Shortly after the formation of the solar system 4.6 billion years ago, the Mars-sized celestial body Theia collided with the primordial Earth. Our moon was formed from the debris that was thrown into space – a mixture of matter from Theia and primordial Earth. But remains of Theia have also been 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 matter, 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 from Arizona State University and his colleagues. The areas deep in the Earth’s mantle therefore differ in their composition from the surrounding material of the Earth’s mantle and are two to three and a half percent denser. To date, there has been no generally accepted scientific explanation for these regions,
Using extensive computer simulations, Yuan and his colleagues now show that such denser regions are a natural consequence of large collisions during planet formation – and that the two anomalies, which lie deep in the Earth’s mantle under the Pacific and under Africa, are remnants can be about Theia. “Our simulations of the collision show that part of Theia’s mantle may migrate into Earth’s lower mantle,” the researchers said.
Fragments are up to fifty kilometers in size
Based on the composition of the moon, which was formed from Earth and Theia debris, Yuan and his colleagues obtain a density for these sinking fragments of the Theia mantle that is two to three and a half percent higher than that of the normal Earth mantle – in good agreement with the results Values derived from seismic studies. As the team’s simulations also show, these are fragments of the Theia mantle up to fifty kilometers in size that sank into the interior of the Earth and merged there, above the Earth’s core, to form larger structures.
An important aspect is that these larger densifications can remain stable in the Earth’s mantle for four and a half billion years, i.e. up to the present day, as the computer models show. And there is another piece of evidence that supports Yuan and his 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 region of Theia matter deep beneath Hawaii.
But Yuan and his colleagues’ model is not only important for the Earth-moon system. “Large collisions are a common occurrence in the final phase of planet formation,” emphasize the researchers. “Similar inhomogeneities are likely to exist in the interior of other planets.” And perhaps traces of even earlier collisions can even be found inside the Earth.
