Mercury has a radius of 2,439.7 kilometers, making it smaller than some moons and the smallest planet in the solar system. Its mass is equal to 0.055 of Earth's.
One of Mercury's most defining features is its atmosphere, which is made up of atoms blasted from the surface by solar wind. Due to Mercury's extreme heat, these atoms quickly travel out into space, which means the planet's atmosphere is constantly being replenished. Because of this thin atmosphere, temperatures on Mercury range from negative 280 degrees Fahrenheit to 800 degrees Fahrenheit. No other planet in the solar system has such extreme swings in temperature.