The early atmosphere probably came from volcanoes

1. Earth formed about 4.6 billion years ago, and for much of its first billion years its surface was covered by active volcanoes.
   1. Gases released by these volcanoes built up around the planet to form the early atmosphere.
2. The early atmosphere is thought to have been mainly carbon dioxide, with water vapour and small amounts of nitrogen, methane and ammonia.
3. It contained little or no oxygen, so it was very different from the air today.
4. Nobody could measure these gases, so the explanation rests on indirect evidence and describes what is thought to have happened.

Evidence for the early atmosphere

1. Gases from present-day volcanoes are mainly water vapour and carbon dioxide, with smaller amounts of sulfur dioxide and nitrogen.
   1. If early volcanoes released similar gases, the early atmosphere would have been rich in water vapour and carbon dioxide.
2. Venus and Mars formed in a similar way to Earth, and their atmospheres are about 95% carbon dioxide.
   1. Neither planet has any known life to change its atmosphere, so they may show what Earth's atmosphere was like before life began.
3. Rocks such as limestone and fuels such as coal hold huge amounts of carbon, which suggests that far more carbon dioxide was once in the atmosphere.
4. The evidence is incomplete, because very few rocks survive from Earth's first few hundred million years and no sample of the early air exists.

Cooling formed oceans that dissolved carbon dioxide

1. As the Earth cooled, water vapour in the atmosphere condensed into liquid water.
   1. The water fell as rain and collected in hollows on the surface to form the oceans.
2. Carbon dioxide from the air dissolved in the oceans, lowering the amount left in the atmosphere.
   1. Some of the dissolved carbon dioxide reacted to form insoluble carbonates, which settled on the sea floor as sediment.
3. Over millions of years these sediments were compressed into rocks such as limestone, locking the carbon away.

Photosynthesis slowly added oxygen to the atmosphere

1. The first organisms able to photosynthesise were algae and bacteria living in the oceans, about 2.7 billion years ago.
2. Photosynthesis takes in carbon dioxide and water and gives out oxygen: $$6{CO}_2+6H_{2}O\to C_6{H}_{12}{O}_6+6O_2$$
3. At first, most of the oxygen reacted with dissolved iron compounds in the oceans, forming iron oxides that settled as layers of rock.
   1. These striped layers, called banded iron formations, record the time when oxygen was first being produced in the oceans.
4. Once the dissolved iron had been used up, oxygen escaped into the air and its percentage gradually rose.
5. Land plants evolved much later and added more oxygen, until the atmosphere reached about 21% oxygen.

Locked-away carbon and unreactive nitrogen shaped today's atmosphere

1. Photosynthesis removed carbon dioxide from the air, and the carbon became part of living things.
   1. When organisms died and were buried, some of this carbon was slowly turned into fossil fuels such as coal, crude oil and natural gas.
2. Sea creatures used dissolved carbon dioxide to build shells of calcium carbonate, which formed limestone after they died.
3. Nitrogen is unreactive and does not dissolve well, so it stayed in the air while carbon dioxide and water vapour were removed.
   1. Its percentage therefore rose, and nitrogen now makes up about 78% of the atmosphere.
4. The rest of today's air is about 21% oxygen, 0.9% argon and 0.04% carbon dioxide, and the carbon cycle now keeps this composition almost constant.