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We present here vertically resolved measurements of the carbon isotopic composition of CO 2 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 CO 2 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 CO 2 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. • The 13C/12C isotopic ratio in CO 2 is derived in the lower atmosphere of Mars with NOMAD data. • There is indication that CO 2 at 20–35 km of altitude is depleted in 13C compared to Earth Standard. • A quantitative method to characterize retrieval uncertainty with spectroscopy is presented. • Surface-atmosphere processes should be investigated to explain the discrepancies between values from NOMAD and Curiosity. [ABSTRACT FROM AUTHOR]

Methane (CH 4) on Mars has attracted a great deal of attention since it was first detected in January 2003. As methane is considered a potential marker for past/present biological or geological activity, any possible detection would require evidence with strong statistical significance. Ethane (C 2 H 6) and ethylene (C 2 H 4) are also relevant chemical species as their shorter lifetimes in the Martian atmosphere make them excellent tracers for recent and ongoing releases. If detected, a CH 4 /C 2 H n ratio could aid in constraining the potential source of organic production. Here we present the results of an extensive search for hydrocarbons in the Martian atmosphere in 240,000 solar occultation measurements performed by the ExoMars Trace Gas Orbiter/NOMAD instrument from April 2018 to April 2019. The observations are global, covering all longitudes and latitudes from 85°N to 85°S, and sampled from 6 to 100 km altitude with a typical vertical resolution of 2 km. There were no statistically significant detections of organics and new stringent upper limits for global ethane and ethylene were set at 0.1 ppbv and 0.7 ppbv, respectively. No global background level of methane was observed, obtaining an upper limit of 0.06 ppbv, in agreement with early results from ExoMars (Korablev et al., 2019). Dedicated searches for localized plumes at more than 2000 locations provided no positive detections, implying that if methane were released in strong and rapid events, the process would have to be sporadic. • A full year of Mars observation with global coverage yield no methane detections. • Search for global background level as well as localized plumes were conducted. • Methane upper limits down to 0.06 ppbv, lower than previous detections, were obtained above an altitude of 6 km. • Search for ethane and ethylene gave upper limits of 0.1 and 0.7 ppbv respectively. [ABSTRACT FROM AUTHOR]