Ученые выяснили, что редкий метеорит произошел от гигантского древнего протопланеты

A University of Colorado Boulder study has found that a rare type of meteorite likely came from a giant, ancient protoplanet that once existed in the solar system. Aaron Bell, a CU Boulder assistant research professor, studied a small sliver of an angrite called the Northwest Africa 12774 that was found in the Sahara Desert in 2019. Angrites, a type of meteorite, are exceedingly rare. Out of more than 80,000 meteorites discovered on Earth, only 68 are angrites. They’re among the oldest known volcanic rocks, forming about the same time the solar system began 4.56 billion years ago. “These are very old, almost as old as the solar system itself,” Bell said. Bell, whose work and salary are funded largely by NASA and the National Science Foundation, used a sample of the 1-pound angrite pressed in a glass slide about 3 inches by 1½ inches, with the sample so thin that light can pass through it. The process of having a meteorite officially recognized by the Meteoritical Society requires that a small subset of the sample be held in a repository that can be requested on loan for research and scientific purposes. Bell requested a sample of the angrite for his work. The entire angrite is held by a private collector. Bell and his colleagues noticed that the sample had a highly unusual composition, even for an angrite. They found that the angrite contained clinopyroxene, a mineral crystal commonly found in Earth’s crust and mantle, that was exceptionally rich in aluminum. That signaled that the rock formed under enormous pressure deep underground. The researchers then reconstructed the pressure conditions that might have been present for the angrite to form. To Bell’s surprise, they found the angrite would’ve needed at least 17.5 kilobars of pressure to form. For comparison, the pressure at the bottom of the Mariana Trench, the deepest point on Earth, is around 1 kilobar. “And so the only place you can generate high pressures like that is in the interior of fairly large planets,” Bell said. That level of pressure could not have existed inside a small asteroid. The researchers calculated that the angrite’s parent body must have been a planet with a radius of at least 1,000 kilometers, or 621 miles. That larger planet body probably broke up, since scientists know it doesn’t exist. “It implies or requires that the angrite parent body had to be big, but the question is, where is it now?” Bell said. “Why is it not here? And no one knows.” William Bottke, the executive director of the Southwest Research Institute in Boulder, said the study is interesting because it could potentially help scientists learn more about the earliest part of solar system formation. “It’s telling us something about how the early solar system may have evolved,” Bottke said. Some meteors come from asteroids, Bottke said, but others may not have formed in the asteroid belt. Those meteors, such as this angrite, may have formed elsewhere. When the solar system was forming, so were protoplanets, or large bodies of matter in the process of becoming planets. After the protoplanets formed, they could have collided and created fragments that might have gotten into the asteroid belt. Rock and material from collisions in the asteroid belt can then travel to Earth. “These meteorites are telling us a story,” Bottke said. “… Rocks tell a story. Rocks are like an ancient book, and if you know how to read them, they’ll tell you about what happened (billions of years ago).” Because of their age, angrites provide unique information about the solar system, Bell said. This research holds implications for how scientists understand the process and timeline of building planets. “It really makes us take a long, hard look at the models of solar system formations that we have and the timelines that we have,” Bell said. “And this suggests that some of those may not be right, or we might have to rethink the way we think about the earliest stages of evolution in ways that we didn’t even know.” Subscribe to our new newsletter to get CU Boulder’s latest news sent straight to your inbox.