The Atmosphere's Composition
Our atmosphere today consists mostly of nitrogen (78%) and oxygen (21%), with small amounts of other gases including carbon dioxide, water vapour and noble gases. This composition has remained relatively stable for about 200 million years, but it wasn't always this way.
During Earth's first billion years, intense volcanic activity released gases that formed the early atmosphere, primarily water vapour and carbon dioxide. As the planet cooled, water vapour condensed to form oceans, but carbon dioxide remained dominant in the atmosphere—similar to what we see today on Mars and Venus.
Carbon dioxide levels decreased over time through several natural processes. The gas dissolved in oceans, creating a weak acid that reacted with minerals to form precipitates. These eventually became sediments of carbonate rock on the sea bed. Marine organisms like corals and mussels also used carbon dioxide to create their shells, which later formed limestone when they died.
Did you know? The limestone formations we see today represent carbon dioxide that was once in the atmosphere, showing how Earth naturally sequestered carbon over millions of years!
