Hadean Earth – The Violent Creation of Our World

The Hadean Aeon

4.54 to 4 Billion Years Ago

NASA

The Earth during the Late Heavy Bombardment was a truly formidable place.

In the first article of a series entitled ‘A Journey through the History of Earth’, we’ll be taking a look at the violent birth of our world, a time of fire and destruction out of which came the first building blocks of life. This week, we’ll be exploring the Hadean aeon, named so after the Greek god of the Underworld. So, without further ado, let’s step back in time 4,600- to 4,000-million years ago.

Part 1: Hadean Earth – The Violent Creation of Our World

Part 2: Archean Earth – Signs of Life

Part 3: Proterozoic Earth – The First Animals

Part 4: Cambrian Earth – An Explosion of Evolution

Part 5: Ordovician Earth – Colonising a Barren Land

Part 6: Silurian Earth – The First Breath of Air

Part 7: Devonian Earth – The Age of Fishes and Forests

Part 8: Carboniferous Earth – The Age Bugs

Part 9: Permian Earth – The Age of Amphibians

Part 10: Triassic Earth – The Rise of the Dinosaurs

Part 11: Jurassic Earth – The Land of Giants

Part 12: Cretaceous Earth – The Reign of Tyrants

Part 13: Paleogene Earth – The Rise of Mammals

Part 14: Neogene Earth – Human Ancestors

Part 15: Quaternary Earth – The Age of Man

That our planet exists is a truly remarkable feat of evolution, a journey that began 14-billion years ago when the Universe was born with the Big Bang. We now know from radiometric dating that the Earth is relatively young at only 4.54-billion years. Some 200 million years after a molecular cloud of hydrogen and helium came together to form the Sun, the primordial Earth formed from some of the heavier elements that made up the protoplanetary disk left behind.

Bound by gravity, the leftover stardust from the Sun’s creation also formed the rest of the planets in our solar system. In these incredibly far-off times, the inner solar system was a violent and crowded place during which the Earth was repeatedly pelted with rocks from outer space. The surface was young and volatile as molten rock covered a globe utterly void of life. But, chaos begets order and, as we’ll see during this journey through our world’s earliest aeon, the building blocks of life.

Highlights of the Hadean

• Birth of Earth and the other planets
• Formation of the Moon
• Meteoric bombardment
• Emergence of plate tectonics and continents
• Formation of the first and second atmospheres
• Creation of the oceans

The Lunar Cataclysm and the Birth of the Moon

We tend to think of the Moon as nothing more than a lifeless hunk of rock. However, without it, Earth would have evolved into a very different place.

The formation of the Moon is just one of many events, which occurred early in the history of our planet, that has had a profound influence on the evolution of life. Without it, there would have been no lunar tides, thus stifling the migration of life from the sea to the land billions of years later.

Radiometric dating of lunar samples have revealed that the Moon is almost as old as the Earth, only around 30 million years younger in fact. Most scientists now agree that the Moon was formed by a colossal impact event between the young Earth and a Mars-sized planet, often dubbed ‘Theia’.

The Hadean aeon is largely characterised by a period known as the Late Heavy Bombardment, during which the young Earth and moon experienced countless impact events. Since the inner solar system was so crowded during these far-off times, the surface was constantly bombarded by asteroids.

During this period, the whole planet was likely covered by an ocean of lava dozens of miles deep. The violent, scorching surface was a product of the constant stream of impacts from outer space coupled with the energy being released from the formation of the Earth’s molten core. At its peak, the temperature was probably around 1,000 °C. Unsurprisingly, the atmosphere was completely different to what we’re used to breathing today. Instead of a healthy mix of nitrogen and oxygen, the Hadean air comprised mostly of hydrogen left over by the formation and accretion of the solar nebula. This early atmosphere stabilised over the course of the aeon as constant volcanic eruptions filled the skies with other gases, such as nitrogen and carbon dioxide. Eventually, most of the lighter gasses that made up the early atmosphere escaped from Earth’s gravity, allowing heavier elements to take their place.

Land Masses Rise from the Magmatic Ocean

The early stages of the Late Heavy Bombardment ended around 4.5-billion years ago as the Sun evolved to become the main-sequence star that we see today. The solar wind helped to clear Earth’s orbit of dust and gas and the violent birth of the solar system gave way to a relatively calmer period that pervades to this day and will continue for at least the next billion years. As the dust settled and the apocalypse drew to an end, the surface started to cool enough for continents to form. Out of the fiery depths of what is now the mantle, a solid crust formed and, finally, our world had a surface that you could stand on.

It is likely that the first oceans on Earth also formed during the Hadean aeon. Nonetheless, they would have been very different to what we know today. With surface temperatures estimated to be over 200 °C, liquid oceans could only exist without vaporising due to the much higher atmospheric pressure combined with an extremely heavy greenhouse effect. Eventually, the evolution of plate tectonics hailed in a new geological aeon by trapping a large portion of these greenhouse gasses inside the Earth and allowing the surface to cool.

Our Most Distant Biological Ancestors Are Born… Perhaps

A controversial study conducted in 2015 discovered the earliest evidence of life on Earth, trapped in the form of biogenic carbon inside 4.1-billion-year-old zircons found in what is now Australia. Since no actual rocks have been discovered from the Hadean aeon, these zircons (which are minerals rather than rocks) give a rare insight into the most distant and little known period of our evolution as a planet and, ultimately, our species. In other words, the discovery introduces strong evidence that the key building blocks for life were already in existence long before the end of the Hadean. This discovery remains disputed, with the oldest undisputed evidence of life on Earth dating back some 3.5-billion years, well into the Archean aeon.

Conclusion

Naturally, this article will be by far the shortest of the series, simply because so little is known about the Hadean. After all, very few geological records exist from this time, simply because Earth didn’t have a geology throughout most of the period. In other words, the rocks that make up the Earth’s crust today didn’t form until quite some time after the formation of the planet.

In fact, the oldest minerals ever discovered are around 4.38-billion years old, while the oldest rocks, being aggregates of minerals, are not much more than 4-billion years old. Fascinatingly, however, these minerals, which are zircons, contain microscopic bubbles of gas that provide us with the first and most definitive insight into the conditions of our planet when it was young.

Part 2: Archean Earth – Signs of Life