Little is known about the long-destroyed moon-forming planet, Theia. But it may have been born in the inner solar system—just like Earth—a new study suggests
"While many scientists have accepted this lunar formation theory, proposed in the 1970s, they don't know much about Theia. It’s long-gone, after all. The run-in with Earth destroyed the small planet, estimated to have been roughly the size of Mars.
A new study published in the journal Science on November 20 sheds light on Theia’s origin. The work, which involved analyses of lunar samples, Earth rocks and meteorites, suggests that Theia formed fairly close to the sun—where Earth was also born.
“Earth and Theia are likely to have been neighbors,” Timo Hopp, a study co-author and geoscientist at the Max Planck Institute for Solar System Research in Germany, says in a statement.
Most computer simulations that model the moon’s formation suggest that it should be mostly made of Theia’s leftovers. But past studies have found that the moon has remarkable chemical similarities to Earth, far more than expected of two distinct planetary bodies, reports Jacek Krywko at Scientific American.
Those findings could hint that Theia resembled the Earth more than once thought. So, Hopp and his colleagues analyzed 15 terrestrial rocks and six lunar samples brought to Earth by NASA’s Apollo missions in the 1960s and 1970s. The researchers looked at the specimens’ iron isotopes, variations of the element. Ratios of these isotopes within an object can provide clues about where it came from.
Researchers suspect that the much of the iron in the Earth’s mantle, the hot rock layer between the crust and core, came from Theia during the collision. So, if terrestrial rocks from the mantle and lunar rocks have similar iron isotope ratios, that would suggest that all three objects—Earth, Theia and the moon—share similar backstories, reports the New York Times’ Robin George Andrews.
The iron signatures of the terrestrial and lunar samples were practically the same, the team found. These chemical fingerprints also closely matched those of meteorites that formed close to the sun.
That data, along with information on a few other elements within the specimens, helped the researchers reverse engineer Theia’s composition and size. The analysis hinted that Theia was a rocky planet that contained just five to ten percent of Earth’s mass, according to Scientific American. It also may have formed closer to the sun than the Earth did.
“The authors make new iron isotope measurements at exceptional levels of precision,” Sara Russell, a planetary scientist at the Natural History Museum, London, who was not involved in the work, says to Scientific American. Russell tells the outlet that “this careful work and insightful modeling help us better understand our origins.”
Paul Byrne, a planetary scientist at Washington University in St. Louis who was also not involved in the research, tells the Times that he agrees. The authors have “certainly done a lot of work, taking a smorgasbord of samples and trying to make sense of it all,” he adds.
Still, Byrne says that many questions remain about Earth and Theia’s collision, and these may never be answered because of the little planet’s destruction.
“The past really is lost to us,” he says."
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