Weird structures near Earth’s core may be scars from a primordial interplanetary collision

A group of mysterious, ultradense structures just outside Earth’s core may be the remnants of an ancient interplanetary collision, new research suggests.

These strange structures are known as ultralow-velocity zones (ULVZs), because seismic waves generated by earthquakes travel about 50% more slowly through these zones than through the surrounding mantle. That means the ULVZs are also much denser than the rest of the mantle, and possibly made of heavier elements. 

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