The universe began 13.7 billion years ago. Scientists believe the universe began with the Big Bang. The Big Bang released particles that quickly combined to form protons, neutrons, and electrons. The protons, neutrons, and electrons then combined to form the first elements. The first element produced was hydrogen. It is the most simple element, just one proton and one electron. Hydrogen is still the most abundant element in the universe today. Helium and lithium formed after hydrogen.

   Hydrogen and helium came together to form gas clouds. These gas clouds condensed until they eventually became the first stars. The first stars formed about 200 million years after the Big Bang.

   As the stars burn, lighter elements such as helium and hydrogen combine. This creates heavier elements such as iron and nickel. Eventually, these first stars exploded. All of the elements that had been formed inside the stars were scattered into space as gas and dust.

   Around 4.6 billion years ago, nearly 10 billion years after the Big Bang, our solar system began to form from the gas and dust released by these exploding stars.

   Scientists have labeled the time of the formation of the Earth the Hadean Eon. It began as the dust and gas of the early solar system came together. As the planet was forming, gravity attracted more material to it. Tiny planets, known as planetesimals, collided with the early planet. These collisions made the Earth even bigger.

   All of the collisions also made the planet so hot that solid rocks did not exist on the surface of the Earth. Convection currents within the mantle brought magma up to the surface of the Earth. Solid rocks sunk into the magma. The world looked like the underworld, so scientists named this eon after Hades, Greek god of the underworld.

   Heavier elements, such as iron, sunk to form the core of the planet. Lighter elements, such as silicon, became a part of the crust. Scientists believe that the first rocks were formed around 4.4 billion years ago, but none of these rocks are left today. Rocks are constantly recycled on Earth when they are subducted into the mantle. The high heat of the mantle melts the rocks, so they become magma.

    Even though they haven’t found rocks from 4.4 billion years ago, scientists have found zircons from that time. Zircons are crystallized minerals. They are usually tiny, and they can remain unchanged in all three types of rock. The age of zircons can be calculated by measuring the amounts of different elements found within them. Zircons form within igneous and metamorphic rock, so scientists know that the crust of the Earth was solid rock 4.4 billion years ago.

   The atmosphere of the Earth during the Hadean Eon was very different than our atmosphere today. At first, it was composed of mostly hydrogen and helium. As the crust cooled ammonia, methane, and neon were released into the air. Then, volcanoes began erupting. These eruptions put water vapor, carbon dioxide, nitrogen, and more helium into the air. Comets filled with ice brought more water vapor to the Earth when they crashed into the planet.

   The water vapor in the atmosphere condensed into clouds, and rain began to fall. All of the water in our oceans, lakes, and rivers fell from the sky during the Hadean Eon.

   The moon also formed early on within the Hadean Eon. Scientists believe that a rock the size of Mars collided with the Earth. So much dust and debris were kicked into space, that it came together to form our moon.

    By the end of the Hadean Eon, the Earth had gone from a version of the underworld to a planet that we would recognize today. Earth had a moon, solid rock, and oceans. The poles may even have had some ice. Proteins had begun to form, but life did not yet exist on Earth. Life would come in the next eon, the Archean Eon.