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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The datasets generated by the ExoMars Trace Gas Orbiter instruments analysed in this study are available in the ESA Planetary Science Archive (PSA) repository, https://archives.esac.esa.int/psa, 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 https://github.com/juanaldayparejo/opendata.
Jakosky, B. M. Mars volatile evolution: evidence from stable isotopes. Icarus 94, 14–31 (1991).
Owen, T., Maillard, J. P., de Bergh, C. & Lutz, B. L. Deuterium on Mars: the abundance of HDO and the value of D/H. Science 240, 1767–1767 (1988).
Webster, C. R. et al. Isotope ratios of H, C, and O in CO2 and H2O of the Martian atmosphere. Science 341, 260–263 (2013).
Villanueva, G. L. et al. Strong water isotopic anomalies in the Martian atmosphere: probing current and ancient reservoirs. Science 348, 218–221 (2015).
Krasnopolsky, V. A. Variations of the HDO/H2O ratio in the Martian atmosphere and loss of water from Mars. Icarus 257, 377–386 (2015).
Aoki, S. et al. Seasonal variation of the HDO/H2O ratio in the atmosphere of Mars at the middle of northern spring and beginning of northern summer. Icarus 260, 7–22 (2015).
Encrenaz, T. et al. A map of D/H on Mars in the thermal infrared using EXES aboard SOFIA. Astron. Astrophys. 586, A62 (2016).
Encrenaz, T. et al. New measurements of D/H on Mars using EXES aboard SOFIA. Astron. Astrophys. 612, A112 (2018).
Khayat, A. S., Villanueva, G. L., Smith, M. D. & Guzewich, S. D. IRTF/CSHELL mapping of atmospheric HDO, H2O and D/H on Mars during northern summer. Icarus 330, 204–216 (2019).
Vandaele, A. C. et al. Martian dust storm impact on atmospheric H2O and D/H observed by ExoMars Trace Gas Orbiter. Nature 568, 521–525 (2019).
Alday, J. et al. Oxygen isotopic ratios in Martian water vapour observed by ACS MIR on board the ExoMars Trace Gas Orbiter. Astron. Astrophys. 630, A91 (2019).
Villanueva, G. L. et al. Water heavily fractionated as it ascends on Mars as revealed by ExoMars/NOMAD. Sci. Adv. 7, eabc8843 (2021).
Carr, M. H. & Clow, G. D. Martian channels and valleys: their characteristics, distribution, and age. Icarus 48, 91–117 (1981).
Baker, V. R. Water and the Martian landscape. Nature 412, 228–236 (2001).
Krasnopolsky, V. On the deuterium abundance on Mars and some related problems. Icarus 148, 597–602 (2000).
Krasnopolsky, V. A. Mars’ upper atmosphere and ionosphere at low, medium, and high solar activities: implications for evolution of water. J. Geophys. Res. Planets 107, 11-1–11-11 (2002).
Fox, J. L. & Hać, A. Isotope fractionation in the photochemical escape of O from Mars. Icarus 208, 176–191 (2010).
Cangi, E. M., Chaffn, M. S. & Deighan, J. Higher Martian atmospheric temperatures at all altitudes increase the D/H fractionation factor and water loss. J. Geophys. Res. Planets 125, e2020JE006626 (2020).
Krasnopolsky, V. A. Photochemistry of water in the Martian thermosphere and its effect on hydrogen escape. Icarus 321, 62–70 (2019).
Clarke, J. T. et al. A rapid decrease of the hydrogen corona of Mars: the Martian hydrogen corona. Geophys. Res. Lett. 41, 8013–8020 (2014).
Chaffin, M. S. et al. Unexpected variability of Martian hydrogen escape. Geophys. Res. Lett. 41, 314–320 (2014).
Chaffin, M., Deighan, J., Schneider, N. & Stewart, A. Elevated atmospheric escape of atomic hydrogen from Mars induced by high-altitude water. Nat. Geosci. 10, 174–178 (2017).
Stone, S. W. et al. Hydrogen escape from Mars is driven by seasonal and dust storm transport of water. Science 370, 824–831 (2020).
Cheng, B.-M. et al. Photo-induced fractionation of water isotopomers in the Martian atmosphere. Geophys. Res. Lett. 26, 3657–3660 (1999).
Merlivat, L. & Nief, G. Fractionnement isotopique lors des changements d’état solide–vapeur et liquide–vapeur de l’eau á des températures inférieures á 0 °C. Tellus 19, 122–127 (1967).
Bertaux, J.-L. & Montmessin, F. Isotopic fractionation through water vapor condensation: the deuteropause, a cold trap for deuterium in the atmosphere of Mars. J. Geophys. Res. Planets 106, 32879–32884 (2001).
Montmessin, F., Fouchet, T. & Forget, F. Modeling the annual cycle of HDO in the Martian atmosphere. J. Geophys. Res. Planets 110, E03006 (2005).
Korablev, O. et al. The Atmospheric Chemistry Suite (ACS) of three spectrometers for the ExoMars 2016 Trace Gas Orbiter. Space Sci. Rev. 214, 7 (2018).
Irwin, P. et al. The NEMESIS planetary atmosphere radiative transfer and retrieval tool. J. Quant. Spectrosc. Radiat. Transf. 109, 1136–1150 (2008).
Rodgers, C. D. Inverse Methods for Atmospheric Sounding: Theory and Practice (Series on Atmospheric, Oceanic and Planetary Physics Vol. 2, World Scientific, 2000).
Shaposhnikov, D. S., Medvedev, A. S., Rodin, A. V. & Hartogh, P. Seasonal water ‘pump’ in the atmosphere of Mars: vertical transport to the thermosphere. Geophys. Res. Lett. 46, 4161–4169 (2019).
