The fraction of carbon dioxide (CO2) in the atmosphere varies for each of the four terrestrial planets in our solar system:
Venus: Venus has the highest fraction of carbon dioxide in its atmosphere, with approximately 96.5% of the atmosphere composed of CO2. It has a dense and primarily carbon dioxide atmosphere, creating a strong greenhouse effect that results in extreme surface temperatures.
Mars: Mars has a relatively higher fraction of carbon dioxide compared to Earth, but significantly lower than Venus. Approximately 95% of the Martian atmosphere is composed of carbon dioxide. The thin atmosphere on Mars is primarily made up of carbon dioxide, with traces of other gases such as nitrogen and argon.
Earth: Earth has a much lower fraction of carbon dioxide in its atmosphere compared to Venus and Mars. Currently, the fraction of carbon dioxide in Earth's atmosphere is around 0.04% (400 parts per million). While carbon dioxide levels have increased due to human activities, it still remains a relatively small fraction of Earth's atmosphere, primarily composed of nitrogen (78%) and oxygen (21%).
Mercury: Mercury has an extremely thin atmosphere, often referred to as an exosphere. Its atmosphere is composed mainly of traces of hydrogen, helium, oxygen, sodium, calcium, and potassium. The fraction of carbon dioxide in Mercury's atmosphere is extremely low or negligible.
Ranking the four terrestrial planets in terms of the mass of their atmosphere, from largest to smallest:
Venus: Venus has the most massive atmosphere among the four terrestrial planets. Its dense atmosphere contributes to its substantial mass.
Earth: Earth has a significant atmosphere with substantial mass, supporting a diverse range of life forms.
Mars: Mars has a much thinner atmosphere compared to Earth and Venus, resulting in a lower mass overall.
Mercury: Mercury has a very thin and tenuous atmosphere, often considered negligible in terms of mass.