Ancient Earth had a thick, toxic atmosphere like Venus – until it cooled down and became liveable

To understand what conditions looked like in Earth’s early years, our research attempted to recreate 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 would have produced. Working with colleagues in France and the United States, we discovered that Earth’s first atmosphere was probably a thick, inhospitable soup of carbon dioxide and nitrogen, much like what we see on Venus today. hui.

How the Earth got its first atmosphere

A rocky planet like Earth is born out of a process called “accretion,” in which small particles begin to clump together under gravity to form larger and larger bodies. The smaller bodies, called “planetesimals”, look like asteroids, and the next size is “planetary embryos”. There may have been many planetary embryos at the start of the solar system, but the only one that still survives is Mars, which is not a full-fledged planet like Earth or Venus.

The later stages of accretion involve giant impacts that release enormous amounts of energy. We believe the last impact of Earth’s accretion involved a Mars-sized embryo hitting the growing Earth, spinning our Moon, and melting most or all of what was left.

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

What was the first atmosphere like

We wanted to know exactly what kind of atmosphere it would have been, and how it would have changed as it, and the ocean of magma below, cooled. The crucial thing to understand is what is going on with the oxygen element because it controls how the other elements combine.

If there was little oxygen around, the atmosphere would have been rich in gases of hydrogen (H₂), ammonia (NH₃) and carbon monoxide (CO). With abundant oxygen, it would have been made up of a much more user-friendly mixture of gases: carbon dioxide (CO₂), water vapor (H₂O) and molecular nitrogen (N₂).

So we had to work on the chemistry of oxygen in the magma ocean. The key was to determine the amount of oxygen chemically bound to the element iron. If there is a lot of oxygen, it binds to iron in a 3: 2 ratio, but if there is less oxygen, we see a 1: 1 ratio. The actual ratio may vary between these. extremes.

When the ocean of magma finally cooled, it became the Earth’s mantle (the layer of rock under the planet’s crust). We therefore hypothesized that the oxygen-iron bond ratios in the magma ocean would have been the same as in the mantle today.

We have many samples of the mantle, some brought to the surface by volcanic eruptions and others by tectonic processes. From these, we could figure out how to set up a corresponding mixture of chemicals in the lab.

In the lab

We determined that this atmosphere was composed of CO₂ and H₂O. The nitrogen would have been in its elemental form (N₂) rather than the toxic ammonia gas (NH₃).

But what would have happened when the ocean of magma cooled? It appears that early Earth cooled enough for water vapor to condense out of the atmosphere, forming oceans of liquid water as we see today. This would have left an atmosphere with 97% CO₂ and 3% N₂, at a total pressure of about 70 times the current atmospheric pressure. Talk about a greenhouse effect! But the sun was less than three-quarters brighter then it is now.

How the Earth avoided the fate of Venus

This ratio of CO₂ to N₂ is surprisingly similar to the current atmosphere on Venus. So why did Venus, but not Earth, keep the hellish hot and toxic environment we see today?

The answer is that Venus was too close to the Sun. It just never cooled enough to form oceans of water. Instead, the H₂O in the atmosphere remained as water vapor and slowly but inexorably lost its way into space.

On early Earth, water from the oceans rather slowly but steadily took CO₂ from the atmosphere by reaction with rock – a reaction known to science for 70 years as the “Urey reaction”. after the Nobel laureate who discovered it – and reducing atmospheric pressure to what we see today.

So although the two planets started out almost identically, it was their differing distances from the Sun that put them on divergent paths. The Earth has become more conducive to life while Venus has become more and more inhospitable.

More information:
Paolo A. Sossi et al. Redox state of the terrestrial magma ocean and its primitive atmosphere similar to that of Venus, Scientific advances (2020). DOI: 10.1126 / sciadv.abd1387

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