Final answer:
Differences in Earth's atmosphere compared to Mars and Venus are likely due to both the presence of life on Earth and the widespread liquid water, which have significantly influenced atmospheric composition, resulting in a distinct Earthly atmosphere with higher oxygen levels. Therefore, the correct answer is 'all of the above'.
Step-by-step explanation:
The differences in the composition of Earth's atmosphere compared to that of Mars and Venus are likely caused by the presence of life on Earth and the widespread presence of liquid water on the surface of the Earth, which has pulled some gases out of Earth's atmosphere and dissolved them. Both life and liquid water have played significant roles in shaping the current atmospheric conditions on Earth. Life, through processes such as photosynthesis, has contributed to decreasing levels of carbon dioxide (CO2) and increasing levels of oxygen.
The presence of liquid water has led to the absorption and sequestration of CO2 as carbonate compounds in marine sediments. In contrast, Venus and Mars, which lack significant quantities of both life and liquid water, ended up with atmospheres composed mainly of CO2 and nitrogen. Venus experienced a runaway greenhouse effect, leading to a dry, CO2-rich atmosphere, while Mars, being smaller and having lower gravity, lost much of its atmosphere to space, resulting in a thin, CO2-dominated atmosphere with minimal greenhouse effect. Consequently, the correct answer is option 3, all of the above.