Image in header: An artist’s impression of the Earth’s surface with atmosphere. Here, the Earth must be at least 1 billion years old, as stromatolites are already present in the shallow waters. Source : http://www.regents-earthscience.com/the-atmosphere.html
The Earth and all other planets originated in the giant cloud of dust and gas from which the Solar System was born, after this cloud had already flattened into a protoplanetary disk, and after most of the matter had already been swallowed by the young Sun. This cloud varied in composition according to its distance from the star, but everywhere the ambient gas was dominated by hydrogen, and to a lesser extent helium. Thus, when the planet clumped together, a “primary” thick atmosphere had immediately stuck around with mostly hydrogen. But hydrogen is very light, and therefore doesn’t need that much speed to escape Earth’s gravity. Extra speed is easy to get as an air molecule on a hot protoplanet, and if you then get a bit higher in the atmosphere, you are also helped by high-energy particles coming in from the solar wind. So most of the hydrogen and helium was lost in space, or the hydrogen reacted with other molecules to form, say, water or ammonium.
At the same time, the newly formed Earth started emitting huge amounts of gases. The planet was still glowing on the outside at first, and even after cooling began to form a solid crust, there was still insane volcanism. This ‘outgassing’ must have been very intense for hundreds of millions of years (today it is still going on in a much milder form, as the planet is still volcanically active). The gases emitted included mostly:
- WaterH2O
- Carbon dioxideCO2
- Carbon monoxide CO
- Methane CH4
- Ammonium NH3
These were the dominant emitted gases (with water and carbon dioxide in the lead), but of course there was a diverse group of other gases in smaller quantities, such as: SO2, CO,H2S, H2SO4, HCl.
Huge amounts of water vapour began to condense massively in the atmosphere of the cooling Earth. Presumably, this is why our planet was covered with liquid water after about the first 170 million years. Imagine pouring rain for tens of millions of years without stopping.
Methane and carbon monoxide also easily oxidised toCO2 on the young Earth, and the atmosphere must have looked well like that of Venus at first: completely full ofCO2 and with an atmospheric pressure around 90 bars. This is confirmed by several scientific models. Today,CO2 levels are around 420 ppm. Where did all thatCO2 go? Well, it binds with minerals in the Earth’s crust to form carbonate rocks. Via active plate tectonics, that carbon is eventually released back into the mantle. Both processes – emission via volcanism and re-absorption into the mantle via plate tectonics – continue today, and were discussed earlier in this course in the sections on plate tectonics.
In the early atmosphere, there was almost no oxygen, and thus no ozone in the upper atmosphere (stratosphere) as there is today. Therefore, much more high-energy UV radiation fell in, as is still the case today on Mars and Venus. This radiation split molecules such as the water (H2O) and ammonium (NH3) of the young atmosphere, a reaction called photodissociation. The result is hydrogen gas (H2), oxygen gas (O2) and nitrogen gas (N2). Hydrogen gas is very light, and can escape from Earth with limited energy. It was therefore largely lost in the empty space outside the atmosphere. Oxygen gas reacted quickly with other molecules (e.g. methane to formCO2 ) and with rocks. So that too gradually disappeared. What always remained was the nitrogen gas, a gas that actually reacts spontaneously with nothing. This explains why today we have an atmosphere dominated by this remaining gas. Later, though, the so-called denitrifying bacteria would emerge, which from then on would convert a small part of the nitrogen gas into nitrate in water and soil, a very important nutrient for living things.
So what about that high atmospheric pressure of the early atmosphere? So it is estimated to be around 90 bar (today we have 1 bar as air pressure), as it still is today on Venus. Where did all that air go?
The molecule that dominated the air wasCO2. However, this gas started disappearing from the air when there were oceans. It is estimated that about half of theCO2 dissolved in seawater. Consequently, the young oceans were very rich in carbon dioxide, and thus quite acidic. We will see later that this could have been important to make emergence of life more possible. The remainingCO2 slowly but surely began to react with metals from the Earth’s crust, forming carbonate rocks. These carbonates can still be found on continental crust in rock layers of all times. By studying the different layers, it was possible to start estimating how muchCO2 was removed from the air during Earth’s history via carbonate formation (and then brought into the mantle by plate tectonics). The graph below shows the trend of atmospheric pressure over the entire history of our planet. Note that at the time of the giant flying dinosaurs (about 200-65 million years ago), the pressure was around 5 bar. Today’s air pressure would not support these giants in the sky.
When Earth was newly created, then our sun radiated about 30% less energy than today. With the same atmosphere today, that would have been a problem. But because there was so much water vapour, methane, and especiallyCO2, the planet benefited from a much stronger greenhouse effect. Conversely, should we have as muchCO2 in the air today as we did then, the climate would have become unlivably warm. The increase in solar energy and the decrease inCO2 in the atmosphere both occurred very gradually over the billions of years. Once again, the Earth exhibits the ‘ideal’ core characteristics for staying within livable limits with the climate all this time: just enough seawater on the surface, and permanent recycling via plate tectonics at the most appropriate rate.