The planet that is nearest to the Sun. It is the smallest planet in the solar system. Mercury is extremely near to the Sun and has no atmosphere. These factors lead to Mercury's surface having the widest temperature variation of any planet or natural satellite in our solar system. The average surface temperature is -180° C, but it can reach as high as 500° C. Earth's average surface temperature is 15° C. Venus' average surface temperature is 48° C. Mars' average surface temperature is -18° C. Mercury's extreme temperatures result from its orbit around the Sun. During the day, mercury is closest to the Sun and experiences a heat wave like Earth, but because it is so small, it gets completely evaporated by midday. At night, it is far from the Sun and cools rapidly. This cycle repeats itself over time, causing the planet's surface to change shape dramatically. As well as being hot during the day, at certain times of the year, parts of Mercury become even cooler due to darkness. These regions are called "maria". The maria on Earth that we know about are visible from space using radar or other methods. Scientists think there might be many more underground formations that we cannot see! The average distance of Mercury from the Sun is 58 million km.