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Photochemical depletion of heavy CO isotopes in the Martian atmosphere


The atmosphere of Mars is enriched in heavy isotopes with respect to Earth as a result of the escape of the atmosphere to space over billions of years. Estimating this enrichment requires a rigorous understanding of all atmospheric processes that contribute to the evolution of isotopic ratios between the lower and upper atmosphere, where escape processes take place. We combine measurements of CO vertical profiles obtained by the Atmospheric Chemistry Suite on board the ExoMars Trace Gas Orbiter with the predictions of a photochemical model and find evidence of a process of photochemistry-induced fractionation that depletes the heavy isotopes of C and O in CO (δ13C = −160 ± 90‰ and δ18O = −20 ± 110‰). In the upper atmosphere, accounting for this process reduces the escape fractionation factor by ~25%, suggesting that less C has escaped from the atmosphere of Mars than previously thought. In the lower atmosphere, incorporation of this 13C-depleted CO fractionation into the surface could support the abiotic origin of recently found Martian organics.

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Fig. 1: Example of ACS MIR spectra from secondary grating position 6.
Fig. 2: Vertical profiles of the C and O isotopic ratios in CO in the atmosphere of Mars.
Fig. 3: Photochemistry-induced fractionation of the C isotopes in the atmosphere of Mars.

Data availability

The datasets generated by the ExoMars TGO 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) can be downloaded from a Zenodo repository57.

Code availability

The spectral fitting and retrievals were performed using the NEMESIS radiative transfer and retrieval algorithm, which can be downloaded from a Zenodo repository58. The photochemical code used to model the isotopic fractionation in the atmosphere of Mars can be downloaded from a Zenodo repository59. Interested users are encouraged to contact the corresponding author for the usage of these tools.


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We thank N. Thomas, M. Read (University of Bern), C. Marriner (The Open University) and the TGO/CaSSIS team for providing supporting images. We thank B. Alday for providing a supporting animation for outreach purposes. 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 the UK Space Agency and Science and Technology Facilities Council (ST/Y000234/1, ST/V002295/1, ST/V005332/1 and ST/X006549/1), Roscosmos, the National Centre for Space Studies of France (CNES) and the Ministry of Science and Education of Russia. Science operations are funded by Roscosmos and ESA.

Author information

Authors and Affiliations



Atmospheric retrievals from the ACS MIR spectra and interpretation of the retrieved parameters were performed by J.A. A.T., M.R.P., A.A.F., F.M., J.P.M., K.S.O., D.A.B. and O.K. provided input and help with spectral fitting and retrievals. Processing of the spectra was done by L.B. at LATMOS and by A.T. at IKI. F.L., M.R.P., J.A.H. and K.R. provided input into the development of the photochemical model. 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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Nature Astronomy thanks Daniel Lo and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Summary of the photolysis cross sections used in this study.

Panel a shows a comparison between the cross sections tabulated in the Mars PCM (blue line) and those calculated by Schmidt et al.19 (red line) at 295 K. In addition, the combined cross sections used in this study are shown with a x10 offset (black line). Panel b shows the cross sections calculated by Schmidt et al.19 for the different isotopologues. Panel c shows the ratio between the cross sections of the major and minor isotopes, convolved with a Gaussian function with a FWHM of 2.5 nm to smooth out high frequency variations.

Extended Data Fig. 2 Photolysis J-values of 12C16O2 and the isotope effect.

Panel a shows the calculated photolysis J-values of 12C16O2 at three different solar zenith angles. Panel b shows the ratio between the photolysis rates of 12C16O2 over those of 13C16O2 and 18O12C16O.

Extended Data Fig. 3 Isotopic fractionation during the recombination of CO and OH into CO2 and H.

The blue and red dots represent the ratio between the reaction rates of C18O/C16O and 13CO/12CO, measured by Stevens et al.25. The black lines are a second-order polynomial fit used to capture the pressure dependence of this fractionation effect. Values of 18k/16k = 1.012 and 13k/12k = 1.006 are applicable to the conditions of the Martian atmosphere.

Extended Data Fig. 4 Evolution of the isotopic ratios in the 1D photochemical model.

The four panels show the carbon (a, b) and oxygen (c, d) isotopic ratios in CO2 (a,c) and CO (b,d) as a function of time as the simulation from the photochemical model progresses, together with the ACS averaged isotopic ratios in CO with uncertainties lower than 0.15 in standard units (black lines). The isotopic ratios in CO2 converge rapidly, as they are mostly affected by diffusive separation, which produces a decrease of the isotopic ratios above the homopause according to their own mass-dependent scale heights. The isotopic ratios in CO are affected by both diffusive separation and by the photochemistry-induced fractionation. The photochemistry of the atmosphere produces a depletion in the 13C/12C and 18O/16O ratios, whose effect is stronger for the former of the two, in line with the ACS observations.

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Supplementary Information

Supplementary Figs. 1–6.

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Alday, J., Trokhimovskiy, A., Patel, M.R. et al. Photochemical depletion of heavy CO isotopes in the Martian atmosphere. Nat Astron (2023).

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