Why Did Earth Nearly Lose Its Atmosphere During the Hadean Era 4 Billion Years Ago? — Here’s What Models Suggest

The planet Earth that we often take for granted in today’s world wasn’t always this life-supporting ball of rock. Around 4 billion years ago, the air and atmosphere were nothing like the air we breathe in now. In fact, some models and research suggest that Earth may have even lost most of its early atmosphere during an era known as the Hadean.

Today, scientists use computer models and simulations to try to find out what really happened in ancient times, back when Earth was still hostile to any lifeforms. But, what exactly are the possible reasons and explanations why, during this era of our dear planet, the atmosphere was mostly stripped and was rebuilt again? Also, how did it set the stage for the eventual emergence of life? Let’s dive deeper!

What Was the Hadean Earth Really Like?

The Hadean era of Earth lasted from 4.6 billion years ago to around 4 billion years ago. For context, life on the planet was estimated to have emerged around 3.8 to 3.7 billion years ago. During the Hadean eon, Earth was still in the process of its formation. It is believed to have happened from the smaller celestial bodies colliding with each other, creating a coalesced mass of rock.

These bodies from outer space are often known as planetesimals. They are estimated to be around a kilometer to hundreds of kilometers in diameter. They are rocky and icy blocks that were thought to have formed planets, moons, asteroids, and more. However, their collision with the early form of Earth may have caused the heating of its surface and the overall crust.

Scientists also estimate that the oceans and the atmosphere may have already formed around 4.4 billion years ago, which was surprisingly early, just a few million years after the Earth started to coalesce. So, in short, imagine the planet during the Hadean era as a ball of rock with heated surfaces, magma, and hot oceans, with a relentless rain of asteroid impacts. The Earth is young, violent, chaotic, and highly unstable. But what exactly are the possible reasons why the atmosphere may have been nearly stripped away completely?

Read more: Britannica

How Giant Impacts Could Rip Away the Early Atmosphere

The Earliest Years Of Earth's 4.6 Billion Year History

One big reason that some models point to for why Earth nearly lost its air was the giant impacts of asteroids that were raining down on the planet. During its early formation, Earth was getting hit hard, not only by asteroids and comets, but also by some planet-sized celestial bodies.

A very famous example was Theia, which was an object estimated to be about the size of Mars. It is thought to have formed our eventual moon after a direct collision with the young Earth. These types of giant impacts could easily heat up a planet, melt the rocks in its surface, and blast away gases and dust to far above the planet, causing some air to be dispersed.

Modern computer models show that these impacts could remove a big chunk of the planet’s atmosphere, especially during the early Hadean era when Earth’s atmosphere was made of hydrogen and helium. Due to these gases being lighter and the hot temperature, they could easily be lost to space, carrying the atmosphere with them during a significant impact on the surface.

How the Young Sun Tried to Blow Earth’s Atmosphere Away

Another possible reason why the atmosphere during the Hadean eon was nearly lost was the young Sun. During this time, it was a bit dimmer in terms of the visible light it produces; however, the radiation it projects is much stronger and higher in energy, such as X-rays and extreme ultraviolet rays.

Since back then, the Earth was comprised of lighter gases as its atmosphere, the radiation of the young Sun could have heated it to the point that they rose up in the sky and were dispersed into outer space, therefore causing the atmosphere to be stripped away. This phenomenon is often referred to as atmospheric escape, and the rate at which this happens is much higher for lighter gases, as they can reach the velocity needed to escape Earth more easily when heated than heavier gases.

Read more: SolarSystem

How Earth Rebuilt Its Atmosphere Again and Again

Alien Earth: The Hadean Eon and Life

So, if the atmosphere of Earth during the Hadean era was mostly stripped away, again and again by the giant impacts and the radiation of the young Sun, why do we still have it today? The answer is because the new gases kept coming from underneath the surface of the Earth and possibly from outer space, forming a new atmosphere, every time it is nearly lost.

The Hadean eon was also marked by volcanic activities. Large and numerous volcanoes erupted, and they were able to release large amounts of water vapor, carbon dioxide, nitrogen, and other gases from the Earth’s interior. Then, over millions of years, as the surface cooled, this water vapor condensed, creating rain that poured over the planet for a very long time, leading to the formation of the first oceans.

The later impacts of asteroids and comets may have also added new gases, water, and other minerals to the planet. Together with the materials being released from the Earth’s volcanic activities, the atmosphere shifted from being composed mainly of lighter gases to heavier ones like nitrogen and carbon dioxide.

By the near the end of the Hadean era, around 4 billion years ago, the planet is estimated to already have a near-solid crust, a thick ocean, and a more stable atmosphere. It was still very different than that of today, but it wasn’t as easy to lose them completely as just a few hundred million years earlier.

Author's Final Thoughts

During the Hadean era, the Earth was young, and so was the sun. The planet was also being constantly hit hard by celestial bodies, but as the mass of the Earth grew, so did its stability over time. The giant impacts, the strong radiation of the Sun, and the lightness of the early atmospheric gases were all factors that worked together, contributing to the stripping away of much of the planet’s first atmosphere, more than once.

Yet, the planet was able to recover and start anew. Volcanoes, oceans, and icy asteroid impacts slowly formed a heavier and thicker atmosphere that is not as easily removed. Over millions of years, these changes set the stage for the chemistry that led to the eventual emergence of life on Earth.

Read next: The Moon Slowly Drifts Farther From Earth Over Time — Could It Ever Break Away Completely? Here’s What Scientists Think

References & Further Reading

Tian, F., Toon, O. B., Pavlov, A. A., & De Sterck, H. (2005). A hydrogen-rich early Earth atmosphere. Science. https://doi.org/10.1126/science.1106983

Catling, D. C. (2006). Comment on “A hydrogen-rich early Earth atmosphere”. Science. https://doi.org/10.1126/science.1118412

Zahnle, K., Schaefer, L., & Fegley, B. (2010). Earth’s earliest atmospheres. Cold Spring Harbor Perspectives in Biology. https://doi.org/10.1101/cshperspect.a004895

Shaw, G. H. (2008). Earth’s atmosphere – Hadean to early Proterozoic. Chemie der Erde / Geochemistry. https://doi.org/10.1016/j.chemer.2008.05.001

de Niem, D., Kührt, E., Morbidelli, A., & Motschmann, U. (2012). Atmospheric erosion and replenishment induced by impacts upon the Earth and Mars during a heavy bombardment. Icarus. https://doi.org/10.1016/j.icarus.2012.07.032