Henry Gee

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.

Less dense materials such as aluminum, silicon, and oxygen combine into a light froth of rocks near the surface. Denser materials such as nickel and iron sink to the core. Today, the Earth’s core is a rotating ball of liquid metal. The core is kept hot by gravity and the decay of heavy radioactive elements such as uranium, forged in the final moments of the ancient supernova. Because the Earth spins, a magnetic field is generated in the core. The tendrils of this magnetic field reach right through the Earth and stretch far out into space. The magnetic field shields the Earth from the solar wind, a constant storm of energetic particles streaming from the Sun. These particles are electrically charged and, repelled by the Earth’s magnetic field, bounce off or flow around the Earth and into space.