And the only material that's common in the solar system
and in the Sun that has the requisite density is iron.
So the idea has been out there since Harold Dury
in the 1950s that Mercury has got a much higher
fraction of iron, probably mostly iron metal than any of
the other planets, including the other inner planets of which Mercury
is a member of a four-planet family.
And so theoreticians have been dealing with this question for half a century.
How do you make the inner planets so that one of them ends up much richer in iron
metal that the others? And all of the ideas pointed to some
peculiarities for the origin of Mercury that imparted
a different outcome to the final composition.
It might have been that because Mercury's the
planet, is the planet closest to the Sun,
that it formed out of material, the gas
and dust that was closest to the early sun.
And in terms of the solid materials
that were around, available to condense into planets was more metal rich.
Or it could be that and this
was another idea by some famous astrophysicists,
that Mercury formed as a planet a
little larger than its current size, maybe the size of Mars, with a composition
closer to that of Mars, and was bathed in a very hot nebula
of gas that was still around the Sun that vaporized the outer silicate shell.
Leaving a central iron core that had
been protected from that thermal environment.
And yet a third idea is that the final stages of the accretion
of the inner planets involves some
collisions of objects nearly the same size.