Water Vapor Vertical Profiles on Mars in Dust Storms Observed by TGO/NOMAD

It has been suggested that dust storms efficiently transport water vapor from the near-surface to the middle atmosphere on Mars. Knowledge of the water vapor vertical profile during dust storms is important to understand water escape. During Martian Year 34, two dust storms occurred on Mars: a global dust storm (June to mid-September 2018) and a regional storm (January 2019). Here we present water vapor vertical profiles in the periods of the two dust storms (Ls = 162-260 degrees and Ls = 298-345 degrees) from the solar occultation measurements by Nadir and Occultation for Mars Discovery (NOMAD) onboard ExoMars Trace Gas Orbiter (TGO). We show a significant increase of water vapor abundance in the middle atmosphere (40-100 km) during the global dust storm. The water enhancement rapidly occurs following the onset of the storm (Ls similar to 190 degrees) and has a peak at the most active period (Ls similar to 200 degrees). Water vapor reaches very high altitudes (up to 100 km) with a volume mixing ratio of similar to 50 ppm. The water vapor abundance in the middle atmosphere shows high values consistently at 60 degrees S-60 degrees N at the growth phase of the dust storm (Ls = 195 degrees-220 degrees), and peaks at latitudes greater than 60 degrees S at the decay phase (Ls = 220 degrees-260 degrees). This is explained by the seasonal change of meridional circulation: from equinoctial Hadley circulation (two cells) to the solstitial one (a single pole-to-pole cell). We also find a conspicuous increase of water vapor density in the middle atmosphere at the period of the regional dust storm (Ls = 322-327 degrees), in particular at latitudes greater than 60 degrees S. Plain Language Summary The most striking phenomenon on Mars is a planet-encircling storm, "global dust storm." Once it starts, the floating dust covers the whole atmosphere for more than several weeks. Recent studies suggest that dust storms effectively transport water vapor from the near-surface to the middle atmosphere. In June to September 2018 and January 2019, a strong global dust storm and a regional storm occurred on Mars, respectively. This study investigates altitude profiles of water vapor in the Mars atmosphere measured during the dust storms, by using brand-new measurements by Nadir and Occultation for Mars Discovery onboard the ExoMars Trace Gas Orbiter. We confirm that the water vapor expanded into the middle atmosphere, and we find that the water vapor reached very high altitudes (up to 100 km) during the dust storms. The dust storms intensify the atmospheric dynamics and heat the atmosphere. As a result, water vapor is lifted to higher altitudes and distributes along the meridional circulation.