How did the Moon get here? Charles Darwin’s grandson, George, theorised that the Moon was spun off from the Earth, leaving a gaping hole we now know as the Pacific. Others theorised the Moon was captured from an independent orbit, or that the Earth and Moon somehow formed together.
Then in 1975, after Apollo and the first round of science results from studying Moon rocks, William Hartmann and colleagues proposed that the Moon was brutally bashed off the Earth by another planet. Since then this theory has come to dominate the debate, and has been refined, with the impactor being dubbed “Theia”. But there’s a problem. Theia had to impact the proto-Earth (call her “Gaia”) with almost no energy excess – in other words it didn’t ram the Earth, but merely fell into its gravity well.
Another puzzle is just how the various isotopic balances of the two, Earth and Moon, became so alike after such violent smashing together – according to the simulations the Moon accreted out of mantle material from Theia, not from Gaia. Yet the two are near identical. A sore puzzle indeed, implying they formed together at the same radial distance from the Sun.
Finally, for the two to form in the same region of space and for Theia to collide at such low energy, then she must have formed as a co-orbital of the other. In Gaia’s L4 or L5 point what become Theia formed, then the orbital arrangement destabilised when Theia’s mass exceeded about 1/24th of Gaia… but it had to accumulate something more like 1/10th of Gaia’s mass before the two collided. How did it survive an unstable orbital arrangement for so long?
How to reconcile the two? According to the work of Rob de Meijer, an ex-nuclear engineer now a nuclear geophysicist, and Wim van Westrenen, experimental geochemist, the solution comes from within Gaia. According to isotopic balances of various rare earth metals, the Core Mantle Boundary formed within 30 million years of Gaia forming. At this point in time natural radioactives – uranium 235 & 238, thorium 232, and plutonium 244, were present at much higher levels than the present day. A massive fast-breeder style runaway reaction occurred which vaporised a vast bubble of mantle material. This rose rapidly, as per Archimedes principle, and flung immense amounts of mantle rock and vapour into space, out to 100,000 kilometres. The debris mostly went into orbit and formed a heavy ring around the Earth which congealed into the Moon, with perhaps an extra Mini-Moon or two that eventually fell back. The theory is detailed in a Cosmos Magazine article from August 2008, but which has just gone online for all to discuss.
According to the study by de Meijer and van Westrenen the proto-Earth object had a spin of 2.3 hours, but once the Moon was expelled a large fraction of the angular momentum was transferred to it by tidal forces. This would have occurred pretty rapidly in the early stages as a semi-molten Earth would’ve allow a massive whole-body tidal response, rather than the much lower oceanic response that has dominated tidal dissipation to the present day. Once the Moon had moved out to ~200,000 km the response would’ve slowed and followed the more sedate tidal dissipation regime recorded in tidal rythmites through-out the Archean, Proterozoic and Phanerozoic geological record.
Speculations:
Only time will tell.
ERRATUM
Contrary to my opinion Venus isn’t enriched in Uranium & Thorium – data from Venera 8 showed enrichment, but this was anomalous compared to all the other landers that have since followed, Veneras and Vegas. Apparently Magellan’s SAR imaging data shows us that Venera 8 landed on a volcanic outflow rather different to all the other lander sites, so the observed enrichment is peculiar to that location, not Venus.