Evidence for the early atmosphere

Scientists cannot directly observe what Earth’s atmosphere was like around 4.6 billion years ago. They must instead use evidence that remains today to infer what conditions may have been like. Much of the original evidence has been lost or altered over time, so these inferences have limitations.

A proposed explanation of the early atmosphere is a scientific theory: an account constructed and assessed using evidence. It is not simply a guess, but neither is it a direct record of the atmosphere. Scientists can revise a theory when new evidence becomes available or when existing evidence is interpreted more effectively.

Judging evidence for the early atmosphere

Evidence can support an explanation without proving it. For example, gases released by volcanoes today provide information about gases that volcanic activity can produce. However, present-day volcanic gases are not a direct measurement of the early atmosphere, and they do not establish its exact composition.

When comparing proposed accounts of the early atmosphere, consider:

• Relevance: Does the evidence relate directly to the claim being made?
• Strength: How strongly does the evidence support that claim? Could it also fit another explanation?
• Limitations: Has the evidence been altered, or does the explanation depend on assumptions about conditions in the distant past?
• Comparison: Which account explains the available evidence more convincingly, and what evidence remains unexplained?

An explanation may be better supported than its competitors while still being uncertain. Incomplete evidence can make it difficult to choose confidently between accounts.

Volcanic origin of the early atmosphere

A key theory is that Earth’s early atmosphere formed from gases released from inside the planet by intense volcanic activity. During the first billion years of Earth’s existence, these gases built up around the planet.

The early atmosphere is thought to have contained mainly carbon dioxide ($$\ce{CO2}$$), with little or no oxygen ($$\ce{O2}$$). In this respect, it was more like the atmospheres of Mars and Venus today than the modern Earth’s atmosphere.

Volcanoes also released water vapour ($$\ce{H2O}$$). As Earth cooled, the water vapour condensed into liquid water, which formed the oceans. Nitrogen ($$\ce{N2}$$) from volcanic gases gradually accumulated in the atmosphere. There may also have been small amounts of methane ($$\ce{CH4}$$) and ammonia ($$\ce{NH3}$$).

Reduction of carbon dioxide in the early atmosphere

The early atmosphere contained a very high concentration of carbon dioxide, $$\ce{CO2}$$. After the oceans formed, some of this gas dissolved from the atmosphere into seawater, reducing the amount in the air.

In seawater, some of the dissolved carbon dioxide became part of carbonate compounds. These carbonates precipitated: they formed solids that settled and were deposited as sediments on the ocean floor.

As carbon was removed from the water into sediments, more carbon dioxide could dissolve from the atmosphere. Over time, this process lowered the amount of carbon dioxide in the atmosphere and changed its composition.