The hypothesis is far from unanimous. “In our simulations, the mantle of Theia and that of the Earth mixed rather well” testifies planetologist Miki Nakajima of the University of Rochester in New York. In recent years, his work has focused on the evolution of the internal structure of the rocky planets of our solar system.
“I don't think the impactor material would have mixed completely, but the degree of homogenization is underestimated in this study,” adds geodynamicist Maxim Ballmer of University College London. Without being associated with the study recently published in the journal NatureBallmer collaborated with Deng on a related study a few years ago.
Scientists recognize that these higher density regions have long occupied the Earth's mantle, but their exact age and origin are still debated.
“There is an alternative explanation for the formation of these superplumes,” adds Ballmer. He notably mentions a theory according to which the solid mantle that we know today was once a thick layer of molten magma, before differentiating to form the current layers. The upper layer solidified quickly, radiating its heat into space. The lower layer solidified more slowly and therefore had time to form more or less dense regions, according to certain studies.
The next step will be to compare the chemical signatures of the materials present in these superplumes and on the Moon, largely composed of Theia. “If they have the same geochemical fingerprint, they must come from the same planet,” says Yuan.
Taking new samples, however, is easier said than done. It is impossible to drill the Earth to superplumes. That said, as Yuan explains to us, it happens that rocks from the lower mantle make their way to the surface, this is particularly the case with basalts from oceanic islands.
The surface of the Moon has been exposed to space erosion for billions of years and is at risk of being contaminated by meteorites; researchers would therefore like to analyze samples from the lunar mantle as well. Those they have to date mainly come from the surface.
To obtain new fragments of the Moon, we will have to wait for a future sample return mission to its South Pole, where the mantle is more exposed and accessible. Until then, scientists will continue to refine their models to try to identify Theia's spectrum.
