Ozone is found in small quantities in the atmosphere of Mars. It is highly reactive, and its abundance indicates other chemical compounds that are not currently measured, such as the hydroxyl radical. In planetary atmospheres containing a non-negligible amount of water vapor, such as the atmospheres of Earth and Mars, the hydroxyl radical is produced by the splitting of water vapor molecules by sunlight (photodissociation). The hydroxyl radical not only destroys ozone but also catalyzes the formation of more hydroxyl radical. As a result, when water vapor is abundant, ozone is not. However, when water ice is present, the hydroxyl radical can adhere to it, rendering it temporarily inert and allowing ozone to persist. This interaction between two compounds of different phases is called heterogeneous chemistry.
To investigate the importance of heterogeneous chemistry to the abundance of ozone in the martian atmosphere, Megan Brown from The Open University and colleagues included the adsorption of hydroxyl radical on water ice in a one-dimensional simulation of the martian atmosphere. The simulation extends up to 70 kilometers (43 miles) from the martian surface. Vertical profiles of ozone from the simulation with and without heterogeneous chemistry are compared to observed ozone profiles from Nadir and Occultation for Mars Discovery (NOMAD) onboard the ExoMars Trace Gas Orbiter (TGO). Brown and colleagues found that without heterogeneous chemistry, their model underpredicts the amount of ozone present in regions of the atmosphere containing ice clouds. Even including heterogeneous chemistry, if water vapor is abundant or ice clouds vaporize, the amount of ozone does not increase because the concentration of hydroxyl radical increases. However, when water vapor is low and ice clouds are present, the amount of ozone increases, with a 43-75% improvement over the model without heterogeneous chemistry. An improved understanding of ozone provides information about the advantages and limitations of ozone observations in planetary atmospheres, including the possible future use of ozone as a biosignature in exoplanet atmospheres. READ MORE