Because of its proximity to the Sun, Mercury's evolution took a slightly different course than that of the other planets. As theSun formed, it pushed much of the lighter gas and dust out of the inner Solar System, leaving behind only heavier elements. As a result, Mercury is made out of alarge percentage of heavier elements, mainly iron. It is essentially a large metal ball of iron with a very thin silicate crust. Mercury's core makes up about 75%of the planet. Its thin mantle, or crust, is only about 300 to 400 miles thick (500 to 600 km). Next to Earth, Mercury is the second most dense planet in our SolarSystem.
The surface of the planet Mercury is covered with craters. These craters have been created by ions of accidental encounters with asteroids and comets.All celestial bodies within the Solar System are subject to these bombardments. However, many of the planets have the ability to heal themselves through naturalgeological processes.
The planet Mercury is too small and has too little gravity to hold onto an atmosphere. Any gases released from the planet quickly escapeinto space. Also, Mercury is so close to the Sun that any atmosphere is quickly blown away by the Sun's solar winds. That means that there is almost no air on Mercury.