CO₂ is the main component of the Martian atmosphere.
It has a mean volume ratio of 94.9%.
In winter polar regions, the surface temperature can be lower than the frost point of CO2.
CO₂ gas in the atmosphere can condense on the surface to form 1–2 m thick solid dry ice.
In summer, the polar dry ice cap can undergo sublimation and release the CO₂ back to the atmosphere.
As a result, significant annual variability in atmospheric pressure (≈25%) and atmospheric composition can be observed on Mars.
The condensation process can be approximated by the Clausius–Clapeyron relation for CO₂.
Despite the high concentration of CO₂ in the Martian atmosphere, the greenhouse effect is relatively weak on Mars (about 5 °C) because of the low concentration of water vapor and low atmospheric pressure.
While water vapor in Earth's atmosphere has the largest contribution to greenhouse effect on modern Earth, it is present in only very low concentration in the Martian atmosphere.
Moreover, under low atmospheric pressure, greenhouse gases cannot absorb infrared radiation effectively because the pressure-broadening effect is weak.

In the presence of solar UV radiation (hν, photons with wavelength shorter than 225 nm), CO₂ in the Martian atmosphere can be photolyzed via the following reaction:
CO
2 + hν ( < 225 nm) ⟶ CO + O
If there is no chemical production of CO₂, all the CO₂ in the current Martian atmosphere would be removed by photolysis in about 3,500 years.
The hydroxyl radicals (OH) produced from the photolysis of water vapor, together with the other odd hydrogen species (e.g. H, HO₂), can convert carbon monoxide (CO) back to CO₂. The reaction cycle can be described as:
CO + OH ⟶ CO
2 + H
H + O
2 + M ⟶ HO
2 + M
HO
2 + O ⟶ OH + O
2
Net: CO + O ⟶ CO
2
Mixing also plays a role in regenerating CO₂ by bringing the O, CO and O₂ in the upper atmosphere downward.
The balance between photolysis and redox production keeps the average concentration of CO₂ stable in the modern Martian atmosphere.

CO₂ ice clouds can form in winter polar regions and at very high altitude (>50 km) in tropical regions, where the air temperature is lower than the frost point of CO₂.
CO₂ is the main component of the Martian atmosphere. It has a mean volume ratio of 94.9%. In winter polar regions, the surface temperature can be lower than the frost point of CO2. CO₂ gas in the atmosphere can condense on the surface to form 1–2 m thick solid dry ice. In summer, the polar dry ice cap can undergo sublimation and release the CO₂ back to the atmosphere. As a result, significant annual variability in atmospheric pressure (≈25%) and atmospheric composition can be observed on Mars. The condensation process can be approximated by the Clausius–Clapeyron relation for CO₂.
Despite the high concentration of CO₂ in the Martian atmosphere, the greenhouse effect is relatively weak on Mars (about 5 °C) because of the low concentration of water vapor and low atmospheric pressure. While water vapor in Earth's atmosphere has the largest contribution to greenhouse effect on modern Earth, it is present in only very low concentration in the Martian atmosphere. Moreover, under low atmospheric pressure, greenhouse gases cannot absorb infrared radiation effectively because the pressure-broadening effect is weak.
In the presence of solar UV radiation (hν, photons with wavelength shorter than 225 nm), CO₂ in the Martian atmosphere can be photolyzed via the following reaction:
CO
2 + hν ( < 225 nm) ⟶ CO + O
If there is no chemical production of CO₂, all the CO₂ in the current Martian atmosphere would be removed by photolysis in about 3,500 years. The hydroxyl radicals (OH) produced from the photolysis of water vapor, together with the other odd hydrogen species (e.g. H, HO₂), can convert carbon monoxide (CO) back to CO₂. The reaction cycle can be described as:
CO + OH ⟶ CO
2 + H
H + O
2 + M ⟶ HO
2 + M
HO
2 + O ⟶ OH + O
2
Net: CO + O ⟶ CO
2
Mixing also plays a role in regenerating CO₂ by bringing the O, CO and O₂ in the upper atmosphere downward. The balance between photolysis and redox production keeps the average concentration of CO₂ stable in the modern Martian atmosphere.
CO₂ ice clouds can form in winter polar regions and at very high altitude (>50 km) in tropical regions, where the air temperature is lower than the frost point of CO₂.
Atmosphere of Mars