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This study analyzes H2O and HDO vertical profiles in the Venus mesosphere using Venus Express/Solar Occultation in the InfraRed data. The findings show increasing H2O and HDO volume mixing ratios with altitude, with the D/H ratio rising significantly from 0.025 at ~70 km to 0.24 at ~108 km. This indicates an increase from 162 to 1,519 times the Earth's ratio within 40 km. The study explores two hypotheses for these results: isotopic fractionation from photolysis of H2O over HDO or from phase change processes. The latter, involving condensation and evaporation of sulfuric acid aerosols, as suggested by previous authors [X. Zhang et al., Nat. Geosci. 3, 834-837 (2010)], aligns more closely with the rapid changes observed. Vertical transport computations for H2O, HDO, and aerosols show water vapor downwelling and aerosols upwelling. We propose a mechanism where aerosols form in the lower mesosphere due to temperatures below the water condensation threshold, leading to deuterium-enriched aerosols. These aerosols ascend, evaporate at higher temperatures, and release more HDO than H2O, which are then transported downward. Moreover, this cycle may explain the SO2 increase in the upper mesosphere observed above 80 km. The study highlights two crucial implications. First, altitude variation is critical to determining the Venus deuterium and hydrogen reservoirs. Second, the altitude-dependent increase of the D/H ratio affects H and D escape rates. The photolysis of H2O and HDO at higher altitudes releases more D, influencing long-term D/H evolution. These findings suggest that evolutionary models should incorporate altitude-dependent processes for accurate D/H fractionation predictions. [ABSTRACT FROM AUTHOR]

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]

We have carried out ground-based seasonal monitoring of hydrogen peroxide on Mars since 2003. This presentation is on the latest set of observations using thermal imaging spectroscopy, with two observations of the planet near opposition, in May 2016 (Ls = 148.5°, diameter = 17") and July 2018 (Ls = 209°, diameter = 23"). Data have been recorded in the 1232 - 1242 cm-1 range (8.1 micron) with the Texas Echelon Cross Echelle Spectrograph (TEXES) mounted at the 3-m Infrared Telescope Facility (IRTF) at Maunakea Observatory in Hawaii. As in the case of our previous analyses, maps of H2O2 have been obtained using line depth ratios of weak transitions of H2O2 divided by a weak CO2 line. The H2O2 map of April 2016 shows a strong dichotomy between the Northern and Southern hemispheres, as predicted by the photochemical model developed in the Mars Climate Database (Forget et al. 1999) and in the Global Environmental Multiscale model (Daerden et al. 2019). The second measurement in July 2018 was taken in the middle of the MY 34 global dust storm. H2O2 was not detected, with a disk-integrated 2-sigma upper limit of 10 ppbv, while both the MCD and the LEM models predicted a value above 20 ppbv that was actually observed by TEXES in 2003 in the absence of dust storm (Encrenaz et al., 2004). This July 2018 depletion is probably the result of the high dust content in the atmosphere at the time of our observations, which led to a decrease of the water vapor column density in the relevant altitude range, as observed by PFS on Mars Express during this period (Giuranna and Wolkenberg, 2019). GCM simulations using the GEM model show that the H2O depletion leads to a drop of H2O2 due to the depletion of HO2 radicals whose self-recombination gives rise to H2O2. Our data provide new constraints to the photochemical modelling of H2O2 in the presence of a high dust content. In parallel, we have reprocessed the whole TEXES dataset of H2O2 measurements using the latest version of the GEISA database (2015). We have recently found that there is a significant difference in the H2O2 line strengths between the 2003 and 2015 versions of GEISA. Therefore, all H2O2 mixing ratios up to 2014 from TEXES measurements have been reduced by a factor of about 1.75. As a consequence, in four cases (Ls around 80°, 100°, 150° and 209°), H2O2 abundances show contradictory values between different Martian years, while, at Ls = 209°, the cause seems to be the increased dust content associated with the global dust storm. The inter-annual variability in the three other cases remains unexplained at this time. [ABSTRACT FROM AUTHOR]