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


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.

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Fig. 1: Example of ACS MIR spectra and summary of the retrieval scheme.
Fig. 2: Climatology from the retrieved atmospheric parameters.
Fig. 3: Seasonal evolution of the isotopic fractionation of D/H during the photodissociation of water vapour.

Data availability

The datasets generated by the ExoMars Trace Gas Orbiter instruments analysed in this study are available in the ESA Planetary Science Archive (PSA) repository,, following a six months prior access period, following the ESA Rules on Information, Data and Intellectual Property. The data products generated in this study (retrieved atmospheric parameters) are available on

Code availability

The spectral fitting and retrievals were performed using the NEMESIS radiative transfer and retrieval algorithm29. The code can be downloaded from


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The ExoMars mission is a joint mission of the European Space Agency (ESA) and Roscosmos. The ACS experiment is led by the Space Research Institute (IKI) in Moscow, assisted by LATMOS in France. This work was funded by Roscosmos, the National Centre for Space Studies of France (CNES), the Ministry of Science and Education of Russia, the Natural Sciences and Engineering Research Council of Canada (NSERC) (PDF 178-516895-2018) and the UK Space Agency (ST/T002069/1, ST/R001502/1 and ST/P001572/1). L.R. acknowledges the support he received from the Excellence Laboratory ‘Exploration Spatiale des Environnements Planétaires (ESEP)’ and from CNES. M.V. is supported by DIM ACAV labelled by the Île-de-France region for the research (Domaine d’Intérêt Majeur, Astrophysique et Conditions d’Apparition de la Vie). Science operations are funded by Roscosmos and ESA.

Author information




Atmospheric retrievals from the ACS MIR spectra and interpretation of the retrieved parameters were performed by J.A. A.T., P.G.J.I., C.F.W., F.M., A.A.F., D.A.B. and K.S.O. provided input and help with spectral fitting and retrievals. Processing of the spectra was done at LATMOS by L.B. and at IKI by A.T. A.A.F. provided ACS NIR profiles for comparison and validation of the dataset. F.L. generated the photolysis rates of water vapour. F.M., F.L., O.K., M.V., L.R. and J.-L.B. provided input on the analysis and interpretation of the retrieved atmospheric parameters. The ACS instrument was operated by A.T., A.P. and A.S. All co-authors contributed to the preparation of the manuscript.

Corresponding author

Correspondence to Juan Alday.

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The authors declare no competing interests.

Additional information

Peer review information Nature Astronomy thanks Shohei Aoki, Vladimir Krasnopolsky and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Histogram of the measured D/H and 18O/16O isotope ratios, as well as their relation with the water vapour mixing ratio and temperature.

Only the points with uncertainties of D/H < 1 VSMOW and 18O/16O < 0.13 VSMOW are shown. The black dashed lines on the histogram plots represent typical measurement uncertainties, centred at D/H = 4.9 ± 0.4 VSMOW and 18O/16O = 1.14 ± 0.11 VSMOW. The colour of the points on the plots in the middle and right columns represents the density of measured points. The red cross in the temperature panels represents the most frequently measured value of the isotope ratios at temperatures > 180 K, which we take as the average non-fractionated isotope ratios in the Martian atmosphere.

Extended Data Fig. 2 Evolution of the water vapour mixing ratio and the D/H and 18O/16O isotopic ratios in the perihelion and aphelion seasons of MY34 and MY35.

The different panels show the retrieved profiles separated in different seasons, with the colour of the lines representing the latitude of the observations. For the clarity of the figure, only the points with uncertainties of D/H < 1 VSMOW and 18O/16O < 0.13 VSMOW are shown.

Extended Data Fig. 3 Comparison of the estimated impact of fractionation during photolysis for the perihelion and aphelion seasons during MY34 and MY35.

a, Calculated H2O dissociation rates, with the colour of the lines representing the latitude of the observations. b, Column-integrated photolysis rates (red dots) as a function of latitude. The yellow dots represent the column-integrated photolysis rates when using the part of the column in which the measurements are sensitive to HDO. c, Fractionation factor R between the D/H ratio representative of water vapour in the lower atmosphere and that of the photolysis products for the cases of photolysis-induced (black line) and condensation-induced fractionation (coloured lines). d, Column-integrated fractionation factor R as a function of latitude for the cases of photolysis-induced (blue dots) and condensation-induced (yellow dots) fractionation, as well as the combination of the two (red dots). The condensation-induced fractionation factor is only plotted if the column-integrated photolysis rate using the part of the column in which the measurements are sensitive to HDO (yellow dots in b) is at least 75% of that calculated when using the whole column (red dots in b).

Extended Data Fig. 4 Evolution of the water vapour photolysis rates and D/H fractionation factor.

a, Latitudinal coverage of the ACS MIR observations, with the colour of the points representing the column-integrated photolysis rate of H2O. b, Column-integrated photolysis rate of H2O. c, Column-integrated photolysis rates of H2O above (green dots) and below (yellow dots) 60 km. d, Column-integrated fractionation factor R for the cases of photolysis-induced (blue dots) and condensation-induced (yellow dots) fractionation, as well as the combination of the two (red dots).

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Alday, J., Trokhimovskiy, A., Irwin, P.G.J. et al. Isotopic fractionation of water and its photolytic products in the atmosphere of Mars. Nat Astron 5, 943–950 (2021).

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