Click the image to see the story.
We don't fully understand how Mercury formed: It is mostly - 65% - iron, about double the other Inner Planets, with much of the balance made up of magnesium and iron silicates. Is its odd composition due to its proximity to the Sun, with the Sun's energy blasting away the lighter materials? Is Mercury perhaps only the core of a world destroyed in an ancient battle?
Perhaps we'll find out soon. I'm personally hoping we find evidence of slagged alien obelisks that had once been buried hundreds of miles below the surface.