Final answer:
Carbon dioxide (CO₂) is the most abundant gas in the atmospheres of both Venus and Mars, making up about 96% of each planet's atmosphere. The prevalence of carbon dioxide on these planets is due to their dry conditions and lack of life processes that reduce CO₂ levels on Earth. Venus's atmosphere is significantly denser than Mars's.
Step-by-step explanation:
The most abundant gas in the atmospheres of Venus and Mars is carbon dioxide (CO₂). Approximately 96% of the atmosphere on Venus consists of carbon dioxide, with the remaining portions being primarily nitrogen. Similarly, Mars also has a high concentration of carbon dioxide comprising about 96% of its atmosphere. Both planets exhibit this characteristic due to their dry conditions and lack of significant quantities of water and life, which on Earth have played critical roles in reducing atmospheric carbon dioxide levels through processes like photosynthesis and the formation of marine sediments.
Comparing Venus and Mars, although they share similarities in their atmospheric compositions, their atmospheres differ greatly in scale. Venus boasts a much more massive atmosphere with a surface pressure about 90 times greater than Earth's, making it over 10,000 times more massive than the atmosphere of Mars. These differences also mean the two planets have vastly different climate conditions, not conducive for human life as we know it.