The inner, or terrestrial, planets—Mercury, Venus, Earth, and Mars—share several key characteristics:  small size and average dimensions; solid, rocky surfaces with high density (3.3 to 5.5); internal structure comprising a silicate mantle and iron core; and few satellites (0, 0, 1, and 2 respectively).  These planets are composed primarily of heavier elements, lacking the abundance of hydrogen and helium found elsewhere.