Now that the current work has highlighted in a quantitative way where the gaps lie in our knowledge, the team will collect more data using other UV instruments operating at Mars and continue their investigations and update the model.

“With Mars Express, we have a completed the longest survey of the martian atmosphere to date, regardless of the mission. We started in 2004, and now have 17 years of data, which has led us to look at almost seven martian years in a row, including four martian years of combined ozone and water vapour measurements before the UV channel of SPICAM, which measured ozone, ceased operating near the end of 2014. This is unique in the story of planetary exploration,” adds Franck Montmessin, also from LATMOS, and the principal investigator of the SPICAM instrument.

Building on the extraordinary dataset from Mars Express, new results are now coming in from ESA’s Trace Gas Orbiter, which has been circling Mars since October 2016. It carries two instruments, ACS (Atmospheric Chemistry Suite) and NOMAD (Nadir and Occultation for MArs Discovery) that are analysing the martian atmosphere. NASA’s Maven mission also carries ultraviolet equipment that monitors ozone abundance. So, the vital piece of information that finally unlocks this mystery could come at any time.

The long-term monitoring of atmospheric parameters and their variations by Mars Express provides a unique data set with which to study the martian atmosphere as a complex dynamic system.

“Maybe adding up all these years together will eventually hold the key to how the HO_X really controls the martian atmosphere, benefiting our understanding of planetary atmospheres in general,” says Franck Montmessin.