Scientists have dated the birth of the Solar System to about 4.57 billion years ago. About 60 million years later a “giant impact” collision between the infant Earth and a Mars-sized body called Theia created the Moon.

Now, new research suggests that the remains of the large object that collided with the young Earth to form the Moon are still identifiable deep within the planet as two large lumps. These lumps make up about 8% of the volume of the Earth’s mantle, which is the rocky zone between the Earth’s iron core and its crust.

The new study, led by Qian Yuan of Arizona State University and Caltech, argues that the heat generated by this collision was not enough to melt the whole of the Earth’s mantle, so the innermost mantle remained solid.

Consequently, the researchers say, the melted mantle of Theia didn’t completely mix with Earth’s mantle. That would have made the Theia remnants indistinguishable from Earth’s mantle as a whole. Instead, a lot of Theia’s mantle ended up as two continent-sized lumps that now sit on top of the Earth’s core-mantle boundary.

Large low-velocity provinces

Yuan argues that these lumps correspond to, and can explain, the existence of the two large low-velocity provinces (LLVPs), that were discovered decades ago: one below the Pacific and another below Africa and the eastern Atlantic.

This discovery was thanks to the observation that the vibrations emanating from earthquakes, known as seismic waves, travel through these regions slightly more slowly than through “normal” lower mantle.

Previous explanations of the LLVPs include that each is a deep accumulation of subducted oceanic plates (where plate tectonics has dragged the ocean floor down beneath a continent). Or that they are a place where anomalously hot lower mantle is beginning to rise as a “superplume” (huge jets of partially molten rock).

However, neither of those models can account for a peculiar enrichment in volatile elements such as helium and xenon in lava that has erupted at oceanic islands above LLVPs. Yuan argues these are “fingerprints” of Theia’s growth within the gas and dust surrounding the young Sun before it collided with Earth.