Why is Mercury’s core so big?

If you study Figure 4.56, once thing that might jump out at you is that Mercury’s core seems unusually large for its size. You might therefore wonder why Mercury has such a large core, and if so, that puts you in good company: Scientists also wonder why this is the case.

One possible explanation for Mercury’s surprisingly large core is that when it was very young, Mercury suffered a “giant impact” much like the one thought to have made our Moon (see Figure 4.23). If such an impact blasted away most of Mercury’s mantle and crust (and didn’t then create a moon), then the planet would have been left with a large core. This explanation seems reasonable, since we expect that lots of large impacts should have occurred in the early solar system, but scientists are still considering a number of other possible explanations as well.