Measuring the 13C/12C in the lower atmosphere of Mars with NOMAD/TGO: challenges and interpretation

The two infrared spectrometers onboard the ExoMars Trace Gas Orbiter (TGO) spacecraft, the Atmospheric Chemistry Suite (ACS) and the Nadir and Occultation for MArs Discovery (NOMAD) have been providing observations since April 2018 and are a unique source of information to unveil the vertical structure of the atmosphere of Mars in a level of detail never previously seen. In this work, we focus on understanding how the NOMAD data can be used to constrain the isotopic composition of atmospheric carbon. This has been measured on several occasions on the surface [1], and in near-surface atmospheric samples [2,3], and is of great interest to trace the history of atmospheric loss and differences with other inner solar system bodies. Additionally, proper characterization of the uncertainties of derived isotopic ratios is a very important proxy towards a better understanding of the instrument limitations and performances.

NOMAD is a 3-channel spectrometer working in the spectral ranges 0.2-0.65 μm (UVIS channel) and 2.2-4.3 μm (SO and LNO channels). The data taken by the Solar Occultation (SO) channel measurements in the infrared channel is used in this work. SO is an echelle grating spectrometer with relatively high spectral resolution (λ/dλ~17,000), and an Acousto Optical Tunable Filter (AOTF) used to instantaneously switch the observed diffraction orders, with a typical cycle of 1 second. This yields a vertical sampling of less than 1 km from the surface to 250 km of altitude. In this work we use NOMAD Full Scan data to gather appropriate information about ¹²C, ¹³C and temperature. We use full scans acquired between Apr/2018 (LS ~160 MY34) and Dec/2021 (LS ~137 MY36), a type of data in which many diffraction orders are measured along the vertical. From these we derive the average rotational temperature along the line of sight with an accuracy of about 5 K [4], which then can be used to derive the column density at a specific tangent altitude – in particular - of ¹²CO₂ and ¹³CO₂. The accuracy of the retrievals is also possible thanks to recent calibration efforts for the AOTF transfer function and the ILS of the SO channel, which are fully parameterized with the diffraction order [5]. Retrievals are performed using the Planetary Spectrum Generator (PSG, [6,7]), a full general purpose radiative transfer code, which contains a dedicated retrieval module based on Optimal Estimation and is configured with all the instrumental parameters of NOMAD SO natively accounted for.

Once all the retrievals are completed, we filter the values by excluding values with low SNR and those spectra where the intensity of the spectral lines is smaller than 5 times the radiometric noise.

By averaging the different measurements, is possible to get an average vertical profile of the C isotopic ratio. In this work we specifically explore analytical criteria to quantify the uncertainty of the ¹³C/¹²C at each altitude and whether the dispersion in the retrieved values is of stochastic nature - meaning that it is not dominated by systematics - or it is related to the limitations of the instrument and the accuracy of spectroscopy. In particular, we conduct specific analyses that quantify the expected uncertainty of a single measurement depending on the observed column of CO₂ and the prescribed radiometric noise.

We find that the ¹³C/¹²C in CO2 below 50 km can be consistently quantified, complementing the current results by ACS [8] and MSL [2,3]. We estimate the ¹³C/¹²C in CO₂ in the lower atmosphere (10-25 km) in 1.027+/-0.030 VPDB, largely consistent with the value observed by MSL in surface minerals and the atmosphere. The value is fairly constant throughout the atmosphere with oscillations within the error bars. Upper atmosphere values (>50 km) do not show any inconsistency with the values reported using ACS observation above 60 km [8]. The precision of the measurement is limited by the radiometric SNR, and by the precision with which the spectral continuum can be quantified, which plays an essential role when spectral lines are close to saturation. By combining several lines, the accuracy of the atmospheric temperature is not the main limiting factor.

References

[1] House et al. (2022), “Depleted carbon isotope compositions observed at Gale crater, Mars”, PNAS, doi: 10.1073/pnas.2115651119.
[2] Webster et al. (2013), “Isotope Ratios of H, C, and O in CO2 and H2O of the Martian Atmosphere”, Science, doi: 10.1126/science.1237961.
[3] Mahaffy et al. (2013), “Abundance and Isotopic Composition of Gases in the Martian Atmosphere from the Curiosity Rover”, Science, doi: 10.1126/science.1237966.
[4] Liuzzi et al. (2021), “First Detection and Thermal Characterization of Terminator CO2 Ice Clouds With ExoMars/NOMAD”, Geophys. Res. Lett., doi: 10.1029/2021GL095895.
[5] Villanueva et al. (2022), Under review, Geophys. Res. Lett.
[6] Villanueva et al. (2018), “Planetary Spectrum Generator: An accurate online radiative transfer suite for atmospheres, comets, small bodies and exoplanets”, J. Quant. Spectrosc. Rad. Trans., doi: 10.1016/j.jqsrt.2018.05.023.
[7] Villanueva et al. (2022), Handbook of the Planetary Spectrum Generator, ISBN: 978-0-578-36143-7.
[8] Alday et al. (2021), Isotopic Composition of CO2 in the Atmosphere of Mars: Fractionation by Diffusive Separation Observed by the ExoMars Trace Gas Orbiter, J. Geophys. Res., doi: 10.1029/2021JE006992.