Hadean

The Earth was very young and hot.

It was made of red, melted rock. A big crash even helped make the Moon.

Soon, the Earth began to cool down. Water in the air turned into rain. This made a huge ocean.

The ocean covered almost the whole planet.

Small bits of rock are hard to find. We find them in a place called Australia.

These tiny rocks are very old. They tell us about our early home.

Some people think life may have started then. It was a wild and busy time.

The Hadean was the very first part of Earth's history. It began about 4.6 billion years ago.

At first, Earth was very hot. Its surface was made of melted lava. A huge crash even helped make the Moon.

As Earth cooled, things began to change. Water vapor in the air turned into liquid water. This made a superocean that covered nearly the whole planet.

Scientists think the ocean came from gas released by the Earth's mantle.

Finding old rocks from this time is hard. Most Hadean rocks are gone. We find tiny, hard crystals called zircons in Australia.

One zircon is 4.404 billion years old. These crystals help us study the past.

Some scientists study plate tectonics. This is the way Earth's outer shell moves in large pieces. They wonder if these plates started moving in the Hadean. Some even think the very first life forms may have started then. Life might have lived in warm, rocky spots near the water.

The Hadean was the very first part of Earth's history. It is the oldest of the four geologic eons. This era began about 4.6 billion years ago.

Scientists believe Earth formed from dust in the solar system. This period ended about 4.031 billion years ago. That is when the oldest known intact rocks appeared. The name comes from Hades, the Greek god of the underworld. This name describes the hot and wild conditions of early Earth. At first, the surface was likely made of molten lava.

Things changed as the planet began to cool down.

Early Earth had a very thick atmosphere. It was filled with water vapor, methane, and ammonia. As the surface cooled, the water vapor turned into liquid. This created a superocean that covered nearly the whole planet. This ocean likely came from gas released by the Earth's mantle. Large asteroid impacts also changed the air over time. These hits helped turn the atmosphere into one with nitrogen and carbon dioxide.

Finding real rocks from this time is a very hard job.

Most Hadean rocks are missing or gone. Scientists mostly find tiny, hard crystals called zircons. Most of these come from one place called Jack Hills in Western Australia. One zircon crystal is dated to 4.404 billion years old. Other zircons in Guyana have been dated to 4.22 billion years. In 2015, researchers found traces of carbon in 4.1-billion-year-old rocks. These might be remains of very early life.

Geologists still debate how the Earth moved back then.

They study plate tectonics, which is the movement of Earth's outer shell. Some think these moving plates started during the Hadean. This movement might have helped form the first continents. One model says continents were only 25% of today's size. Another model says they grew to full size by 4.2 billion years ago. The high heat in the mantle made this a complex process. The presence of oceans may have helped these plates start moving.

Even in this wild era, life might have found a way.

Some studies suggest the last common ancestor of all life emerged then. This happened between 4.09 and 4.33 billion years ago. Tiny life forms could have lived in warm, rocky spots. These spots had water and heat to help them grow. Large asteroid impacts were a danger to any early life. These hits could boil off much of the global ocean. Still, there were long periods where life could have survived.

The Hadean is the first and oldest geologic eon in Earth's history. It began when the planet formed about 4.6 billion years ago. This era ended approximately 4.031 billion years ago. That date marks the age of the oldest known intact rock formations on Earth. The name comes from Hades, the Greek god of the underworld. This name describes the hellish conditions of the early planet. Scientists believe the surface was once a mass of molten lava.

During the early Hadean, the atmosphere was very different from today. It was a thick, hydride-rich atmosphere. This air likely resembled the solar nebula or the gas giants. It consisted mostly of water vapor, methane, and ammonia. As the Earth's surface cooled, a major transformation occurred. Vaporized water in the atmosphere condensed into liquid water. This process eventually created a superocean that covered nearly the entire planet. This turned the early Earth into an ocean planet.

Scientists use specific processes to understand how this atmosphere changed. Volcanic outgassing released gases from the Earth's interior. Additionally, asteroid bombardments struck the surface. These events eventually altered the atmosphere into a nitrogen- and carbon dioxide-rich environment. This new air is known as a weakly reducing Paleoarchean atmosphere. Some water molecules escaped into space through a process called photodissociation. Short-wave ultraviolet sunlight split water molecules into oxygen and hydrogen. The light hydrogen then escaped the atmosphere into space.

Finding physical evidence from the Hadean is extremely difficult. Hadean rocks are very rare because most were destroyed over time. Most of what we know comes from granular zircons. These are tiny, durable crystals found in one specific locality. This place is called Jack Hills in Western Australia. One zircon crystal there is dated to 4.404 billion years old. Other zircon cores in the Guiana shield have been dated to 4.22 billion years. In 2015, researchers found carbon minerals in 4.1-billion-year-old rocks. These might be the remains of early biotic life.

Geologists study many models to understand Hadean physics. There is much debate regarding plate tectonics. This is the movement of large plates of the Earth's crust. Some models suggest plate tectonics and the growth of cratons started in the Hadean. The presence of oceans might have triggered this movement. Mantle convection was likely very vigorous during this time. This was due to lower viscosity, or how easily the mantle flows. High levels of radiogenic heat kept the interior very active.

Models also predict how much land existed during this eon. The amount of continental crust was likely very small. One model predicts the crust was only 25% of its current area. Another model suggests continents grew to present-day volume between 4.2 and 4.0 billion years ago. The high heat of the mantle made it hard to support high elevations. Continents may have appeared in the mid-Hadean and then disappeared. They could have been covered by a thick ocean again.

Even in these harsh conditions, life may have begun. A 2024 study suggests the last common ancestor of all life emerged during the Hadean. This would have been between 4.09 and 4.33 billion years ago. Life might have used geothermal microenvironments to survive. These are warm, rocky spots with water and heat. Porous rock systems could have allowed for RNA replication. However, life faced constant danger from the Late Heavy Bombardment. Large asteroid impacts could boil off much of the global ocean.