Mercury - Size, Mass, and Density - Mercury is the smallest planet in both mass and diameter. It is about 18 times less massive than Earth. With a diameter of about 3,032 miles (4,879 kilometers), Mercury is not quite two fifths the size of Earth. It is only about a third larger than Earth’s Moon. In fact, two moons in the solar system—Jupiter’s moon Ganymede and Saturn’s moon Titan—are larger than Mercury. However, Mercury is larger than the dwarf planet Pluto. For…