Hadean

The Hadean is the very first and oldest part of Earth's long history. It began when our planet was just forming, about 4.6 billion years ago, and lasted until around 4 billion years ago. This time period is named after Hades, the Greek god of the underworld, because conditions on Earth were extremely hot and harsh, much like a hidden, fiery world.

During the Hadean, Earth was still cooling down from its fiery beginning. It had a thick atmosphere filled with gases like water vapor, methane, and ammonia. As the planet cooled, water in the air turned into liquid and formed a huge ocean that covered almost the entire surface. This made Earth an ocean planet for a time.

We know very little about the Hadean because rocks from this time are extremely rare. Scientists have found only a few tiny pieces of a special type of mineral called granular zircons in Western Australia. These tiny pieces give us important clues about what Earth was like back then. Some scientists think that processes like the movement of Earth's plates and the formation of the first continents may have started during the Hadean, but we are still learning more about this ancient time in our planet's story.

Etymology

The name "Hadean" comes from Hades, the Greek god of the underworld. It describes the very hot conditions on early Earth when the planet was just forming from bits of space material. At that time, the surface was thought to be covered in glowing, melted lava.

An American scientist named Preston Cloud first used the name Hadean to describe the time before the oldest known rocks on Earth were formed. Another scientist, W. B. Harland, suggested a similar name called the Priscoan period. Some older books call this time the Pre-Archean.

Rock dating

Further information: Oldest dated rocks

Before the 1980s, scientists mostly used computer models to understand the very early Earth.

Later, geologists found some rocks from the Hadean time in western Greenland, northwestern Canada, and Western Australia. In 2015, they found signs of very old carbon in rocks from Western Australia that were about 4.1 billion years old.

The oldest known tiny crystals, called zircons, were found in Western Australia and date back to about 4.4 billion years ago. These crystals are clues that tell us about the early Earth. Other older pieces of crystals found in younger rocks show that the younger rocks formed on top of much older land.

Atmosphere

Earth formed with a lot of water, but some of it might have escaped into space because the planet was smaller back then. Sunlight can break water into oxygen and hydrogen. The oxygen would mix with other things in the air, while the lighter hydrogen and helium would float away.

A big crash that created the Moon likely melted parts of Earth. This crash may have turned some of Earth's material into gas, which then cooled and formed clouds. Over millions of years, Earth developed a thick atmosphere made mostly of carbon dioxide, with some hydrogen and water vapor. This early atmosphere was very hot and heavy.

Oceans

Studies of tiny rocks called zircons suggest that liquid water might have existed on Earth between 4.0 and 4.4 billion years ago, very soon after the planet formed. Even though the surface was very hot, water could stay liquid because the air pressure was much higher—about 27 times what we have today.

The water in these early oceans probably came from deep inside the Earth, called the mantle. Big space rocks hitting Earth likely did not bring much water, because the types of elements in water from space don’t match what we find on Earth. These impacts would have sometimes heated up and disrupted the oceans, but scientists are still learning how often this happened.

Plate tectonics

A study from 2008 found minerals in very old rocks from Australia that suggest plate tectonics might have started as early as 4 billion years ago, just 600 million years after Earth formed. However, some scientists think these minerals might have come from meteorite impacts instead. We don’t have much direct evidence from this time because most of these special rocks are found in just one place in Australia.

The movement inside Earth during this time was likely very strong because of heat and because there wasn’t as much water in the deep parts of our planet yet. It’s still debated whether this strong movement caused plate tectonics or if Earth was covered by a solid layer. Oceans from this time may have played a role in starting plate tectonics.

If plate tectonics happened back then, it would have created the early land areas we call continents. Different ideas suggest different sizes of land areas during this time, with some thinking there was only about a quarter of today’s land area by the end of this period.

Continents

During the Hadean time, Earth may have had very little land above the water. This is because the amount of land depends on how much crust formed and how high the oceans were. Because Earth was very hot back then, it might have been hard for land to stay high above the water. If any land did appear in the middle of the Hadean, it might have disappeared under a big ocean by the end of that time. This small amount of land could have affected how life first began on Earth.

Possible life

Scientists have found that conditions similar to those in the Hadean could have helped create and copy RNA, a molecule important for life. This suggests that very simple life might have started to develop during this time, between 4.09 and 4.33 billion years ago.

Even though there were many space impacts during the Hadean, they happened far apart in time. Since Earth already had oceans, life could have existed, though it might have been wiped out by very large impacts. Some clues on the Moon show that some of these space rocks were even bigger than the one that later caused the extinction of dinosaurs.