Formation and evolution of the Solar System through further collisions, growing at the rate of centimetres per year over the course of the next few million years. The inner Solar System, the region of the Solar System inside 4 AU, was too warm for volatile molecules like water and methane to condense, so the planetesimals that formed there could only form from compounds with high melting points, such as metals (like iron, nickel, and aluminium) and rocky silicates. These rocky bodies would become the terrestrial planets (Mercury, Venus, Earth, and Mars). These compounds are quite rare in the Universe, comprising only 0.6% of the mass of the
Lucky Starr and the Moons of Jupiter of Eternity". Consequently, there are small inconsistencies between the various novels. In the first, Ganymede is mentioned as a settled world of the Solar System, although in later novels Ganymede has no permanent settlement, and has even served as the location of a secret Sirian base. The first man on Mars is named Wingrad in "David Starr, Space Ranger", but is named Ching in "Moons of Jupiter". The name of the state that rules the Solar System also changes from novel to novel. In "Pirates of the Asteroids", it is the Terrestrial Empire; in "Oceans of Venus", it is the