Origin of the Atmosphere, Hydrosphere, and Oxygen
If you could travel back 4 billion years, you could not breathe the air, and you would not find a single drop of rain that had fallen on a living thing. So where did the air we breathe and the water in the oceans come from? Let's follow the evidence.
A second atmosphere from volcanoes
Earth's very first atmosphere, mostly hydrogen and helium left over from the solar nebula, was thin and light. Gases that light escaped into space, partly because the young Sun blasted them away and Earth's gravity could not hold them.
The air that replaced it came from the inside of the planet. Outgassing is the release of gases trapped in molten rock, which escape when magma erupts or cools. Early Earth was far more volcanically active than today, so outgassing was intense.
The gases were roughly these:
| Gas | Formula | Role |
|---|---|---|
| Water vapor | H₂O | Most abundant volcanic gas; later condensed into oceans |
| Carbon dioxide | CO₂ | Dominant carbon gas; strong greenhouse gas |
| Nitrogen | N₂ | Chemically stable, so it built up over time |
| Sulfur gases | SO₂, H₂S | Small amounts; volcanic smell |
Notice what is missing: free oxygen (O₂). Oxygen is highly reactive. Any that was released quickly combined with iron and other elements in rocks and seawater. So the early atmosphere was mostly water vapor, CO₂, and N₂.
Earth's first atmosphere was replaced by a second one produced mainly by volcanic outgassing. Define outgassing* in your own words, and name two gases that volcanoes…
Earth's first atmosphere was replaced by a second one produced mainly by volcanic outgassing. Define outgassing in your own words, and name two gases that volcanoes release in large amounts.
Mateo's notes say: "Early Earth's atmosphere was mostly oxygen and nitrogen, like today's." Explain why this statement is incorrect. In your answer, name the gas that…
Mateo's notes say: "Early Earth's atmosphere was mostly oxygen and nitrogen, like today's." Explain why this statement is incorrect. In your answer, name the gas that dominated the atmosphere's oxygen-free composition besides water vapor and nitrogen, and say where it came from.
Building the oceans
Where did the water come from? Scientists point to two sources, and the best evidence supports a mix.
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Volcanic outgassing. Magma released huge amounts of water vapor. This is water that was already part of Earth's rock.
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Water-bearing comets and asteroids. Impacts delivered icy and water-rich material from the outer solar system, especially during the heavy bombardment of the early Earth.
Once the surface cooled below the boiling point of water (100 °C at sea level), condensation turned water vapor into liquid. It rained for a very long time, and the water collected in basins. This formed the hydrosphere, and the oceans hold most of Earth's liquid water today.
| Source | What it supplied | Why it matters | Limitation |
|---|---|---|---|
| Outgassing | Water vapor (and CO₂, N₂) | Explains the air and water together; volcanism is well documented | Some scientists question whether Earth's interior held enough water |
| Comets and asteroids | Water-rich material | Some asteroids have water chemistry (hydrogen isotope ratios) that matches Earth's ocean water | Many comets have water that differs from Earth's, so comets alone cannot explain it |
This is why neither source alone satisfies every scientist. Each explains part of the data, and the debate continues as new measurements arrive.
Scientists think that water-bearing comets and asteroids contributed to Earth's oceans. Identify the Earth system (geosphere, hydrosphere, atmosphere, or biosphere) that…
Scientists think that water-bearing comets and asteroids contributed to Earth's oceans. Identify the Earth system (geosphere, hydrosphere, atmosphere, or biosphere) that holds most of Earth's liquid water today, and state the process by which water vapor in the early atmosphere became liquid water on the surface.
Compare volcanic outgassing and comet impacts as sources of Earth's water. For each source, describe what it supplied and give one reason scientists consider it…
Compare volcanic outgassing and comet impacts as sources of Earth's water. For each source, describe what it supplied and give one reason scientists consider it important. Then explain why neither source alone is considered a complete explanation by every scientist.
