Hadean

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.