Revealing a High Water Abundance in the Upper Mesosphere of Mars with
ACS onboard TGO
Abstract
We present the first water vapor profiles encompassing the upper
mesosphere of Mars, 100–120 km, far exceeding the maximum altitudes
where remote sensing has been able to observe water to date. Our results
are based on solar occultation measurements by Atmospheric Chemistry
Suite (ACS) onboard the ExoMars Trace Gas Orbiter (TGO). The observed
wavelength range around 2.7 μm possesses strong CO2 and H2O absorption
lines allowing sensitive temperature and density retrievals. We report a
maximum H2O mixing ratio varying from 10 to 50 ppmv at 100–120 km
during the global dust storm (GDS) of Martian Year (MY) 34 and around
southern summer solstice of MY 34 and 35. During other seasons water
remains persistently below ~2 ppmv. We claim that
contributions of the MY34 GDS and perihelion periods into the projected
hydrogen escape from Mars are nearly equivalent.