Last weekend Anton Petrov linked the wonders of Mercury's history. Mercury looks like... Chile. Namely, it has salt deserts, caused by saltwater eruptions, to the point of blowing a sodium coma like a comet.
Which suggests that, also like a comet, Mercury didn't form where it exists now. It formed with a large iron core but lost its original crust and mantle to an impact. Also pointed out is boninite, found in Cyprus and (especially) Mars. So Mercury seems like Mars' twin with a larger core.
Several commenters are asking if Mercury's old crust might be sailing the present asteroid belt. I am unaware of this. We know what a piece of broken crust from a differentiated body should look like; there's a lot of upper Vesta floating around, even landing on Earth. But I've not heard of any differentiated crustal rocks as would match Mercury. Aubrite meteors look like the rocks which helped form Mercury, but not kicked off of Mercury-having-formed.
To make all this work, do we have: Venus, Mars, Mercury, Theia, Earth; with something whacking Mercury around the same time Theia hit Earth? That's ... a Velikovsky level of reshuffling. Theia's mantle was lately boosted to 13% iron so I don't even know.
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