Abstract

Mars is a cold and arid planet, with an atmosphere less than 1% the surface pressure of Earth’s. Yet it undergoes excessive seasonal changes, from thick water ice clouds at high latitudes (aphelion – furthest from the Sun) to global dust storms engulfing the entire planet (perihelion – closest to the Sun). The chemical components in the atmosphere are similar to those known on Earth – CO2 , O2, H2O – but in different quantities and under extreme conditions on Mars. Hydroxyl radicals (OH and HO2 ) catalyse the reformation of CO2 and are responsible for carbon dioxide being the primary constituent of the atmosphere (95%). Ozone is a key trace gas for understanding these chemical reactions: it can be detected via UV spectrometers aboard satellites and is sensitive to short-lived hydroxyl radicals. A discrepancy between observed ozone and ozone modelled through global climate models implies missing or inaccurate chemistry in our knowledge of the martian atmosphere. One possible explanation for this ozone deficit could be the heterogeneous uptake of hydroxyl radicals on water ice particles. This causes a depletion in HOx and an increase in ozone. Using 1D and 3D atmospheric models, we show evidence that heterogeneous reactions occur, but have a greater impact on ozone under conditions when water vapour abundance is minimal. The effect of heterogeneous reactions is also greater when water ice clouds persist for several sols, as HOx remain adsorbed and also when dissociation of adsorbed species occurs at the surface.

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