The main difference is between the four outer gas giant planets and the four inner terrestrial planets. The four gas giants; Jupiter, Saturn, Uranus, and Neptune; are composed of hydrogen and helium. It is believed that they have solid iron-based cores. The terrestrial planets; Mercury, Venus, Earth, and Mars; are smaller and are closer to the sun. They have cores, mantles, and crusts; with a relative paucity of hydrogen and helium. The Earth differs from the other terrestrial planets in that it contains abundant ice and water, along with a nitrogen and oxygen atmosphere. Venus has a much thicker atmosphere of sulfur dioxide. Mercury has no atmosphere while Mars has a very thin atmosphere.
