Icarus (2024) https://doi.org/10.1016/j.icarus.2024.116121
Giuliano Liuzzi, Geronimo L. Villanueva, Shohei Aoki, Shane W. Stone, Sara Faggi, Loïc Trompet, Lori Neary, Frank Daerden, Sébastien Viscardy, Guido Masiello, Carmine Serio, Ian R. Thomas, Manish R. Patel, Giancarlo Bellucci, Jose-Juan Lopez-Moreno, Bojan Ristic, Ann Carine Vandaele
We present here vertically resolved measurements of the carbon isotopic composition of CO2 in the Martian atmosphere between the surface and 50 km of altitude. The results are based on data taken by the ExoMars Trace Gas Orbiter with the NOMAD instrument and have been aggregated to derive an average vertical profile of 13C/12C. We find no seasonal or spatial trends with variabilities beyond the sensitivity of the measurements. For the analysis, we developed a method that allows us to estimate whether the observed variability of isotopic measurements is beyond their intrinsic accuracy, by exploiting ab-initio spectroscopy and the radiometric noise of the instrument. Applying this method to our data, we find that atmospheric CO2 is depleted in 13C compared to the Earth standard by 30‰ to 45‰, in line with previous ground-based measurements values of the atmosphere and in contrast with the average value obtained by Curiosity at the surface (46 ± 4‰). These differences in isotopic signatures of CO2 as measured across the atmosphere and near-surface pose new questions when inferring the evolution and history of carbon on Mars, and suggest that processes such as a strong atmosphere-surface interactions may be fractionating the carbon reservoirs on the planet.
A: Single retrievals color-coded by the average SNR of spectral lines in orders 141, 145 and 146. It also shows the 3-σ uncertainty at all altitudes from 5 to 55 km retrieved from the SPEX method (dashed lines). The plot also shows the average profile (black solid line) computed by weighted average of the retrievals in bins of 5 km, together with its uncertainty (grey area). B: histograms of the distribution of retrieved 13C/12C ratios in different altitude intervals with their standard deviation. The vertical lines represent the medians (solid) and the means (dashed). The grey histogram represents the values at 55–70 km, and the average is not reported given the high dispersion of the points. C: retrievals of synthetic spectra with 10 different noise realizations simulated using the VPDB standard for 3 different temperature biases. Note: all panels have the isotopic ratio with respect to the VPDB standard on the x-axis.
