The formation and evolution of Earth's atmosphere

There are no direct measurements of the Earth’s earliest atmosphere, so its composition must be inferred from evidence. Volcanoes release gases from inside the Earth, supporting the explanation that volcanic activity supplied the early atmosphere. The carbon-dioxide-rich atmospheres of Mars and Venus also provide useful comparisons, although they do not prove that the Earth had exactly the same atmosphere.

Minerals in ancient rocks provide another line of evidence. Their chemical composition helps scientists work out which gases were present when the minerals formed. To interpret this kind of evidence, connect the conditions needed to form a mineral with what its presence suggests about the atmosphere at that time.

These are indirect clues rather than samples of the whole early atmosphere. Several lines of evidence can support a proposed composition, but they do not establish exact percentages of every gas. This is why the early atmosphere is described as thought to contain these gases.

Cooling allowed oceans to form

While the Earth was very hot, water remained in the atmosphere as water vapour. As the Earth cooled sufficiently, the vapour condensed: it changed from a gas into liquid water. Water fell to the surface and accumulated to form oceans.

$$\mathrm{H_2O(g) \rightarrow H_2O(l)}$$

The state symbols show the change clearly: $(g)$ means gas and $(l)$ means liquid. The substance remained water; condensation was a physical change, not the production of a new substance.

The new oceans also changed the atmosphere. Carbon dioxide is soluble in water, so some atmospheric carbon dioxide dissolved in the oceans. It moved from the atmosphere into the water, decreasing the amount left in the air. Carbonate substances also precipitated from the water, forming solid sediments and storing carbon outside the atmosphere.

Photosynthesis changed the balance of gases

Primitive plants and algae began to carry out photosynthesis. Using light energy, they took in carbon dioxide and water to make glucose, releasing oxygen.

$$\text{carbon dioxide + water} \xrightarrow{\text{light}} \text{glucose + oxygen}$$

The balanced symbol equation is:

$$\mathrm{6CO_2 + 6H_2O \xrightarrow{light} C_6H_{12}O_6 + 6O_2}$$

Carbon dioxide is a reactant, so photosynthesis removes it. Oxygen is a product, so photosynthesis releases it. As primitive plants and algae grew and spread, these processes gradually reduced atmospheric carbon dioxide and increased atmospheric oxygen. This happened over a very long period, rather than in one sudden change, and eventually allowed oxygen-dependent animals to evolve.

Two different processes therefore reduced carbon dioxide: dissolving in the oceans and its use in photosynthesis. Of these, photosynthesis also supplied oxygen.

Identifying oxygen in a gas sample

Oxygen is identified using a glowing splint. Light a wooden splint, then blow out the flame so that its tip is still glowing. Insert the glowing tip into a test tube containing the gas.

If the gas is oxygen, the glowing splint relights. The starting condition matters: the splint is glowing, not already burning with a flame.

Today’s atmosphere

Today’s atmosphere is mainly nitrogen and oxygen. The approximate composition of dry air is:

There are also very small amounts of other gases. Air contains a variable amount of water vapour, so water vapour does not have one fixed percentage.

The contrast with the early atmosphere is substantial: carbon dioxide is now only a small proportion, while oxygen makes up about one fifth of the air. Ocean formation and photosynthesis help explain this change.

How it changed

• Cooling → condensation → oceans: $\mathrm{H_2O(g) \rightarrow H_2O(l)}$.
• Carbon dioxide dissolved in the oceans, reducing atmospheric carbon dioxide. Carbonates formed solid sediments.
• Primitive plants and algae photosynthesised: carbon dioxide decreased and oxygen gradually increased.

$$\mathrm{6CO_2 + 6H_2O \xrightarrow{light} C_6H_{12}O_6 + 6O_2}$$

Oxygen test

Insert a glowing splint into the gas. It relights in oxygen.

Today’s atmosphere

Dry air: approximately 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide. Water vapour varies; other gases occur in very small amounts.