Ancient Earth had a thick toxic atmosphere like Venus – until it cooled off and became livable



Earth is the only planet we know contains life. Is our planet special? Scientists have researched over the years what factors are essential for or beneficial for life. The answers help us identify other potentially inhabited planets elsewhere in the galaxy.

To understand what the conditions were like in the first years of Earth, our scientists tried to restore the chemical balance of the boiling magma ocean that covered the planet billions of years ago, and conducted experiments to see what kind of atmosphere it was. He would have formed. We were working with colleagues in France and the United States, and the Earth’s first atmosphere was probably a thick, inhospitable soup of carbon dioxide and nitrogen, just like what we see on Venus today.



Read more: If there is life on Venus, how could it have got it? The beginning of life experts explain


As the earth got the first atmosphere

A rocky planet like Earth is born by a process called “accretion”, in which initially small particles clamp together under the pull of gravity to form larger and larger bodies. The smaller bodies, called “planetesimals”, look like asteroids, and the next size up are “planetary embryos”. There may have been many planetary embryos in the early solar system, but the only one that still survives is Mars, which is not a fully fledged planet like Earth or Venus.

The late stages of accretion involve giant impacts that release huge amounts of energy. We think that the recent impact in the Earth’s accretion involved a Mars-sized embryo hitting the growing earth, spinning off our moon and melting most or all that was left.

The impact would have left land covered in a global sea of ​​molten rock called a “magma ocean”. The Magma Ocean would have leaked hydrogen, carbon, oxygen and nitrogen gases to form the Earth’s first atmosphere.

What the first atmosphere was like

We wanted to know exactly what kind of atmosphere this would be, and how it would have changed like it, and the magma ocean below, cooled down. The crucial thing to understand is what was happening with the element oxygen because it controls how the other elements combine.

If there was little oxygen around, the atmosphere would be rich in hydrogen (H₂), ammonia (NH₃) and carbon monoxide (CO) gases. With abundant oxygen, it would be made from a much friendlier mix of gases: carbon dioxide (CO₂), water vapor (H₂O) and molecular nitrogen (N₂).



Read more: The rise and fall of oxygen


So we have to work the chemistry of oxygen in the magma ocean. The key was to determine how much oxygen was chemically bonded to the element iron. If there is a lot of oxygen, it is connected to iron in a 3: 2 ratio, but if there is less oxygen, we see a ratio of 1: 1. The actual ratio may vary between the extremes.

When the magma ocean eventually cooled down, it became the mantle of the earth (the layer of rock under the crust of the planet). We have therefore made the assumption that the oxygen-iron bonding ratios in the Magma Ocean would be the same as they are in the mantle today.

We have many samples of the mantle, some brought to the surface by volcanic eruption and others by tectonic processes. From these, we can find out how to put a matching mix of chemicals in the laboratory.

In the laboratory

In these experiments we levitated a miniature magma ocean on a stream of gases, kept molten by the heat of a powerful laser. Allow us to calibrate the chemical reaction between iron and oxygen in the magma and connect this to the composition of the atmosphere.
IPGP, Author provided

We have determined that the atmosphere is composed of CO₂ and H₂O. Nitrogen would be in its elemental form (N₂) rather than the toxic gas ammonia (NH₃).

But what would have happened when the Magma Ocean cooled down? It seems that the early Earth was cooled enough for the water vapor to condense from the atmosphere and form oceans of liquid water as we see today. This would have left an atmosphere with 97% CO₂ and 3% N₂, at a total pressure approximately 70 times the current atmospheric pressure. Talk about a greenhouse effect! But the sun was less than 3/4 as bright as it is now.

How Earth avoids the fate of Venus

An ultraviolet view shows cloud bands in the atmosphere of Venus.
ISAS / JAXA, CC BY

The ratio of CO₂ to N₂ is strikingly similar to the present atmosphere on Venus. So why did Venus, but not Earth, retain the hellishly hot and toxic environment we observe today?

The answer is that Venus was too close to the sun. It simply never cooled down enough to form water oceans. Instead, the H₂O in the atmosphere stayed as water vapor and was slowly but inexorably lost to space.

On Earth early, the water oceans instead slowly but steadily descend CO ₂ from the atmosphere by reaction with rock – a reaction known to science for the last 70 years as the “Uri reaction”, after the Nobel Preacher who discovered it – and Reducing atmospheric pressure to what we observe today.

So, although both planets started almost identically, there are very different distances from the sun that place them on divergent paths. Earth became more conducive to life while Venus was increasingly inhospitable.



Read more: Ancient minerals on earth can help explain the early solar system



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