> That debris went into orbit, then coalesced to form the Moon. And that raises an intriguing counter-theory: maybe that Mars-size impactor, which planetary scientists call Theia, is the object that became Mercury. If that’s correct, the answer to where Mercury’s rocky outer layer went might not be under our feet after all.

Source: http://time.com/2958731/mercury-peculiar-solar-system/

Thanks.

> Earth, Venus, Mars and asteroids such as Vesta and, perhaps, Lutetia have chondritic bulk compositions with massive silicate mantles surrounding iron cores. Anomalies include Mercury with its abundant metallic iron (about 70% by mass), the Moon with its small iron core, and metal-dominated asteroids. Although a giant impact with proto-Earth can explain the Moon’s small core, a giant impact origin for Mercury is problematic. Such a scenario requires that proto-Mercury was blasted apart with far greater specific energy than required for lunar formation, yet retained substantial volatile elements and did not reaccrete its ejected mantle. Here we present numerical hydrocode simulations showing that proto-Mercury could have been stripped of its mantle in one or more high-speed collisions with a larger target planet that survived intact.

http://www.nature.com/ngeo/journal/v7/n8/full/ngeo2189.html

So now we have evidence that lunar formation involved far greater specific energy than thought. It seems plausible that Theia's core could have survived a near head-on collision with Earth, which vaporized both mantles.