The Great Impact Theory is the most accepted among the scientific community to explain how our natural satellite, the Moon, was formed. This theory states that the Moon arose as a result of a great collision about 4.5 billion years ago between a young Earth and a world the size of Mars, which we often refer to as Theia. However, despite being one of the most widely examined theoretical models, direct evidence indicating the existence of the Theia impactor remains elusive.
Now, a team of scientists offers a tantalizing new proposition : Theia’s remains could still be present in two layers of rock the size of a continent buried deep within the Earth’s mantle.
Certainly for many years seismologists have wondered about the origin of these two spots or layers of rock found under West Africa and the Pacific Ocean, straddling the Earth’s core as if it were a pair of headphones. At up to 1,000 kilometers high or even more , “they are the most important factor within the Earth’s mantle,” says Qian Yuan, academic in seismology at Arizona State College and co-author of the paper presented at the Lunar and Planetary Science virtual conference .
These anomalous structures are so massive that, in turn, they generate their own disturbances. Seismic waves from earthquakes decelerate abruptly after crossing through these layers, suggesting that they are denser and chemically completely different from the surrounding mantle rock. An example of this is the phenomenon known as the South Atlantic Anomaly.
LLSVPs are the guts of Theia
“Here, we show that the Theia mantle can be intrinsically several times denser than the Earth’s mantle, allowing materials from the Theia mantle to sink to the lowest mantle on Earth and accumulate in thermochemical piles that they can cause LLSVPs (large provinces with low shear velocity or super feathers) observed seismically, ”the researchers explain.
Thus, although we do not know what happened to the rest of Theia after that great impact, this model theorizes that LLSVPs may represent ancient fragments of the dense and iron-rich mantle of Theia, which would now lie in the Earth’s own mantle when the two developing worlds came together billions of years ago.
Is it reliable?
The theory of a massive planet Theia would also explain the scale of the LLSVPs, which together contain six times more mass than the Moon, as it would have implied that Theia’s core would have rapidly fused with Earth’s. However, there are caveats to raise : diffuse evidence for LLSVPs is one of them, for example, since their pile-like structure could be a mere illusion created by internal models based on low-frequency seismic waves that blur small differences. Be that as it may, the findings are consistent with previous research suggesting that certain LLSVP-related chemical signatures are at least as old as the Theia impact.
This mystery still has more pieces to fit, but it is a plausible option.
Reference: GIANT IMPACT ORIGIN FOR THE LARGE LOW SHEAR VELOCITY PROVINCES.Q. Yuan1 *, MM Li1, SJ Desch1andB.Ko1, 1Arizona State University. These findings were presented at the 52nd Lunar and Planetary Science Conference, held as a virtual event last week. The study is currently under review, prior to its future publication in Geophysical Research Letters.
