~2–3 Billion

Earth’s Core Solidifies

The interior of the Earth is hot because of Earth’s original heat of formation, as well as the heat released from the decay of radioactive elements over time. The deep Earth is hot enough, in fact, for the outer core of iron-nickel metal to melt (temperatures above 4,000°C to 6,000°C, or 7,200°F to 10,800°F), although the inner core remains solid. Earth’s molten, spinning, electrically conducting outer iron core creates a strong magnetic dynamo with a field that extends deep into space around our planet, helping to protect the surface and atmosphere from harmful solar and cosmic radiation. Without the molten outer core and the protective magnetic field it creates, life on Earth might never have been able to form. And even if it did, it would be difficult or impossible for it to survive on the radiation-bombarded surface.

The Earth’s interior is slowly cooling, however, partly because the abundance of radioactive elements is decreasing with time, and partly because heat is transferred from the core to the mantle to the crust and then to space, via mantle convection, volcanic eruptions, and thermal radiation. As the interior cools, the liquid outer core is slowly freezing (solidifying), enlarging the diameter of the solid inner core by about 0.04 inches (1 millimeter) per year. Inexorably, over time as the planet continues to cool, Earth’s liquid outer core will completely solidify. By most geophysical estimates, this will happen between about 2 and 3 billion years from now.

Once the core solidifies, the magnetic dynamo engine deep inside the Earth will stop working, and the Earth’s magnetic field and magnetosphere will dissipate relatively quickly. Decreasing heat flow will slow mantle convection and could potentially cease plate tectonics on the surface. Without the protection from the magnetic field, the solar wind (the stream of high-energy radiation coming from the Sun) will begin to directly impact and erode the Earth’s upper atmosphere, breaking apart CO₂, O₂, H₂O, and other molecules and driving light elements like hydrogen and other volatiles off into space. Loss of the magnetic field is the primary explanation for how early Mars lost its once-thicker and warmer atmosphere; a similar fate likely awaits our own world in the distant future.

Artist’s rendering of a solar storm hitting Mars and stripping ions from the planet’s upper atmosphere. Does the same fate await the Earth billions of years in the future, when our planet’s magnetic field shuts down?