The remnants that make up the Moon would be on Earth

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If they were solid and were on the Earth’s surface, the International Space Station (ISS) would have to bypass the two gigantic bubbles of liquid rock that lie two thousand kilometers below West Africa and the Pacific Ocean. Discovered in the 1970s, they may be remnants of Theia, the planet that hit the primitive Earth 4.5 billion years ago, according to two scientists at Arizona State University (ASU).

With the size of two continents, the two masses extend for thousands of kilometers, influencing even the seismic waves of earthquakes: when they pass through what are called “large low-shear-velocity provinces , or LLSVPs) their intensity drops dramatically, which may mean that the material from which they are made is more dense than the rest of the Earth’s mantle.

To arrive at this hypothesis (suggested for years but never defended by someone who gathered enough elements and evidence to present it), the geochemist Qian Yuan, one of the authors of the research presented in mid-March at the Lunar Science Conference and Planetária, based on a study by the study’s co-author, astrophysicist Steven Desch, on the impact theory that formed the Moon.

Among the many explanations for the absence of water and a larger iron core on our satellite, one suggests that the body that would have hit Earth would have to be as big as Mars, capable of causing such a catastrophic impact that the water on both planets would immediately evaporate. Less dense rocks would be launched into space and then be grouped together on what is now the Moon.

Desch’s work, published in mid-2019, suggests that Theia was almost as big as Earth. To reach this conclusion, their research group measured the ratio between hydrogen and one of its heaviest isotopes, deuterium, in samples from Earth and the Moon. What they found is that our planet has much less hydrogen in its rocks than moon stones.

Poor in iron

“To capture and retain so much light hydrogen, Theia must have been huge and quite dry: the water, enriched with heavy hydrogen during its formation in space, would have raised overall levels of deuterium. Such a large, dry protoplanet would have a core that is poor in iron and a mantle rich in that element, 2% to 3.5% denser ”, says Desch.

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The two ASU scientists modeled the impact between Earth and Theia, and the result showed that, after the shock, the cores of the two planets merged; Theia’s mantle, formed by denser rocks, would have formed the LLSVPs that today rest near the center of the Earth – a scenario that Desch’s research with deuterium predicted.

The collision with such a large planet would have sent a large amount of dense rock into the bowels of the Earth, just like the LLSVPs. Seismological studies carried out in Iceland and Samoa point to the antiquity of LLSVPs. “They have been around since the time of the moon’s formation impact,” says geochemist Sujoy Mukhopadhyay, from the University of California.

To reach this conclusion, Mukhopadhyay’s team analyzed, in the last decade, the levels of radioactive isotopes, formed in the first hundred million years of Earth’s history, following the plumes of magma that feed the volcanoes of the two islands and that descend until the LLSVPs.

Future missions

The hypothesis can still be tested by the geochemical analysis of the lava expelled by the volcanoes above the bubbles and rocks of the lunar mantle, in search of similarities. This, however, can only be done in the future: the existing samples are those brought by the Apollo missions from the surface of the satellite.

Not all scientists studying the Earth’s past agree with the scenario envisioned by Yuan and Desch, like the seismologist at Durham University Jennifer Jenkins.

“Yuan’s idea is not inconsistent with what we know, but I am not entirely convinced. The LLSVPs are perhaps the remains of nuclei from other miniature planets that hit the primitive Earth, ”she says, adding that Theia may just be a tomb in a planetary cemetery inside our planet.

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