Composition and Chemistry of the Martian Atmosphere as Observed by Mars Express and ExoMars Trace Gas Orbiter

The atmosphere of Mars has been studied for many years now by a long series of missions. The paper focuses on the results obtained by two of these that are led by European researchers overseen by the European Space Agency, i.e., Mars Express which was launched in 2003 and ExoMars Trace Gas Orbiter launched in 2016. Both missions are still providing high-quality data about the atmosphere of Mars, such as abundances of its key species – CO, CO, HO, O - playing an important role in the different cycles existing on the planet, as well as other trace gases – O (mixing ratio of 3.1 to 5.8 × 10 above 90 km), the recently discovered HCl (up to 4 ppbv below 30 km), and the elusive CH (stringent detection limit of 20 pptv). Some instruments are also sensitive enough to provide information on isotopologues of the key elements and have delivered for some of these the first and unique vertical profiles available today (δC and δO in CO and CO, D/H, δO and δO in water vapour). The paper retraces the history of the exploration of the Martian atmosphere putting the results from both missions in perspective.