4.9.1.1,4.9.1.2,4.9.1.3,4.9.1.4 - The atmosphere and its evolution

The present atmosphere

The present atmosphere is mainly nitrogen and oxygen, with smaller amounts of several other gases.

The main composition is approximately four-fifths nitrogen, or about $80\%$, and one-fifth oxygen, or about $20\%$. Small proportions of other gases include carbon dioxide, water vapour and noble gases such as argon. These rounded figures add to $100\%$, but that rounding does not mean the other gases are absent. Water vapour varies with conditions, so the approximations are not an exact analysis of every air sample.

For roughly the last 200 million years, the broad proportions have been much as they are today. This is a statement about the main composition over geological time. It does not imply that every trace gas has stayed exactly constant or that changes in small proportions cannot matter.

Using the nitrogen fraction

For a $500\,\mathrm{cm^3}$ air sample, convert four-fifths to a multiplier and use it to find the nitrogen volume.

The $400\,\mathrm{cm^3}$ result is smaller than the total and is about four times the oxygen share.

Using the oxygen percentage

For a $750\,\mathrm{cm^3}$ sample, first convert $20\%$ to a fraction.

The complementary approximate nitrogen volume is $750-150=600\,\mathrm{cm^3}$, giving the expected $4:1$ ratio.

The approximate nitrogen-to-oxygen ratio is $4:1$. This ratio compares the two main gases; it is not evidence that air is a compound with a chemical formula. Air is a mixture, and the gases are not all chemically combined with one another.

The early atmosphere and its evidence

Scientists reconstruct the early atmosphere from limited evidence because Earth formed about 4.6 billion years ago and no direct sample of that whole atmosphere remains.

One theory is that intense volcanic activity during Earth's first billion years released the gases that formed the early atmosphere. It was probably mainly carbon dioxide, with little or no oxygen gas. It may have resembled the carbon-dioxide-rich atmospheres of Mars and Venus today. Volcanoes also released nitrogen, which gradually accumulated, and there may have been small proportions of methane and ammonia.

NASA's Curiosity rover measured 95.9% carbon dioxide by volume at Gale Crater on Mars in October 2012. This is a measured modern planetary example, rather than a measurement of ancient Earth. It shows that a carbon-dioxide-dominated atmosphere is possible, while the different histories of the two planets limit the comparison.

Water vapour released by volcanic activity condensed as Earth cooled. Liquid water collected to form oceans. Carbon dioxide dissolved in that water, and carbonates precipitated and became sediments. These processes began removing carbon dioxide from the atmosphere before the later contribution of widespread photosynthesis.

Modern volcanic gases and the atmospheres of other planets provide useful comparisons, but neither proves Earth's ancient composition. Volcanic conditions can differ, planets have different histories, and rocks may have been altered or destroyed. The long timescale limits the surviving evidence. Theories develop when improved measurements or new observations change how well an explanation fits.

Illustrative evidence set: volcanic gases contain carbon dioxide and water vapour; very old rocks provide no clear evidence for abundant atmospheric oxygen; another planet has a carbon-dioxide-rich atmosphere. These observations are consistent with the volcanic model. They do not establish the exact proportion of carbon dioxide or rule out every other source of gas. A proposal predicting abundant oxygen from the beginning fits the old-rock evidence less well than a low-oxygen proposal. Missing evidence for abundant oxygen is not proof that no oxygen existed: judge the strength and preservation of the evidence as well as the prediction. Compare each theory's predictions with the observations, identify limitations, and distinguish “supports” from “proves”.

Photosynthesis and atmospheric oxygen

Photosynthesis by algae and plants supplied the oxygen that accumulated in Earth's atmosphere.