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

Theia, perhaps as big as the proto-Earth, would have delivered its densest rocks inside the planet.

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 seismology 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.

The large low-shear velocity provinces (LLSVPs), as seismologists call them, might simply have crystallized out of the depths of Earth’s primordial magma ocean. Or they might be dense puddles of primitive mantle rock that survived the trauma of the Moon-forming impact. But based on new isotopic evidence and modeling, Yuan believes the LLSVPs are the guts of the alien impactor itself. “This crazy idea is at least possible,” says Yuan, who presented the hypothesis last week at the Lunar and Planetary Science Conference.