Moon rocks dating
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.In virtually all of those simulations, most of what ends up in the Moon came from the impactor rather than from Earth.But the Apollo (and Luna) lunar samples, not to mention lunar meteorites, show that the Moon and Earth have very similar compositions.
Instead, they propose that Earth endured dozens of lesser (but still potent) impacts with object ranging from 1% to 10% of its mass, each of which ejected debris into an orbiting disk.
This might not seem like much — but if proven true there'd be significant implications.
"A wet start of the Moon, coupled with the strong similarities between the composition of the Moon and the composition of the silicate Earth," Lin's team concludes, "suggests that equally high concentrations of water were present in the Earth at the time of the Moon-forming event." References: Raluca Rufu et al.
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"?
Several years ago a different research group had analyzed these same grains, and it also came up with an early formation age.