Britannica Educational Publishing 
Inner Planets 

Despite their proximity to Earth, much remains to be learned about the origins and conditions of Mercury, Venus, and Mars. The three smallest planets in our solar system and the three closest to the Sun-Mercury, Venus, and Mars-differ greatly from each other and from the place we call home. This far-reaching volume travels to each of the inner planets in turn and details the physical characteristics as well as the exploration efforts made in order to learn more about our closest planetary neighbors.