The surface was an ocean of molten lava, perpetually stirred up by the impacts of asteroids, comets, and even other planets. One of these was Theia, a planet about the same size as today’s Mars.1 Theia struck the Earth a glancing blow and disintegrated. The collision blasted much of the Earth’s surface into space. For a few million years, our planet had rings, like Saturn. Eventually the rings coalesced to create another new world—the Moon.2 All this happened approximately 4,600,000,000 (4.6 billion) years ago.