a. Origin of Earth and Primitive Atmosphere

The universe itself is thought to have originated through the Big Bang described by Georges Lemaitre (1931) — a single, immense, titanic explosion after which the universe expanded, cooled, and its scattered matter condensed into galaxies of solid objects. Our own Milky Way is one such galaxy, and the Sun's solar system is a small part of it; within that system, Earth formed roughly 4.6 billion years ago, beginning life as a rotating cloud of hot gases and cosmic dust called a nebula. As this nebula condensed and cooled, its material separated out by density: heavier elements such as nickel and iron sank inward to form the planet's core, while lighter elements — helium, hydrogen, nitrogen, oxygen and carbon — remained near the surface and went on to form Earth's atmosphere. Cruciall …