The Early Atmosphere

Volcanoes also produced large quantities of nitrogen gas. Because nitrogen is unreactive—meaning it does not easily take part in chemical reactions—it gradually built up over billions of years. Today, nitrogen is the most abundant gas in our atmosphere, making up approximately 80% of the air we breathe.

How the Oceans Formed

How did a boiling planet covered in erupting volcanoes eventually become a blue world covered in water? The answer lies in the changing temperature of the Earth's surface.

• Initially, the Earth was too hot for liquid water to exist, so all water remained as a gas in the atmosphere.
• Approximately 4 billion years ago, volcanic activity subsided and the Earth began to cool.
• Once the surface temperature dropped below 100°C, the water vapour underwent condensation to form liquid water.

This liquid water pooled in the deep basins of the Earth's crust, forming the first oceans. This physical change of state was the critical first step in changing the composition of the atmosphere.

Reducing Carbon Dioxide

Every time you open a fizzy drink, you see dissolved carbon dioxide gas escaping from a liquid. A similar process in reverse helped save our planet from extreme greenhouse heat.

Because carbon dioxide is a water-soluble gas, the formation of the oceans gave the atmospheric $CO_2$ somewhere to go. Massive amounts of carbon dioxide underwent dissolution, physically absorbing into the seawater:

$$CO_{2(g)} \rightleftharpoons CO_{2(aq)}$$

Once dissolved in the oceans, the carbon dioxide reacted with dissolved minerals and metal ions, such as calcium. These chemical reactions formed insoluble solid particles called carbonate precipitates:

$$Ca^{2+}_{(aq)} + CO_{3(aq)}^{2-} \rightarrow CaCO_{3(s)}$$

These heavy precipitates sank to the seabed, settling as sediments. Over millions of years, the intense pressure compressed these sediments to form sedimentary rock, such as limestone.

Early marine organisms, like corals and shellfish, also used dissolved carbonates from the water to build their hard shells and skeletons. When they died, their remains were buried and compressed alongside the sediments. This entire process successfully locked up huge amounts of carbon in the Earth's crust, drastically reducing the amount of $CO_2$ in the atmosphere.

Evaluating Theories of the Early Atmosphere

Understanding the past helps us predict the future, but reconstructing the history of a 4.6-billion-year-old planet is extremely difficult. When evaluating theories about the early atmosphere, you must weigh the different arguments and provide a structured concluding judgement.

Arguments FOR the Volcanic Theory:

• The proposed early atmosphere (high $CO_2$, low/no $O_2$) closely aligns with the modern atmospheres of Mars and Venus, which are known to be heavily influenced by volcanic activity.
• Erupting volcanoes provide a clear, observable mechanism for releasing the massive amounts of carbon dioxide, water vapour, and nitrogen thought to be present.

Arguments AGAINST / Alternative Theories:

• Some scientists argue that volcanic activity alone cannot account for the vast volume of water required to form the Earth's oceans.
• Alternative theories suggest that a bombardment of icy comets colliding with the early Earth may have delivered the missing water.

Concluding Judgement: While the volcanic theory effectively explains the presence of high levels of carbon dioxide and nitrogen, our ability to confirm the exact source of water is hindered by the vast geological timescale. Because the Earth is 4.6 billion years old, direct physical evidence (like pristine early air samples or undisturbed rocks) is incredibly limited. Therefore, scientists must rely on evolving models, accepting that no single theory can be proven as absolute fact.