C13.01: The history of our atmosphere

• Scientists use evidence from gas bubbles trapped in ancient rocks and also use data gathered from the atmospheres of other planets and their moons in the solar system to construct theories about the Earth's early atmosphere.
• One theory suggests that volcanoes released carbon dioxide, water vapour and nitrogen, and that these gases formed the early atmosphere. Water vapour in the atmosphere condensed as the Earth gradually cooled down, and fell as rain. Water collected in the crust as the rock solidified and the first oceans were formed.
• One theory suggests that comets could have also brought water to the Earth. As icy comets rained down on the surface of the Earth, they melted, adding to its water supplies.
• As the Earth began to stabilise, the atmosphere was probably mainly carbon dioxide. There could also have been some water vapour and nitrogen gas, and traces of methane and ammonia. There would have been very little or no oxygen at the time. This resembles the atmospheres that are known to exist today on the planets Mars and Venus
• Scientists think that life began about 3.4 billion years ago, when the first simple organisms appeared. These could use the breakdown of chemicals as a source of energy. Then, about 2.7 billion years ago, bacteria and other simple organisms like algae evolved. Algae could use the energy from the sun to make their own food by photosynthesis. This produced oxygen as a waste product. Over the next billion years, the levels of oxygen rose steadily as algae and bacteria thrived in the seas. Plants increasingly evolved - all of them were photosynthesising, removing carbon dioxide and making oxygen
• As plants evolved, they successfully colonised most of the surface of the Earth. So the atmosphere became richer in oxygen. This made it possible for the first animal forms to evolve. These animals could not make their own food like algae and plants could. They relied on algae and plants for their food and on oxygen to respire. However, many of the earliest microorganisms could not tolerate a high oxygen concentration, because they had evolved without it. They largely died out as there were fewer places where they could survive.
• An alternative theory:

  Scientists have found evidence from some of the oldest rocks on Earth that question assumptions that the early gases originated from the volcanoes. Some scientists suggest that the mixture of gases could have been formed from solar debris, similar to comets, crashing into the Earth and vaporising around 500 million years after its formation.