Pasadena (dpa) – Shortly after the formation of the solar system 4.6 billion years ago, the celestial body Theia, about the size of the planet Mars, collided with the primitive Earth. Our Moon was formed from debris thrown into space – a mixture of Theia’s matter and primordial Earth. But the remains of Theia are also preserved deep in the Earth’s mantle, as a research team from the USA and Great Britain has now discovered. Two previously puzzling regions of increased density can be explained as accumulations of Theia, the scientists write in the journal Nature.
“Seismic studies of the Earth’s interior show two continent-sized regions in which seismic waves propagate unusually slowly,” explain Qian Yuan and colleagues from Arizona State University. Therefore, the deep regions of the Earth’s mantle differ in composition from the material surrounding the Earth’s mantle and are 2 to 3.5 percent denser. To date, there is no generally accepted scientific explanation for these regions.
Using large-scale computer simulations, Yuan and his colleagues now show that such denser regions are a natural result of large collisions during planetary formation — and that the two anomalies, located deep in Earth’s mantle beneath the Pacific Ocean and beneath Africa, are detectable remnants. About Thea. “Our simulations of the collision show that part of Theia’s mantle may migrate into the Earth’s lower mantle,” the researchers said.
The size of the fragments reaches fifty kilometers
Based on the composition of the Moon, which was formed from Earth and Theia debris, Yuan and his colleagues obtained a density for these sinking parts of Theia’s mantle that is 2 to 3.5% higher than the normal density of Earth’s mantle. The values are in good agreement with the results derived from seismic studies. As the team’s simulations also show, these are fragments of Theia’s mantle up to fifty kilometers in size that have sunk into the Earth’s interior and coalesced there, above the Earth’s core, to form larger structures.
An important aspect is that these larger densities could remain stable in the Earth’s mantle for four and a half billion years, that is, to the present day, as computer models show. Another piece of evidence supports Yuan and colleagues’ hypothesis: The Hawaiian Islands contain a form of volcanic basalt whose composition is surprisingly similar to the rocks of the lava plains on the Moon. This rock may have originated in the Theia material region deep in Hawaii.
But Yuan and colleagues’ model is not only important for the Earth-Moon system. “Large impacts are common in the final stage of planet formation,” the researchers assert. “Similar inhomogeneities likely exist in the interiors of other planets.” Perhaps traces of previous collisions can be found inside the Earth.
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