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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The datasets generated by the ExoMars TGO 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) can be downloaded from a Zenodo repository57.
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.
Baker, V. R. Water and the Martian landscape. Nature 412, 228–236 (2001).
Head, J. W. et al. Possible ancient oceans on Mars: evidence from Mars Orbiter Laser altimeter data. Science 286, 2134–2137 (1999).
McElroy, M. B., Yung, Y. L. & Nier, A. O. Isotopic composition of nitrogen: implications for the past history of Mars’ atmosphere. Science 194, 70–72 (1976).
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 (1988).
Hu, R., Kass, D. M., Ehlmann, B. L. & Yung, Y. L. Tracing the fate of carbon and the atmospheric evolution of Mars. Nat. Commun. 6, 10003 (2015).
Hu, R. & Thomas, T. B. A nitrogen-rich atmosphere on ancient Mars consistent with isotopic evolution models. Nat. Geosci. 15, 106–111 (2022).
Scheller, E. L., Ehlmann, B. L., Hu, R., Adams, D. J. & Yung, Y. L. Long-term drying of Mars by sequestration of ocean-scale volumes of water in the crust. Science 372, 56–62 (2021).
Krasnopolsky, V. A., Mumma, M. J. & Gladstone, G. R. Detection of atomic deuterium in the upper atmosphere of Mars. Science 280, 1576–1580 (1998).
Yung, Y. L. et al. HDO in the Martian atmosphere: implications for the abundance of crustal water. Icarus 76, 146–159 (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).
Mahaffy, P. R. et al. Abundance and isotopic composition of gases in the Martian atmosphere from the Curiosity Rover. Science 341, 263–266 (2013).
Fox, J. L. & Hać, A. Velocity distributions of C atoms in CO+ dissociative recombination: implications for photochemical escape of C from Mars. J. Geophys. Res. 104, 24729–24737 (1999).
Fox, J. L. & Hać, A. Isotope fractionation in the photochemical escape of O from Mars. Icarus 208, 176–191 (2010).
Jakosky, B. M. The CO2 inventory on Mars. Planet. Space Sci. 175, 52–59 (2019).
Cui, J., Wu, X.-S., Gu, H., Jiang, F.-Y. & Wei, Y. Photochemical escape of atomic C and N on Mars: clues from a multi-instrument MAVEN dataset. Astron. Astrophys. 621, A23 (2019).
Gröller, H., Lichtenegger, H., Lammer, H. & Shematovich, V. I. Hot oxygen and carbon escape from the martian atmosphere. Planet. Space Sci. 98, 93–105 (2014).
Lo, D. Y., Yelle, R. V., Lillis, R. J. & Deighan, J. I. Carbon photochemical escape rates from the modern Mars atmosphere. Icarus 360, 114371 (2021).
House, C. H. et al. Depleted carbon isotope compositions observed at Gale crater, Mars. Proc. Natl Acad. Sci. USA 119, e2115651119 (2022).
Schmidt, J. A., Johnson, M. S. & Schinke, R. Carbon dioxide photolysis from 150 to 210 nm: singlet and triplet channel dynamics, UV-spectrum, and isotope effects. Proc. Natl Acad. Sci. USA 110, 17691–17696 (2013).
McElroy, M. B. & Donahue, T. M. Stability of the Martian atmosphere. Science 177, 986–988 (1972).
Parkinson, T. D. & Hunten, D. M. Spectroscopy and aeronomy of O2 on Mars. J. Atmos. Sci. 29, 1380–1390 (1972).
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).
Fedorova, A. et al. Climatology of the CO vertical distribution on Mars based on ACS TGO measurements. J. Geophys. Res. Planets 127, e2022JE007195 (2022).
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).
Stevens, C. M. et al. The kinetic isotope effect for carbon and oxygen in the reaction CO + OH. Int. J. Chem. Kinet. 12, 935–948 (1980).
Krasnopolsky, V. A. Solar activity variations of thermospheric temperatures on Mars and a problem of CO in the lower atmosphere. Icarus 207, 638–647 (2010).
Alday, J. et al. Isotopic composition of CO2 in the atmosphere of Mars: fractionation by diffusive separation observed by the ExoMars Trace Gas Orbiter. J. Geophys. Res. Planets 126, e2021JE006992 (2021).
Cheng, B.-M. et al. Photo-induced fractionation of water isotopomers in the Martian atmosphere. Geophys. Res. Lett. 26, 3657–3660 (1999).
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).
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. Variations of the HDO/H2O ratio in the martian atmosphere and loss of water from Mars. Icarus 257, 377–386 (2015).
Wright, I. P., Carr, R. H. & Pillinger, C. T. Carbon abundance and isotopic studies of Shergotty and other shergottite meteorites. Geochim. Cosmochim. Acta 50, 983–991 (1986).
Kass, D. M. Change in the Martian Atmosphere (California Institute of Technology, 1999).
Jakosky, B. M. et al. Loss of the Martian atmosphere to space: present-day loss rates determined from MAVEN observations and integrated loss through time. Icarus 315, 146–157 (2018).
