According to NASA, Mercury is considered a terrestrial planet, meaning it has a core, mantle and crust, much like Earth. Mercury's atmosphere contains oxygen, sodium, hydrogen, helium and potassium.
Mercury is the smallest planet in the solar system, and it's also the closest to the sun. This terrestrial planet is one of three in that category; Earth and Mars are also terrestrial. Its surface is rocky and cratered and is said to resemble Earth's moon. Because Mercury is so close to the sun, its orbit path is smaller than most, and it takes only 88 Earth days for this terrestrial planet to make a full trip around the sun.