Final answer:
The atmospheres of Venus and Mars have similar chemical compositions, primarily composed of CO₂, though their atmospheric masses, surface pressures, and temperatures differ significantly.
Step-by-step explanation:
The atmospheres of Venus and Mars are similar in terms of chemical composition. While their total atmospheric masses differ greatly, with Venus having a much thicker atmosphere and higher surface pressure, both planets have atmospheres composed predominantly of carbon dioxide (CO₂). Venus has about 96% CO₂, and Mars has a high concentration of CO₂ as well. However, their surface pressures and temperatures are dramatically different; Venus has a surface pressure of 90 bars and temperatures high enough to melt lead, while Mars has a much thinner atmosphere with a surface pressure of only 0.006 bars and much colder temperatures.