HERE’S THE STORY — Unlike any other known planet, Earth’s surface has both continents and oceans. Continental crust is less dense and much thicker than oceanic crust, which causes it to float substantially higher than oceanic crust in the Earth’s dense underlying mantle. The presence of the continents on Earth greatly affected the planet’s atmosphere, oceans, climate and the proliferation of life. For example, runoff from the continents is the main source of a number of key nutrients for the oceans, such as phosphorus, which is needed to make DNA and other biological building blocks. Previous research has detected cycles in the production of continental crust. Scientists generally believed that these cycles were associated with the regular birth and destruction of supercontinents on Earth’s surface due to plate tectonics—the drifting, crashing, and sinking of the giant slabs of rock that make up Earth’s surface today. However, these cycles are also seen in some of Earth’s most ancient rocks, when plate tectonics may not have existed. Our position in the Galaxy was affected when comets hit Earth and shaped our continents. Kirkland et al. WHAT DID THE SCIENTISTS DO? — In the new study, the researchers analyzed data from two places where the earliest history of continents on Earth is preserved – the North American craton in Greenland and the Pilbara craton in Western Australia. (Cratons are giant solid blocks of the Earth’s crust that form the hearts of the continents.) The decay of uranium into zircon crystals helped scientists pinpoint details about continental formation at both sites, spanning a time period from about 2.8 billion to 3.8 billion years ago. Hafnium isotopes within these crystals also helped identify times that experienced magma inflows associated with crust production. By looking at vast amounts of data from large amounts of rock, the researchers identified a rhythmic pattern in the formation of the continental crust that spans about 170 to 200 million years. They found a similar pattern when they looked at oxygen isotopes, supporting their results. “Only through large datasets can this pattern be detected,” study lead author Chris Kirkland, a geochronologist at Curtin University in Perth, Australia, tells Inverse. WHAT DID THEY FIND? — This pattern corresponds to the time it takes for the Solar System to pass through one of the four main spiral arms of the Galaxy, where the density of stars and interstellar clouds is high, as it completes an orbit around the heart of the galaxy. Gravitational influence from the spiral arms could drag comets from the Oort cloud in the outer regions of the Solar System inward to Earth and the other planets. Scientists argue that such cometary bombardment may have excavated vast amounts of rock from the Earth’s surface, causing the underlying mantle rock to decompress and melt, sort of like popping a cork in a champagne bottle. The creation of this buoyant molten rock may, in turn, have permeated the production of the continents. “While the impacts are considered cataclysmic events, they have also shaped the evolution of our planet, and it appears that our continents would not have developed as they have without them,” says Kirkland. “Our planet is connected to the structure of the galaxy.” The researchers found more evidence for this idea within spheroids, which are rock formations produced by cosmic impacts. These hold deposits of tiny spheres formed either from molten rock ejected during an impact or from vaporized rock that condensed and rained down after the impact. They observed that the ages of the globule beds in Australia and South Africa match the motion of the Solar System in the Norma spiral arm about 3.25 billion to 3.45 billion years ago. Determining the ages of more pellet beds could add more evidence, they noted. The continents we know today could have been shaped by outside forces. ilbusca/DigitalVision Vectors/Getty Images “It’s becoming clear that the Solar System and Earth did not form on their own, but with outside influence,” study co-author Phil Sutton, a planetary scientist at the University of Lincoln in England, tells Inverse. “We need to redirect our gaze outward rather than focusing purely on internal mechanisms.” Although the Earth is hit much more often by rocky bodies in the asteroid belt than by comets in the Oort cloud, collisions from the Oort cloud would hit with much more energy. That’s because near-Earth asteroids “travel in the same direction as Earth,” Sutton says. “So when there’s an impact from that group, the difference in speed is much smaller.” Imagine cars traveling on a multi-lane road in the same direction. If a car hits another car next to it, “the relative impact speed is pretty low,” he explains. However, if there was an intersection on that road and a car hit another at that 90-degree intersection, “the damage would be orders of magnitude greater,” Sutton says. “That’s essentially what happens with comets compared to the asteroids”. Additionally, the gravity of the Sun and Jupiter makes it difficult to knock near-Earth asteroids out of their orbits. In contrast, the sun exerts a weak influence on comets in the Oort cloud, making them more susceptible to perturbations, Sutton explains. The researchers emphasize that the timescales over which these giant impacts occur “are huge and in no way pose a risk to us,” says Sutton. “For example, dinosaurs were on the planet for about the same amount of time it takes to go from one spiral arm to another, which is hundreds to thousands longer than humans were around.” Paradoxically, “life is thought to originate from the times we report periodic crustal production,” says Sutton. “Coincidence or was it the spark that ignited the explosion of life on the planet?” WHAT’S NEXT? — The role of impacts in the production of continental crust likely declined over time due to the exponential decrease in the average size and number of potential impactors as the Solar System evolved. The emergence of tectonic plates would also have begun to play a more important role in continent production on Earth, the scientists note. “It would be great to see a similar pattern in the timing of lunar impacts,” says Kirkland. “You would predict the same impact frequency for the moon.” LEARN SOMETHING NEW EVERY DAY.
