Remains of impact that created the Moon may lie deep within Earth

Source:  By Paul Voosen

Excerpt: Scientists have long agreed that the Moon formed when a protoplanet, called Theia, struck Earth in its infancy some 4.5 billion years ago. Now, a team of scientists has a provocative new proposal: Theia’s remains can be found in two continent-size layers of rock buried deep in Earth’s mantle. For decades, seismologists have puzzled over these two blobs, which sit below West Africa and the Pacific Ocean and straddle the core like a pair of headphones. Up to 1000 kilometers tall and several times that wide, “they are the largest thing in the Earth’s mantle,” says Qian Yuan, a Ph.D. student in geodynamics at Arizona State University (ASU), Tempe. Seismic waves from earthquakes abruptly slow down when they pass through the layers, which suggests they are denser and chemically different from the surrounding mantle rock. ...Evidence from Iceland and Samoa suggests the LLSVPs have existed since the time of the Moon-forming impact, says Sujoy Mukhopadhyay, a geochemist at the University of California (UC), Davis, who considers Yuan’s idea plausible but is open to other explanations. Seismic imaging has traced plumes of magma that feed volcanoes on both islands all the way down to the LLSVPs. Over the past decade, Mukhopadhyay and others have discovered that lavas on the islands contain an isotopic record of radioactive elements that formed only during the first 100 million years of Earth history. Moreover, a new picture of the Moon-forming impactor suggests it could have delivered a cargo of dense rock deep inside Earth. The impact theory was developed in the 1970s to explain why the Moon is dry and doesn’t have much of an iron core: In a cataclysmic impact, volatiles like water would have vaporized and escaped, while a ring of less dense rocks thrown up in the collision would have eventually coalesced into the Moon. The theory invoked an impactor the size of Mars or—in recent variants—much smaller. But recent work from Yuan’s co-author, ASU Tempe astrophysicist Steven Desch, suggests Theia was nearly as big as Earth....  

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