Mercury is the smallest planet in our solar system, and also the closest to the sun. It is named after the Roman messenger god, known for his speed and agility. Mercury has a very thin atmosphere, which is primarily made up of oxygen, sodium, and hydrogen. The surface of Mercury is covered in craters, mountains, and large expanses of smooth plains. Due to its proximity to the sun, temperatures on Mercury can vary greatly, ranging from extremely hot during the day to freezing cold at night.