9.1 The Earth's early atmosphere and how it changed

The early atmosphere

Scientists think that, in the first billion years, intense volcanic activity released gases that formed the early atmosphere. It was probably:

• mainly carbon dioxide, with little or no oxygen
• with water vapour and nitrogen
• and small amounts of methane and ammonia

This is like the atmospheres of Mars and Venus today. The evidence is limited, because it was so long ago.

How it changed

1. Oceans formed: as the Earth cooled, the water vapour condensed to form the oceans.
2. Carbon dioxide decreased: carbon dioxide dissolved in the oceans. It formed carbonate compounds that were precipitated as sediments.
3. Oxygen increased: about 2.7 billion years ago, algae began to photosynthesise, and later plants evolved. Photosynthesis removed carbon dioxide and released oxygen: carbon dioxide + water → glucose + oxygen (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂)
4. Carbon was locked away: carbon from living things was trapped in sedimentary rocks (such as limestone) and fossil fuels (coal, crude oil and natural gas).

Over the next billion years, the amount of oxygen built up until animals could evolve.

The atmosphere today

For the last 200 million years, the composition has been much as it is today:

Test for oxygen

A glowing splint relights in oxygen.