Mercury is the smallest planet of the Solar System. Mercury's distance from the Sun is 58.117 million kilometers. It has a diameter of 4879.4 kilometers, making the radius 2439.7 kilometers. Mercury's inner composition consists of a metallic core, an intermediate rocky layer and a brittle crust. Mercury has a very thin, almost non-existant atmosphere, consisting of very little bits of helium and oxygen and even smaller bits of calcium, magnesium, etc. Mercury has very long days. One day on Mercury is equal to 58 Days, 15 hours and 30 minutes on Earth. Since Mercury is close to the Sun, it has a smaller orbit compared to the earth. It takes 88 days for Mercury to make one complete revolution around the sun. One Year, therefore, on Mercury will be of 88 days. That means, by the time it finishes one whole year, it will only have finished almost one and a half days.