Ancient Earth had a dense and toxic atmosphere like Venus, until it cooled and became livable

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

To understand what conditions were like in Earth’s early years, our research sought 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 produce. Working with colleagues in France and the United States, we found that Earth’s first atmosphere was likely a thick and inhospitable soup of carbon dioxide and nitrogen, just like the one we see today on Venus.

How the Earth got its first atmosphere

A rocky planet like Earth is born through a process called “accretion”, in which small particles initially group under the pull of gravity to form larger and larger bodies. The smaller bodies, called “planetesimals”, look like asteroids, and the next dimensions 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 full-fledged planet like Earth or Venus.

The latter stages of accretion involve gigantic impacts that release enormous amounts of energy. We believe the latest impact in Earth’s accretion involved a Mars-sized embryo that hit the growing Earth, rotated our Moon, and melted most or all of what was left.

The impact would have left Earth covered with a global sea of ​​molten rock called the “magma ocean”. The magma ocean would have lost 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 be, and how it would change when the magma ocean cooled. The crucial thing to understand is what was happening with the oxygen element, because it controls how the other elements combine.

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

So we had to work out the oxygen chemistry in the magma ocean. The key was to determine how much oxygen was 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 can vary between these extremes.

When the magma ocean eventually cooled, it became Earth’s mantle (the layer of rock beneath the planet’s crust). So we hypothesized that the oxygen-iron bond ratios in the magmatic ocean would be the same as those found 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 work out how to put together a matching mix of chemicals in the laboratory.

In the laboratory

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

But what would happen when the magma ocean cooled? It appears that the 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 today’s atmospheric pressure. Talk about a greenhouse effect! But the Sun was less than three-quarters brighter then as it is now.

How the Earth avoided the fate of Venus

This ratio of CO₂ to N₂ is strikingly similar to the current atmosphere on Venus. So why has Venus, but not Earth, kept the hellishly hot and toxic environment we observe today?

The answer is that Venus was too close to the sun. It simply never cooled enough to form oceans of water. Instead, the H₂O in the atmosphere remained in the form of water vapor and was slowly but surely dispersed into space.

On the early Earth, however, oceans of water slowly but steadily absorbed CO₂ from the atmosphere by reaction with rock, a reaction known to science for the past 70 years as the “Urey reaction”, after the Nobel Prize who discovered it, and reducing atmospheric pressure to what we observe today.

So, although both planets started almost identically, it is their different distances from the Sun that put them on divergent paths. The Earth has become more conducive to life as Venus has become increasingly inhospitable.

More information:
Paolo A. Sossi et al. Redox state of the Earth’s magma ocean and its primordial atmosphere similar to Venus, Advances in science (2020). DOI: 10.1126 / sciadv.abd1387

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