The planet Mercury, the closet planet to our Sun, is something of an exercise in extremes. It’s days last longer than it’s years and at any given time, it’s sun-facing side is scorching hot while its dark side is freezing cold. It is also one of the least understood planets in our solar system. While it is a terrestrial (i.e. rocky) planet like Earth, Venus and Mars, it has a significantly higher iron-to-rock ratio than the others.