Distinct, terrestrial geochemical reservoirs may therefore have survived Moon formation." And, indeed, researchers have identified portions of Earth's mantle that are compositional mismatches to the rest of our planet.
The piecemeal assembly envisioned by the Israeli team would have taken a long time, perhaps even 100 million years — and that opens up another aspect of the lunar-formation debate.
, maintains that the Moon came together in a hurry and had mostly solidified by 4.51 billion years ago, or 60 million years after the solar system's birth.
The evidence, say Mélanie Barboni (University of California, Los Angeles) and six colleagues, is found in eight tiny grains of the mineral zircon (Zr Si O), collected by Apollo 14's astronauts, in which they found traces of uranium, lead, and hafnium used for isotopic age-dating.
Apart from their lack of iron and extreme lack of water, Moon rocks match Earth's isotopic ratios for the geochemically diagnostic elements titanium, calcium, silicon, and (especially) oxygen and tungsten.
This really pins the dynamicists in a corner — only in rare cases, 1% or 2% of the time, do their simulations yield a Moon with an Earthlike composition.
More pointedly, the Apollo 14 zircon grains presumably crystallized from the deep lunar magma ocean (LMO) that existed right after the Moon came together.
There's also a problem of fine-tuning the impact to yield the angular momentum of the current Earth-Moon system.
I've written about possible solutions to these conundrums (or is it "conundra"?
The rings quickly coagulated into moonlets, and tidal interactions with the young, mostly molten Earth then drove each of them outward. This approach yields a lunar composition that's an amalgam of many compositions, which eases the unyielding isotopic constraints.
The most Earth-like contributions came from nearly head-on collisions that drilled deeply into our planet's mantle.