Planets

3.1 Mercury Mercury is the closest planet to the sun. Its average distance from the sun is 36 million miles. Mercury is about 3,030 miles (4,878 km) across. That makes it the second-smallest planet in the solar system; only Pluto is smaller. In fact, Mercury is just a little bigger than Earth\'s moon. It\'s also covered with craters just like the moon. Mercury\'s year (the time it takes to orbit the sun one time) is 88 Earth days long, but one day (the time it takes to rotate from sunrise to sunrise) is 176 Earth days long. Because Mercury is so close to the sun, it has the smallest orbit of all the planets. That means it goes around the sun very quickly (88 Earth days or about 3 Earth months). Although Mercury goes around the sun quickly, it spins very slowly on its axis. For every year on Mercury, the planet spins 1 1/2 times (about 59 Earth Days or 2 Earth months). But while Mercury is slowly spinning, it is still speeding its way around the sun. Mercury has to spin another 1 1/2 times for the sun to rise over the same spot. That means one day (sunrise to sunrise) on Mercury is about 6 Earth months long, and that\'s twice as long as Mercury\'s year! So every day on Mercury means 2 years! Mercury has a very, very thin atmosphere of helium and hydrogen. On Mercury you would either freeze or roast. The highest surface temperature is 870°F, while the lowest temperature is -300°F. Scientists who study the planets have found ice at Mercury\'s poles. Like Earth\'s moon, Mercury is covered with craters. At one time, Mercury may have had volcanoes. Scientists believe that Mercury has a very thin crust, followed by a rocky layer, with a metallic core, probably made of iron, at its center. To escape Mercury\'s gravity you have to travel at 9,600 miles per hour. Because Mercury moves so fast around the sun, early Roman sky watchers named it after their speedy-messenger god. To the ancient Greeks, Mercury is identified with the god Hermes.





Mercury is covered with craters