Final answer:
Mars is cold because its thin atmosphere, mostly composed of carbon dioxide, has little capacity for greenhouse warming since it has lost most of its atmospheric gases, including water vapor that contributes to this effect.
Step-by-step explanation:
Mars is cold even though its atmosphere is mostly made up of greenhouse gas because of several key factors. Initially, Mars probably had a warmer surface temperature thanks to the greenhouse effect. However, over time, Mars has lost much of its atmosphere, particularly water vapor, which contributes substantially to greenhouse warming. This loss occurred as Mars is a smaller planet with lower gravity, making it easier for atmospheric gases to escape into space. The resulting thin atmosphere means very little greenhouse warming occurs on the planet, with estimates suggesting it contributes about 2 °C to the surface temperature. In contrast, Earth experiences significant warming of about 33 °C because of its greenhouse gases, while Venus has extreme greenhouse warming of about 510 °C due to its thick carbon dioxide atmosphere. Mars's current cold and dry state is largely because it has a much thinner atmosphere that retains less heat.