The outer regions of the young solar system, like most of the universe, consisted mainly of the primordial elements, hydrogen and helium. But the inner regions, where the rocky planets—Mercury, Venus, Earth, and Mars—would form, lost so much hydrogen and helium that they had a rare chemical diversity. Oxygen, silicon, aluminum, and iron make up over 80 percent of Earth’s crust, with elements such as calcium, carbon, and phosphorus playing lesser roles. On Earth, hydrogen plays only a medium-size role, and helium is hardly ever sighted.
