Water heavily fractionated as it ascends on Mars as revealed by ExoMars/NOMAD

Abstract

Isotopic ratios and, in particular, the water D/H ratio are powerful tracers of the evolution and transport of water on Mars. From measurements performed with ExoMars/NOMAD, we observe marked and rapid variability of the D/H along altitude on Mars and across the whole planet. The observations (from April 2018 to April 2019) sample a broad range of events on Mars, including a global dust storm, the evolution of water released from the southern polar cap during southern summer, the equinox phases, and a short but intense regional dust storm. In three instances, we observe water at very high altitudes (>80 km), the prime region where water is photodissociated and starts its escape to space. Rayleigh distillation appears the be the driving force affecting the D/H in many cases, yet in some instances, the exchange of water reservoirs with distinctive D/H could be responsible.

Annual Appearance of Hydrogen Chloride on Mars and a Striking Similarity With the Water Vapor Vertical Distribution Observed by TGO/NOMAD

“…One of the most remarkable capabilities of NOMAD is its high spectral resolution in the infrared range. This supports investigations of: (a) vertical profiles for atmospheric constituents such as carbon dioxide, carbon monoxide, water vapor, and their isotopic ratios (Aoki et al, 2019;Vandaele et al, 2019;Villanueva et al, 2021); and (b) sensitive searches for organic species (such as CH 4 , C 2 H 4 , C 2 H 6 , H 2 CO; Korablev et al, 2019;Knutsen et al 2021) and other trace gases (such as HCl, HCN, HO 2 , H 2 S, N 2 O, and OCS) by solar occultation measurements with the SO channel. In this study, we analyze the solar occultation measurements acquired by the NOMAD SO channel during the period from April 22, 2018 to December 18, 2020 (from a solar longitude (L s ) of 163° in MY 34 to L s = 333° in MY 35), which covers southern summer periods in MY 34 and 35.…”

Revealing a High Water Abundance in the Upper Mesosphere of Mars with ACS onboard TGO

“…The new ACS/TGO dataset confirms those conclusions for altitudes below 100 km in MY34-MY35 (Fedorova et al 2020, Alday et al, 2021. In parallel, during the perihelion season, the D/H ratio in water decreases with altitude from 4-6 times SMOW (Standard Mean Ocean Water) in the lower atmosphere to 2-3 times in the middle one (50-70 km) as measured by ACS MIR (Alday et al, 2021) and NOMAD (Villanueva et al, 2021) spectrometers. Alday et al (2021) show that ultraviolet H 2 O photolysis dominates the production of H relative to D atoms in the upper atmosphere.…”

Revealing a High Water Abundance in the Upper Mesosphere of Mars With ACS Onboard TGO

“…10.1029/2021GL093411 6 of 9 ] ions measured under two scenarios: low water, corresponding to 2 ppm prescribed at 80 km in a non-GDS case; and high water of 40 ppm in a GDS case (Stone et al, 2020;as corrected in March 2021). Notably, all the solar occultation observations performed by TGO and MEX to date (Aoki et al, 2019;Fedorova et al, 2018Fedorova et al, , 20202021;Villanueva et al, 2021), including the present data set (Figures 3e and 3f), report even higher water vapor VMRs at 80 km, of 50-80 ppm during the GDS. Stone et al (2020) indicate a systematic uncertainty of 69% on their neutral H 2 O inference, which is consistent with observed MAVEN, TGO, and MEX values within such error bars.…”

“…The new ACS/TGO data set confirms those conclusions for altitudes below 100 km in MY34–MY35 (Alday et al., 2021; Fedorova et al., 2020). In parallel, during the perihelion season, the D/H ratio in water decreases with altitude from 4 to 6 times SMOW (Standard Mean Ocean Water) in the lower atmosphere to 2–3 times in the mesosphere (50–70 km) as measured by ACS MIR (Alday et al., 2021) and NOMAD (Villanueva et al., 2021) spectrometers. Alday et al.…”