Fox, J. L. & Hać, A. B. Photochemical escape of oxygen from Mars: a comparison of the exobase approximation to a Monte Carlo method. Icarus 204, 527–544 (2009).
Irwin, P. G. J. et al. The NEMESIS planetary atmosphere radiative transfer and retrieval tool. J. Quant. Spectrosc. Radiat. Transf. 109, 1136–1150 (2008).
Alday, J. et al. Isotopic fractionation of water and its photolytic products in the atmosphere of Mars. Nat. Astron. 5, 943–950 (2021).
Gordon, I. E. et al. The HITRAN2020 molecular spectroscopic database. J. Quant. Spectrosc. Radiat. Transf. 277, 107949 (2022).
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).
Quémerais, E. et al. Stellar occultations observed by SPICAM on Mars Express. J. Geophys. Res. 111, E09S04 (2006).
Holmes, J. A., Lewis, S. R. & Patel, M. R. OpenMARS: a global record of martian weather from 1999 to 2015. Planet. Space Sci. 188, 104962 (2020).
Belyaev, D. A. et al. Thermal structure of the middle and upper atmosphere of Mars from ACS/TGO CO2 spectroscopy. J. Geophys. Res. Planets 127, e2021JE006992 (2022).
Krasnopolsky, V. A. Photochemistry of the Martian atmosphere (mean conditions). Icarus 101, 313–332 (1993).
Lefèvre, F. et al. Relationship between the ozone and water vapor columns on Mars as observed by SPICAM and calculated by a global climate model. J. Geophys. Res. Planets 126, e2021JE006838 (2021).
Lefèvre, F., Lebonnois, S., Montmessin, F. & Forget, F. Three-dimensional modeling of ozone on Mars. J. Geophys. Res. Planets 109, E07004 (2004).
Cariolle, D. et al. ASIS v1.0: an adaptive solver for the simulation of atmospheric chemistry. Geosci. Model Dev. 10, 1467–1485 (2017).
Hobbs, R., Shorttle, O., Madhusudhan, N. & Rimmer, P. A chemical kinetics code for modelling exoplanetary atmospheres. Mon. Not. R. Astron. Soc. 487, 2242–2261 (2019).
Verwer, J. G., Spee, E. J., Blom, J. G. & Hundsdorfer, W. A second-order Rosenbrock method applied to photochemical dispersion problems. SIAM J. Sci. Comput. 20, 1456–1480 (1999).
Forget, F. et al. Improved general circulation models of the Martian atmosphere from the surface to above 80 km. J. Geophys. Res. 104, 24155–24175 (1999).
Hunten, D. M. The escape of light gases from planetary atmospheres. J. Atmos. Sci. 30, 1481–1494 (1973).
Chaffin, M. S., Deighan, J., Schneider, N. M. & Stewart, A. I. F. Elevated atmospheric escape of atomic hydrogen from Mars induced by high-altitude water. Nat. Geosci. 10, 174–178 (2017).
Huestis, D. L. & Berkowitz, J. Critical evaluation of the photoabsorption cross section of CO2 from 0.125 to 201.6 nm at room temperature. Bull. Am. Astron. Soc. 42, 972 (2010).
Lewis, B. R. & Carver, J. H. Temperature dependence of the carbon dioxide photoabsorption cross section between 1200 and 1970 Å. J. Quant. Spectrosc. Radiat. Transf. 30, 297–309 (1983).
Parkinson, W. H., Rufus, J. & Yoshino, K. Absolute absorption cross section measurements of in the wavelength region 163–200 nm and the temperature dependence. Chem. Phys. 290, 251–256 (2003).
Stark, G., Yoshino, K., Smith, P. L. & Ito, K. Photoabsorption cross section measurements of CO2 between 106.1 and 118.7 nm at 295 and 195 K. J. Quant. Spectrosc. Radiat. Transf. 103, 67–73 (2007).
Yoshino, K. et al. Absorption cross section measurements of carbon dioxide in the wavelength region 118.7–175.5 nm and the temperature dependence. J. Quant. Spectrosc. Radiat. Transf. 55, 53–60 (1996).
Alday, J. Isotopic composition of CO, CO2 and H2O in the atmosphere of Mars from ACS/TGO. Zenodo https://doi.org/10.5281/ZENODO.7499466 (2023).
Irwin, P. nemesiscode/radtrancode: NEMESIS. Zenodo https://doi.org/10.5281/ZENODO.5816714 (2022).
Alday, J. juanaldayparejo/pchempy-dist: 1D photochemical model with isotopic fractionation for Mars’ atmosphere. Zenodo https://doi.org/10.5281/ZENODO.7807731 (2023).
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.
The authors declare no competing interests.
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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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Alday, J., Trokhimovskiy, A., Patel, M.R. et al. Photochemical depletion of heavy CO isotopes in the Martian atmosphere. Nat Astron (2023). https://doi.org/10.1038/s41550-023-01974-2