[U-08] Biogeochemistry of CO world

[U08-04] Carbon and Oxygen isotope ratio in CO in the Martian atmosphere observed by TGO/NOMAD

*Kimie Shiobara¹, Shohei Aoki^2,3, Nao Yoshida¹, Hiromu Nakagawa¹, Naoki Terada¹, Yasumasa Kasaba¹, Isao Murata¹, Tatsuya Yoshida¹, A. C. Vandaele³, L. Trompet³, G. Liuzzi⁴, I. R. Thomas³, G. L. Villanueva⁵, M. A. Lopez-Valverde⁶, A. Brines⁶, M. R. Patel⁷, S. Faggi^5,8, F. Daerden³, J. T. Erwin³, B. Ristic³, G. Bellucci⁹, J. J. Lopez-Moreno⁶, Hiroyuki Kurokawa¹⁰, Yuichiro Ueno¹⁰ (1.Tohoku University, 2.The University of Tokyo, 3.Royal Belgian institute for space aeronomy, 4.Scuola di Ingegneria, Universita degli Studi della Basilicata, 5.NASA Goddard Space Flight Center, 6.Instituto de Astrofisica de Andalucia, Glorieta de la Astronomia, 7.School of Physical Sciences, The Open University, 8.American University, 9.Istituto Nazionale di Astrofisica, 10.Tokyo Institute of Technology)

Keywords:isotope, Mars, CO, TGO, NOMAD, isotope fractionation

Carbon isotope ratios in the Martian atmosphere are important tracers to constrain the atmospheric evolutionary history and origin of organics at surface of Mars (Jakosky et al., 1994; House et al., 2021). The carbon isotopic ratio can be fractionated by degassing, atmospheric escape to space (e.g., Lammer et al, 2020), and recent photochemical research shows that it is also strongly affected by isotope fractionation effects via photodissociation of CO₂. Theoretical calculations by Schmidt et al. (2013) suggest that the absorption cross sections for CO₂ photodissociation differ by several hundred per mil, and our Mars atmospheric photochemical calculations, which are based on these results, also show that CO carbon isotope ratio is significantly depleted compared to that of CO₂. (Yoshida et al., under review). We need observations of the carbon isotope ratio in both CO₂ and CO to understand the carbon isotope fractionation processes. However, the previous observations of carbon isotope ratios are limited to CO₂ (delta13C=46+/-4 per mil, Webster et al.,2013; delta13C~0 per mil below 100 km, Alday et al.,2021). In addition, Schmidt et al. (2013) also showed that oxygen as well as carbon is fractionated during photodissociation of CO₂, and derivation of oxygen isotope ratio is also expected to be an indicator of photodissociation. ExoMars Trace Gas Orbiter (TGO), which started science operations in 2018, has multiple high-spectral resolution spectrometers that carry out solar occultation measurements, which allows us to perform a sensitive measurement of isotopic ratios on Mars. In this study, we attempt to derive the vertical distributions of carbon and oxygen isotopic ratios in Martian CO, using infrared spectral data observed by Nadir and Occultation for MArs Discovery (NOMAD) on board TGO. This is the first attempt to derive carbon and oxygen isotope ratios in CO on Mars simultaneously and we also attempt to constrain the CO isotopic fractionation processes.

We perform the retrievals within two different spectral ranges: order 185 (4157-4190 cm^-1) and order 186 (4180-4213 cm^-1), the full spectral range respectively. For the retrieval, we use a radiative transfer and inversion code, ASIMUT (Vandaele et al., 2006), which uses Optimal Estimation Method (OEM) (Rodgers, 2000) to find the best parameters to fit the data. A-priori profiles of pressure and CO₂ volume mixing ratio in the Martian atmosphere for radiative transfer calculations are obtained from the theoretical predictions by GEM-Mars model (Daerden et al., 2019), and those for temperatures are obtained from the CO₂ absorption lines (Trompet et al. 2023), which are observed simultaneously. We consider the total amount of ¹²C¹⁶O, ¹³C¹⁶O and ¹²C¹⁸O volume mixing ratio along the line of sight as the free parameters and perform the retrievals for each altitude independently. We only selected the results retrieved in 30~50 km altitude with a confidence level of at least 3-sigma.

First, we analyzed 9 orbits data from 2022/3/1 to 2022/4/8 as an initial result. About the retrieved isotope ratios, when we take the weighted average from 30 to 50 km in all of the 9 orbits, the averaged value and the standard derivation are delta13C=-498+/-140 per mil (order 185), -138+/-88 per mil (order 186), and delta18O=-195+/-187 per mil. We think the reason delta13C values are not consistent in the two wavelength ranges is due to the assumed atmospheric temperature uncertainties. Even if we take into account those large errors, 13C depletion is suggested, and the results provide observational evidence that CO isotope distribution due to CO₂ photodissociation, as predicted by Schmidt et al. (2013) and Yoshida et al. (under review). Second, we increased the number of analyzed data, and performed similar calculations for 61 orbits from 2022/3/1 to 2022/12/24 at the same altitude range, 30~50 km. The derived isotope ratios were delta13C= -457+/-393 per mil (order 185), -183+/-208 per mil (order 186), and delta18O=70+/-307 per mil. Unlike the results of the 9 orbits analysis as mentioned above, we could find variations in the isotopic ratios. Especially carbon and oxygen isotope ratio tend to increase with altitude. We plan to analyze in detail the causes of these variations.