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Earth has been slowly cooling since it formed 4.5 billion years ago, and sometime between 1.5 and 0.5 billion years ago, the core began to crystallise into a solid ball of mainly iron and nickel. The core is growing by around one millimetre per year, and at that rate, Earth won’t have time to fully cool and solidify before the Sun reaches the end of its life in around five billion years, when it’ll expand and potentially engulf the planet we live on. If some currently unknown mechanism caused Earth to cool much sooner, it would have serious long-term consequences for most life on the planet. Without the electric dynamo of the molten outer core, Earth’s magnetic field would fade to zero, and the stream of charged particles from the Sun, known as solar wind, would begin stripping away the atmosphere, as may have happened to Mars long ago.
For Earth to have already cooled by now, it would have to be much smaller. Gravitational compression and friction will heat any planet-sized body, and radioactive decay from elements in the mantle will add to this heat. The Moon is thought to have been formed from the impact of a Mars-sized body called Theia with the early Earth, around 4.5 billion years ago. The energy of the collision would have meant that Earth and the Moon would have both started out mostly molten, but the Moon cooled much more rapidly because it’s smaller. A smaller, cold Earth would have lacked the volcanoes and plate tectonics that have recycled carbon and minerals in the crust and added gases to the atmosphere. Without a thick atmosphere, the surface temperature would drop low enough to freeze the ocean. It’s doubtful that life – at least, complex life – would have evolved in these conditions.
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