NEWS: Does Uber-Ancient Earth Water Mean Life Started Earlier?
The simulation showed that during this process, the moon picks up about half of its mass "from melt condensed in the inner portions of the disc", Canup said, which resided between the Earth and the moon's first orbit. As the protomoon grew larger, it continued to collide with this material, causing its orbit around the Earth to expand.
"When the Mmoon is distant enough, it can no longer efficiently accumulate inner disk melt, which is instead scattered inward and assimilated by the Earth," Canup added. "We find that this transition occurs prior to the condensation of the depleted volatile elements, so that the final half of the Moon's mass forms from volatile-poor melt."
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Coincidentally, Canup's group's work was published around the same time an article in Science suggested that volcanic rock may be responsible for at least some of the Earth's water - as opposed to say, comets bringing water to the surface.
"We have not followed water explicitly yet in our models," she said. "But in order for the moon to retain water at depth, one would need to invoke a water-rich Earth and/or giant impactor before the impact, which I believe is consistent with this new paper."
Canup acknowledged a few limitations in the model - such as how the moon ended up with so much heavy zinc under this scenario - but said that they are planning further research to probe our neighbor's origins. You can read the published research in Nature.