Mercury is the closet planet to the Sun and is smaller than all the other planets apart from Pluto. It was name for the Roman god of travel because it seems to move so quickly across the sky. Mercury is a planet in an extreme environment. It orbits at the average of 40 percent of Earth- Sun distance,and the heat received from being so close to a star raises its surface temperature to as much as 700 K (800 degrees F/427 degrees C) The solar wind consisting of charged particals blown from the Sun, is constantly striking the planet, and the atomic debris crates a thin atmosphere.The surface of Mercury is covered in crates and fault lines. There is little evidence of large-scale resurfacing resulting from lava flows, suggesting a planet whose geological structures have experienced no significant movement for most of its lifetime. Mercury has a high density and the largest proportional metal core of any planet in the solar system. Metal makes of between 60 and 70 percent of its mass, with rock acconting for the rest. Mercury has very little tilt to its axis,the north pole would see the Sun continually circling on the horizon. In 1991 the north pole of Mercury revealed a bright reflection which is characteristic of water ice.  Craters could therefore contain areas of permanent shadow,maintaining a temperature below the freezing point of water. Ice from cometary impacts would be able to collect in these areas. Anywhere else on the planet the water would boil and evaporate into space.  Also Mercury is the only planet that does not have any moons.