Scientists proposed a new theory Wednesday that could solve two mysteries in one fell swoop, one circling the Earth every day and the other concerning the innards of our planet.
The first enigma is the origin of the Moon, for which the most commonly accepted theory is its appearance after the impact of a forming planet with the future Earth 4.5 billion years ago.
The collision with Theia, a protoplanet the size of Mars, would have propelled enough material into space for its agglomeration to form the Moon.
It remained to find remains of Theia. By looking not in the air, but underground, according to the study published in Nature by a team of scientists from mainly American institutions.
Because 2900 km below the surface, two large “blobs” have intrigued scientists since their discovery using seismic waves in the 1980s. Laid at the bottom of the Earth’s mantle, the layer separating the Earth’s core from its crust , these masses, each the size of a continent, lie beneath Africa and the Pacific Ocean.
They are warmer and denser than the environment surrounding them. And the researchers’ computer simulations suggest that these masses are “buried relics” of Theia, which entered Earth at the time of the collision.
This collision was “the most violent event suffered by the Earth” in its history, Qian Yuan, a geodynamics researcher at the California Institute of Technology (CalTech) and first author of the study, told AFP. .
Which makes it “very, very strange” that there is no visible trace left, according to him. And what motivated his thinking: “Where is the impactor? My answer: underground.”
Between space and geology
The research led experts from two very distinct specialties, space and geology, to collaborate.
Theia hit the Earth, then in formation, at more than 36,000 km/h, a speed sufficient for part of the impactor to penetrate “very deep into the lower mantle of the Earth”.
These pieces of essentially molten rock, several tens of kilometers wide, cooled and, solidifying, descended to the limit of the mantle and the earth’s core. Helped in this by a greater proportion of iron oxide than that of the terrestrial environment, which made them heavier.
They accumulated into two distinct masses, each of which is larger than the Moon, according to Mr. Yuan, who also insists that these conclusions remain the fruit of necessarily imperfect models and simulations.
An expert in Earth sciences and planetary exploration at Scotland’s University of Stirling told AFP that the theory put forward by Mr. Yuan “agrees with several existing clues.” “This is a significant finding,” according to Christian Schroeder, who did not participate in the study.
Even if, according to him, it does not resolve the question of the origin of the Moon, this theory provides “a credible explanation for the anomalies observed at the boundary between the mantle and the core”.
As for the remains of Theia, they could well “be responsible for important ongoing processes on Earth”.
The masses are known to carry plumes from the mantle, rising magma, to the surface of the Earth’s crust. A phenomenon linked to volcanic eruptions but also the evolution of supercontinents.
For Mr. Yuan, Theia’s impact “played a role in the evolution of the Earth over 4.5 billion years.” And this is what according to him would make it “unique […] different from other rocky planets.