The early atmosphere of Earth was radically different from the one we breathe today. Many planetary geochemists believe Earth's primitive atmosphere was very similar to the current atmosphere of Venus, which has experienced a runaway greenhouse effect.
The table below shows the approximate percentages of carbon dioxide and oxygen in the atmospheres of Venus and modern Earth:
GasPercentage (%) in Venus’s atmospherePercentage (%) in Earth’s atmosphere todayCarbon dioxide96.500.04OxygenTrace (< 0.01)20.95 \begin{array}{|l|c|c|} \hline \textbf{Gas} & \textbf{Percentage (\%) in Venus's atmosphere} & \textbf{Percentage (\%) in Earth's atmosphere today} \\\hline \text{Carbon dioxide} & 96.50 & 0.04 \\\hline \text{Oxygen} & \text{Trace (< 0.01)} & 20.95 \\\hline \end{array} GasCarbon dioxideOxygenPercentage (%) in Venus’s atmosphere96.50Trace (< 0.01)Percentage (%) in Earth’s atmosphere today0.0420.95Describe and explain the geological and biological processes that caused the percentage of carbon dioxide in Earth's atmosphere to decrease from its early volcanic levels to its modern value.
Explain how the evolution of early photosynthetic organisms, such as cyanobacteria, altered the levels of both carbon dioxide and oxygen in Earth's atmosphere.
State one reason why scientists cannot be completely certain about the precise composition of Earth's atmosphere billions of years ago.
7 exam-style questions on AQA GCSE Chemistry 9.1 The composition and evolution of the Earth's atmosphere, covering 9.1.1 The proportions of different gases in the atmosphere, 9.1.2 The Earth's early atmosphere, 9.1.3 How oxygen increased, and 9.1.4 How carbon dioxide decreased. Each one has a worked solution and a mark scheme showing where the marks go.