Mercury has a large metallic core with a relatively thin rocky crust and mantle that has a high abundance of sulfur and potassium. Its magnetic field is off-center, which indicates that its internal structure is complex and unusual. After Earth, it is the second densest planet in the solar system.
Mercury does not have an atmosphere, but it does have gases surrounding it in what is known as its exosphere. This is produced by solar wind that strips atoms off Mercury's surface and sends the into space, where they collide with tiny meteoroids. This causes Mercury to have a tail similar to a comet.