Photochemical Stability and Atmospheric Escape
It seems likely that the Martian atmosphere has always consisted largely of CO2 with minor amounts of H2O. This raised the question of the stability of such as atmosphere against photochemical decomposition and escape of volatiles. The abundance and stability of carbon dioxide present similar problems on Earth. How do we explain the stability of CO2 with respect to conversion to CO, O and O2? Because there is so little CO2 on Mars, the time required for the Martian atmosphere to be converted to a predominantly CO-O2 atmosphere in the absence of recombination mechanisms is very short, as little as 3500 years. Even more troublesome is the fact that the observed amount of CO and O2 on Mars could be supplied by UV photolysis of carbon dioxide in only 3 and 8 years, respectively.
There are two principal constraints that applied to the photochemistry theories of Martian atmosphere.
The first constraint
The second constraint
MARS AT OPPOSITION
These NASA Hubble Space Telescope views provide the most detailed
complete global coverage of the red planet Mars ever seen from Earth.
The pictures were taken on February 25, 1995, when Mars was at a
distance of 65 million miles (103 million km).
To the surprise of researchers, Mars is cloudier than seen in previous
years. This means the planet is cooler and drier, because water vapor in the atmosphere freezes out to form ice-crystal clouds. Hubble
resolves Martian surface features with a level of detail only exceeded
by planetary probes, such as impact craters and other features as small
as 30 miles (50 kilometers) across.