The slowing of seismic waves that encounter LLSVPs is an indication that the material they’re made of is denser than the rest of the mantle, and indeed they seem to rest on the rim of the outer core, which is itself telling. The new work on the Moon’s formation is less photogenic, but it draws on a detectable feature within the mantle of the Earth, so-called Large Low Shear Velocity Provinces (LLSVPs), which have been confirmed through seismic wave detections. Neither has anything to do with the Moon but vast objects running into each other offer possibilities Hollywood was bound to seize at some point. Better known, of course, is the 1951 film of the same name, produced by George Pal. Think When Worlds Collide, the 1933 science fiction novel written by Philip Wylie and Edwin Balmer, whose cover is irresistible and thus must be reproduced here. The impact of the protoplanet called Theia would have been a fearsome thing, blasting pieces of both worlds into space that later coalesced into the Moon. Whatever the name, the event offers a model for the formation of the Moon, one that explains the latter’s small iron core and the anomalous high degree of angular momentum of the Earth-Moon system. Or we can call it the ‘Giant Impact,’ as Arizona State scientists did in a presentation at the virtual Lunar and Planetary Science Conference recently concluded. The ‘Big Whack,’ as I’ve heard it called, is the impact of a planetary embryo of perhaps Mars-size (or larger) that is thought to have struck the Earth during the latter era of planet formation.
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