Earth glows like a campfire with no fresh wood on it. Heat escapes into the dark. A planet built from crashes is starting to cool.
You might expect the melted rock to harden fast. But Earth wears a blanket of steam and carbon dioxide, gases that trap heat. Models show that this blanket can keep the magma ocean, a deep sea of melted rock, hot for millions of years.
Scientists still argue about how long the cooling took. A thick blanket traps more heat than a thin one, so the answer depends on what air the young planet had.
At last, patches of crust form. Steam turns to rain that can stay on the ground. Water spreads over rock that has cooled enough to hold it.
Yet Earth never goes cold all the way through. Some heat is left from its birth. More comes from atoms that slowly break down inside rock and release warmth. The mantle, the thick rock beneath the crust, is mostly solid today. It flows slowly and melts in places. Below it, the outer core is liquid metal.
Small worlds shed heat more easily: they have more surface for the warmth they hold inside. Earth kept a hot heart beneath its new skin.
