temperatures and pressures formed in a planet.
Once they had an idea of what an exoplanet’s mantle and crust might look like, geochemically speaking, the scientists were able to determine whether that exoplanet’s crust would be dense enough to sink into the mantle, just as Earth’s oceanic plates do at places like the Cascadia subduction zone—North America’s 1,000-kilometer-long chain of volcanoes built as one plate takes a deep dive beneath another. Making the calculation involved rigorous modeling: As pressures and temperatures mount during a plate’s descent, atoms in the plate undergo a reorganization that makes the plate denser. Should the plate remain denser than the surrounding mantle then the plate would continue to sink. If that is the case, plate tectonics might thrive for billions of years. But if it does not and the plate stalls, then plate tectonics would shut down, crippling life’s chances.
The results paint a rather depressing result as far as habitability is concerned: At least two thirds of the simulated planets build a crust that is too buoyant to sink. “If subduction were to happen, and [the plate] were to go down, it would just pop back up,” Unterborn says. “It’s like trying to push an inner tube underwater.” If these plates are on the move, they might crash into each other and crumple upward to form mountain chains as tall as the Himalayas, Unterborn says. But one plate will never subduct below another to remove excess carbon dioxide or form the volcanoes that spew more carbon dioxide into the atmosphere. As such, the planet will not be able to regulate its own temperature and will easily escalate into a world that resembles a snowball or a sauna.
THE NEW FIELD OF EXOGEOLOGY
The results highlight that a planet’s habitability cannot be defined only by the Goldilocks zone—that sweet spot in a planetary system where a planet’s orbital distance from its star keeps it neither too hot nor too cold. Nor can density alone determine what counts as an “Earth-like” planet. “Density is not destiny when it comes to planets,” Unterborn says. “The Earth is much more than a one-Earth mass, one-Earth radius planet” in the sun’s habitable zone. Just think back 2.5 billion years: Earth would not have been considered habitable to alien astronomers unless they took its geology into account.
Bradford Foley, a geologist at The Pennsylvania State University who was not involved in the study, agrees with the paper’s ultimate point—that the majority of rocky exoplanets likely cannot host plate tectonics—but he argues that finer details, such as the exact percentage of those planets, cannot yet be pinned down. “I would take everything beyond the big-picture view with a grain of salt because there are uncertainties wrapped up in there that are subject to change as more studies come out,” he says.
One of those uncertainties, Foley notes, is geologists still argue over how plate tectonics ignited on Earth and what continues to drive it today. The issue is that even if a plate is dense enough to sink into the mantle, the lithosphere—the strong and rigid outer shell of the planet—has to crack first. But what causes the lithosphere to crack is hotly debated in the field. Unterborn sidestepped this complication by looking for planets that might be able to undergo plate tectonics for billions of years—should it begin in the first place. Foley agrees that it is a clever workaround, and Unterborn argues that it is more interesting from a scientific point of view because we are more likely to find life where it has evolved over billions of years. But the assumption plate tectonics magically begins does show that even the proper elemental cocktail does not guarantee a shifting and rumbling surface. Still, Unterborn argues it does maximize our chances of finding plate tectonics and therefore life.
Unterborn views the work as a step forward in a new field—one where geology meets astronomy in a discipline one might call exogeology—that began just 10 years ago with Valencia’s paper. Just last week Foley, Unterborn and Driscoll (who agrees that exogeology will be a “hot topic in the future”) submitted a proposal to the NASA Astrobiology Institute to further assess how materials of different compositions react under high pressures and temperatures. Whereas Unterborn’s study was based on theoretical calculations, the new team would like to synthesize these rocks in the lab and physically subject them to those conditions. That would allow them to paint a more accurate picture and even explore how changing the composition might crack the lithosphere—the other important criterion for kick-starting plate tectonics. “I think it’s definitely the future,” Unterborn says. “I'm glad to be at the forefront of it.”
Quelle: Scientific American