Chemistry 8.18 - 8.23 - Early atmosphere and oxygen

Reconstructing the early atmosphere

The young Earth was hot and volcanically active. Gases released by volcanic activity collected around the planet and formed its early atmosphere. This process is often called outgassing: material from inside Earth escaped as gases at the surface.

The early atmosphere is thought to have contained:

• little or no oxygen;
• a large amount of carbon dioxide;
• water vapour;
• small amounts of other gases.

These descriptions are deliberately qualitative. Scientists do not have a bottle of unchanged air from the early Earth, so exact percentages cannot simply be measured. The phrase is thought to have contained signals a model supported by evidence, not a perfectly known recipe.

From observation to inference

Scientists combine different lines of indirect evidence.

Evidence that is consistent with a model supports it, but does not prove every detail. A modern volcanic gas sample can make volcanic outgassing plausible; it cannot by itself establish the exact composition of the early atmosphere. Evidence in ancient rocks can test predictions about oxygen conditions, but its interpretation must account for the rock's age and later changes.

Cooling, oceans and dissolved carbon dioxide

The early atmosphere contained water as a gas because Earth was very hot. As Earth cooled, it eventually became cool enough for water vapour to condense. Condensation is a physical change from gas to liquid:

$\ce{H2O(g) -> H2O(l)}$

At the particle level, the particles remain $\ce{H2O}$ molecules. They lose energy and become much closer together as liquid water; they do not change into a new substance. Liquid water fell and collected in low areas over a long time, forming the oceans.

Ocean formation also changed the carbon dioxide in the atmosphere. Carbon dioxide is soluble in water, so carbon dioxide molecules moved from the air into the new liquid oceans:

$\ce{CO2(g) -> CO2(aq)}$

The state symbol (aq) means dissolved in water. It does not mean that carbon dioxide vanished or that its atoms were destroyed. The carbon dioxide moved from the atmosphere into an ocean store, so the amount of carbon dioxide remaining in the atmosphere decreased.

Keep the two changes separate:

1. water vapour condensed to make liquid water and oceans;
2. carbon dioxide then dissolved in that ocean water, reducing atmospheric carbon dioxide.

Condensation and dissolving are physical processes. Neither one explains the later rise in oxygen; that requires photosynthesis.

Photosynthesis and rising oxygen

Primitive plants and other early photosynthetic organisms began to grow. They used carbon dioxide and water in photosynthesis and released oxygen. At the macroscopic scale, continued growth meant that photosynthesis occurred across more organisms and over immense periods of time.

The overall word equation is:

$\text{carbon dioxide}+\text{water}\rightarrow\text{glucose}+\text{oxygen}$

Using formulae, the balanced overall equation is:

$\ce{6CO2 + 6H2O -> C6H12O6 + 6O2}$

Light supplies the energy for the process. The equation is balanced: there are 6 carbon atoms, 12 hydrogen atoms and 18 oxygen atoms on each side. Matter is rearranged rather than created or destroyed.

The atmospheric consequences follow from the substances used and released:

• photosynthesis uses carbon dioxide, helping its atmospheric amount to decrease;
• photosynthesis releases oxygen;
• as primitive plants grew and photosynthesis continued, oxygen accumulated, so the amount of oxygen in the atmosphere gradually increased.