Cyanobacteria add oxygen
Evidence suggests that cyanobacteria were living in the oceans by about 2.7 billion years ago, perhaps earlier. They carried out photosynthesis, using sunlight to make sugar:
$$6\,\text{CO}_2 + 6\,\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2$$The raw materials (reactants) are carbon dioxide and water. Oxygen is a by-product. For the cyanobacteria, it was waste.
Here is the surprising part: oxygen did not pile up in the air right away. The oceans held huge amounts of dissolved iron. Oxygen reacted with it and formed iron oxide, which sank and made layers of rock called banded iron formations (BIFs). Many large BIFs formed about 2.5 to 1.8 billion years ago. Oxygen was being used up as fast as it was made.
Once most dissolved iron and other "oxygen sinks" were used up, oxygen finally began to build up in the atmosphere. This rise is called the Great Oxidation Event, starting around 2.4 billion years ago. Ga means billion years ago.
Cyanobacteria were among the first organisms to release oxygen as a by-product. Name the process they used, and write the two main raw materials (reactants) of that…
Cyanobacteria were among the first organisms to release oxygen as a by-product. Name the process they used, and write the two main raw materials (reactants) of that process.
Banded iron formations are layered rocks that formed in ancient oceans about 2.5 to 1.8 billion years ago. Dissolved iron reacted with oxygen and settled out as iron…
Banded iron formations are layered rocks that formed in ancient oceans about 2.5 to 1.8 billion years ago. Dissolved iron reacted with oxygen and settled out as iron oxide layers. Explain what this evidence tells you about when and where oxygen was first produced, and why oxygen did not build up in the atmosphere immediately.
Oxygen changes every Earth system
Oxygen changed all four Earth systems.
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Atmosphere: O₂ rose from almost 0% to about 21%. Some O₂ formed ozone (O₃) in the stratosphere. The ozone layer absorbs much of the Sun's ultraviolet radiation.
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Hydrosphere: Ocean chemistry changed. Dissolved iron was removed, and the water became oxygenated, at least near the surface.
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Geosphere: BIFs formed, and oxygen-rich weathering made new minerals, such as iron oxides ("rust").
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Biosphere: Many organisms that lived without oxygen were harmed, while oxygen-using life evolved. Ozone allowed more life to live in shallow water and eventually on land.
The rise was neither steady nor good for everyone. Oxygen was toxic to many anaerobic organisms, and some scientists link the rise to a major crash in life and to climate change. Oxygen destroyed methane, a strong greenhouse gas, which may have contributed to long ice ages. Atmospheric oxygen has also gone up and down since.
Oxygen also made aerobic respiration possible, which releases much more energy from food than living without oxygen. Over time this energy supported complex cells and large organisms.
Aaliyah is writing a CER (claim, evidence, reasoning) response to the question: "How did early life change Earth's atmosphere?" Write a complete response. Your claim…
Aaliyah is writing a CER (claim, evidence, reasoning) response to the question: "How did early life change Earth's atmosphere?" Write a complete response. Your claim should address oxygen. Use at least two pieces of evidence (for example, banded iron formations, fossil stromatolites, or the rise of an ozone layer), and in your reasoning explain how the changing atmosphere affected the hydrosphere, the geosphere, and the biosphere.
Suppose atmospheric oxygen rose from almost 0% to about 21% of the atmosphere over roughly two billion years. Evaluate this statement: "The rise of oxygen was a gradual,…
Suppose atmospheric oxygen rose from almost 0% to about 21% of the atmosphere over roughly two billion years. Evaluate this statement: "The rise of oxygen was a gradual, steady increase that benefited all life." Give at least two reasons why it was neither purely steady nor beneficial to all organisms, and describe one way the new oxygen made a later development in life, such as complex cells that use oxygen, possible.
Back to the start: you could not breathe the air 4 billion years ago because it came from volcanoes and had almost no oxygen. The oceans came from outgassed water vapor and water-rich impactors, and oxygen arrived later, thanks to cyanobacteria.