Isotopic fractionation of water and its photolytic products in the atmosphere of Mars

Abstract

The current Martian atmosphere is about five times more enriched in deuterium than Earth's, providing a direct testimony that Mars hosted vastly more water in its early youth than nowadays. Estimates of the total amount of water lost to space from the current mean D/H value depend on a rigorous appraisal of the relative escape between deuterated and non-deuterated water. Isotopic fractionation of D/H between the lower and the upper atmospheres of Mars has been assumed to be controlled by water condensation and photolysis, although their respective role in influencing the relative proportion of atomic D and H populations has remained speculative. Here we report HDO and H 2 O profiles observed by the Atmospheric Chemistry Suite (ExoMars Trace Gas Orbiter) in orbit around Mars that, once combined with expected photolysis rates, reveal the prevalence of the perihelion season for the formation of atomic H and D at altitudes relevant for escape. In addition, while condensation-induced fractionation is the main driver of variations of D/H in water vapour, the differential photolysis of HDO and H 2 O is a more important factor in determining the isotopic composition of the dissociation products.

Explaining NOMAD D/H Observations by Cloud‐Induced Fractionation of Water Vapor on Mars

“…However, given the abundances of water vapor and HDO in the lower and middle atmosphere, we expect these processes to have only an impact on the D/H variability in the upper atmosphere, and so not to show up in total column D/H (see also Alday et al. (2021) for a recent prediction on HDO photolysis). Also regolith adsorption is not yet included in our GCM simulations.…”

Isotopic Composition of CO2 in the Atmosphere of Mars: Fractionation by Diffusive Separation Observed by the ExoMars Trace Gas Orbiter

“…Based on the differential photolysis cross sections of $${\mathrm{H}}_{2}^{16}$$O and $${\mathrm{H}}_{2}^{18}$$O (Miller & Yung, 2000), Alday et al. (2021) estimated that the 18 O/ 16 O isotopic ratio of the formed OH molecules would be approximately 0.975 times lower than that of the parent H 2 O molecules. The recombination of the OH molecules into CO 2 might therefore result in the transfer of oxygen depleted in 18 O from H 2 O to CO 2 .…”

“…The atmospheric temperature profile can be retrieved in this spectral range using the CO 2 absorption under the assumptions of a known CO 2 volume mixing ratio and an atmosphere in hydrostatic equilibrium (e.g., Quémerais et al, 2006). Such an approach, previously used for the derivation of the isotopic ratios in H 2 O from ACS MIR spectra (Alday et al, 2019(Alday et al, , 2021, requires the simultaneous retrieval of all tangent heights. While this methodology is optimal when using relatively narrow spectral windows, another method can be applied when using a broader spectral range.…”

“…In this study, the ILS is parameterized using a double Gaussian function as described by Alday et al (2019), in which the tuning parameters are fitted in parallel with the rest of the atmospheric properties. This approach has been validated against simultaneous solar occultation measurements of H 2 O, CO, and CO 2 made by ACS NIR (Alday et al, 2019(Alday et al, , 2021Fedorova et al, 2020;Olsen et al, 2021).…”

“…The isotopic composition of H 2 O in the Martian atmosphere was reported byAlday et al (2021) using ACS MIR solar occultation observations also made with secondary grating position 4. This analysis revealed an average non-fractionated isotopic ratio of 18 O/ 16 O (H 2 O) = 1.140 ± 0.080 VSMOW (δ 18 O (H 2 O) = 140 ± 80‰), which is therefore more enriched in the heavy isotope than that in CO 2 (δ 18 O (CO 2 ) = −29 ± 38‰).…”