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

Juan Alday,Margaux Vals,Alexey Shakun,Franck Montmessin,Colin F Wilson,Franck Lefévre,Andrey Patrakeev,Anna A Fedorova,Jean-Loup Bertaux,Lucio Baggio,Patrick G J Irwin,Denis A Belyaev,Kevin S Olsen,Loïc Rossi,Alexander Trokhimovskiy,Oleg Korablev

doi:10.1038/s41550-021-01389-x

Abstract

The current Martian atmosphere is about five times more enriched in deuterium than Earth’s, providing 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 roles in influencing the proportions of atomic D and H populations have remained speculative. Here we report HDO and H2O 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 H2O is a more important factor in determining the isotopic composition of the dissociation products. Three years of observations with the ExoMars TGO Atmospheric Chemistry Suite have clarified the mechanism of atmospheric water loss from Mars, and particularly the fraction of deuterium compared with hydrogen. Analysis of several isotopologues of water in the mid-atmosphere of Mars shows that atomic H and D are produced at perihelion in relative amounts controlled by photolysis. These atoms are able to escape from the atmosphere.

Highlights

The isotope composition of the Martian atmosphere has often been used as a means to understand its evolution[1 ]
Earth and Mars were assembled out of the same material, the relative enrichment of heavy isotopes in Martian atmospheric water vapour with respect to Earth suggests that the escape of water throughout the Martian history has been substantial2–12, consistent with the geomorphological and mineralogical imprints on the surface, which require the presence of liquid water on ancient Mars[13, 14]
This production was assumed to be sourced by long-lived molecular hydrogen formed in the lower atmosphere and slowly ascending to the upper atmosphere, where it is dissociated into HCO+

Summary

Introduction

The isotope composition of the Martian atmosphere has often been used as a means to understand its evolution[1 ]. Estimations of the size of the ancient water reservoir on Mars from its isotope composition depend on the present-day atmospheric ratio, and on the net escape fractionation factor, f .

Results

Conclusion