Fragments of the planet that formed the Moon may be buried by the Earth’s core

The Moon is believed to have formed in a gigantic impact when a planet the size of Mars collided with primordial Earth. This object is nicknamed Theia, in honor of the titan and goddess of precious metals and gems, and mother of Selene (the Moon) in Greek mythology. New research now suggests that Theia has not completely disappeared; large portions may still exist within our planet.

Around the Earth’s core, scientists believe that there are two large structures called low shear velocity provinces (LLSVP). One is below West Africa and the other is below the Pacific. They are 1,000 kilometers (600 miles) thick and stretch for several thousand kilometers. Some researchers now believe that these spots are pieces left over from Theia.

“This crazy idea is at least possible,” Qian Yuan, a geodynamics doctoral student at Arizona State University, told Science. Yuan presented his research at the 52nd Lunar and Planetary Science Conference last week.

Animation of LLSVPs based on the clustering analysis of Cottaar & Lekic (2016). Image credit: Sanne.cottaar / Wikimedia Commons; CC BY-SA 4.0

The idea that these structures were pieces of Theia has been discussed before, but this new work combines several pieces of evidence into a coherent setting. Theia merged with Earth and their cores merged into one. Simulations carried out by the researchers show that Theia’s mantle was slightly denser than Earth’s, and this would allow a large part of its mantle to survive. The range of the simulations suggests rocks 1.5 to 3.5 percent denser than those on Earth. Previous studies have suggested that the density of LLSVP is within that range.

The simulation also suggests that Theia could be larger than previously thought, perhaps as big as Earth. After all, LLSVPs contain six times more mass than the entire Moon. So the impactor, if these structures are from outer space, it must have been quite massive.

While the evidence presented provides a good picture, there are still many open questions regarding these structures that could affect this interpretation. The interior of our planet is studied using seismic waves and models, so there could be errors lurking there, creating an inaccurate view. LLSVPs may not be as thick or dense or constructed the way we think they are. So a large Theia densa might not be behind these spots.

The work has been submitted for peer review to Geophysical Research Letters and will certainly continue to be discussed. LLSVPs are a source of continuing interest. They have even been considered as a potential source of a future super eruption.

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