The Second Constraint

The second constraint on Martian photochemistry involves ozone. Odd oxygen (O and O₃) is produced mainly by CO₂ photodissociation. In the dry Martian atmosphere these species are removed mainly by O + O₃ ® O₂ + O₂ or by surface reaction. This reaction acting alone cannot keep O₃ concentrations below observable levels outside of the polar regions. But the presence of water vapor provides an alternative route for removal of odd oxygen:

H + O₃ ® OH + O₂

O + OH ® O₂ + H

The required hydrogen level is only a few parts in 10¹⁰, and this is easily provided by the H₂O photodissociation. Note that this abundance of odd hydrogen is not sufficient to catalyze recombination of O or O₂ with CO.

Because water vapor is the main source of odd hydrogen, the dry atmospheric regions near the Martian poles would expect to contain much larger quantities of ozone. This is in fact observed; indeed, ozone levels at the poles appear to be close to those expected in an almost completely dry Martian atmosphere.