Olsen, K. S. et al. The vertical structure of CO in the Martian atmosphere from the ExoMars Trace Gas Orbiter. Nat. Geosci. 14, 67–71 (2021).
Neary, L. et al. Explanation for the increase in high-altitude water on Mars observed by NOMAD during the 2018 global dust storm. Geophys. Res. Lett. 47, e2019GL084354 (2020).
Trokhimovskiy, A. et al. Mars’ water vapor mapping by the SPICAM IR spectrometer: five Martian years of observations. Icarus 251, 50–64 (2015).
Fedorova, A. A. et al. Stormy water on Mars: the distribution and saturation of atmospheric water during the dusty season. Science 367, 297–300 (2020).
Aoki, S. et al. Water vapor vertical profiles on Mars in dust storms observed by TGO/NOMAD. J. Geophys. Res. Planets 124, 3482–3497 (2019).
Fedorova, A. et al. Multi-annual monitoring of the water vapor vertical distribution on Mars by SPICAM on Mars Express. J. Geophys. Res. Planets 126, e2020JE006616 (2020).
Montmessin, F., Forget, F., Rannou, P., Cabane, M. & Haberle, R. M. Origin and role of water ice clouds in the Martian water cycle as inferred from a general circulation model. J. Geophys. Res. Planets 109, E10004 (2004).
Liuzzi, G. et al. Strong variability of Martian water ice clouds during dust storms revealed from ExoMars Trace Gas Orbiter/NOMAD. J. Geophys. Res. Planets 125, e2019JE006250 (2020).
Luginin, M. et al. Properties of water ice and dust particles in the atmosphere of Mars during the 2018 global dust storm as inferred from the Atmospheric Chemistry Suite. J. Geophys. Res. Planets 125, e2020JE006419 (2020).
Lamb, K. D. et al. Laboratory measurements of HDO/H2O isotopic fractionation during ice deposition in simulated cirrus clouds. Proc. Natl Acad. Sci. USA 114, 5612–5617 (2017).
Majoube, M. Fractionation factor of 18O between water vapour and ice. Nature 226, 1242–1242 (1970).
Casado, M. et al. Experimental determination and theoretical framework of kinetic fractionation at the water vapour–ice interface at low temperature. Geochim. Cosmochim. Acta 174, 54–69 (2016).
Lefévre, F. Three-dimensional modeling of ozone on Mars. J. Geophys. Res. 109, E07004 (2004).
González-Galindo, F. Extension of a Martian general circulation model to thermospheric altitudes: UV heating and photochemical models. J. Geophys. Res. 110, E09008 (2005).
Lefévre, F. & Krasnopolsky, V. in The Atmosphere and Climate of Mars (eds Haberle, R. M. et al.) 405–432 (Cambridge University Press, 2017).
Bhattacharyya, D., Clarke, J. T., Bertaux, J.-L., Chaufray, J.-Y. & Mayyasi, M. A strong seasonal dependence in the Martian hydrogen exosphere. Geophys. Res. Lett. 42, 8678–8685 (2015).
Halekas, J. S. Seasonal variability of the hydrogen exosphere of Mars. J. Geophys. Res. Planets 122, 901–911 (2017).
Clarke, J. T. et al. Variability of D and H in the Martian upper atmosphere observed with the MAVEN IUVS echelle channel. J. Geophys. Res. Space Phys. 122, 2336–2344 (2017).
Krasnopolsky, V. A., Mumma, M. J. & Gladstone, G. R. Detection of atomic deuterium in the upper atmosphere of Mars. Science 280, 1576–1580 (1998).
Miller, C. E. & Yung, Y. L. Photo-induced isotopic fractionation. J. Geophys. Res. Atmos. 105, 29039–29051 (2000).
Jakosky, B. M. & Phillips, R. J. Mars’ volatile and climate history. Nature 412, 237–244 (2001).
Jakosky, B. M., Pepin, R. O., Johnson, R. E. & Fox, J. Mars atmospheric loss and isotopic fractionation by solar-wind-induced sputtering and photochemical escape. Icarus 111, 271–288 (1994).
Gordon, I. et al. The HITRAN2016 molecular spectroscopic database. J. Quant. Spectrosc. Radiat. Transf. 203, 3–69 (2017).
Régalia, L. et al. Laboratory measurements and calculations of line shape parameters of the H2O–CO2 collision system. J. Quant. Spectrosc. Radiat. Transf. 231, 126–135 (2019).
Devi, V. M. et al. Line parameters for CO2- and self-broadening in the ν1 band of HD16O. J. Quant. Spectrosc. Radiat. Transf. 203, 133–157 (2017).
Devi, V. M. et al. Line parameters for CO2 broadening in the ν2 band of HD16O. J. Quant. Spectrosc. Radiat. Transf. 187, 472–488 (2017).
Devi, V. M. et al. Line parameters for CO2- and self-broadening in the ν3 band of HD16O. J. Quant. Spectrosc. Radiat. Transf. 203, 158–174 (2017).
Forget, F. et al. Improved general circulation models of the Martian atmosphere from the surface to above 80 km. J. Geophys. Res. Planets 104, 24155–24175 (1999).
Quémerais, E. et al. Stellar occultations observed by SPICAM on Mars Express. J. Geophys. Res. 111, E09S04 (2006).
Montmessin, F. et al. Stellar occultations at UV wavelengths by the SPICAM instrument: retrieval and analysis of Martian haze profiles. J. Geophys. Res. 111, E09S09 (2006).
Irwin, P. G. et al. Probable detection of hydrogen sulphide (H2S) in Neptune’s atmosphere. Icarus 321, 550–563 (2019).
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.
The authors declare no competing interests.
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 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).
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). https://doi.org/10.1038/s41550-021-01389-x