1 and 2), leading to a big uncertainty in abundance. The H 37 Cl absorption line is several times weaker than H 35Cl (as can be seen in Figs. We have more than 80 sessions with clear HCl detections in Position 11, but not every orbit with confident HCl observations can be used to study the isotopic fractionation. Isolated in time, these detections likely happened very close to the moment of HCl formation but are unfortunately too weak to derive chlorine isotopic ratios accurately. 2021), but the two aphelion detections suggest it may be even shorter. Solar occultation observations revealed seasonal increases of the volume mixing ratio (VMR) up to 5 ppbv during the dusty perihelion season, followed by a drop in abundance down to undetectable values (1000 sol ( Aoki et al. Hydrogen chloride gas (HCl) was recently discovered by the Atmospheric Chemistry Suite mid-infrared channel (ACS MIR) instrument and found to be sporadically present in the atmosphere of Mars ( Korablev et al. 2021) provide initial insight into the relative role of these mechanisms. Recent D/H profiles in H 2O, measured by the ExoMars Trace Gas Orbiter (TGO) instruments ( Vandaele et al. ![]() 1999) or fractionation during a mass exchange with the regolith ( Hu 2019). Besides the loss to space, several mechanisms are responsible for fractionating the D/H ratio of the Martian middle atmosphere at present, such as condensation and photolysis-induced fractionation ( Montmessin et al. ![]() D/H in the Martian atmosphere is on average five times the terrestrial ratio, relative to the Vienna Standard Mean Ocean Water (VSMOW). The most pronounced and discussed fractionation relative to terrestrial values is that of hydrogen isotopes in water vapour ( Owen et al. Nitrogen was also found to be enriched in heavy isotope 15N by a factor of 1.57 ( Wong et al. Estimates suggest a significant loss of argon of >50%, perhaps up to 85–95%, from the atmosphere of Mars in the past 4 billion years. 2013) the ratio 38Ar/36Ar in the atmosphere of Mars is 1.26 times larger than on Earth ( Lee et al. The same instrument measured atmospheric argon isotope fractionation ( Atreya et al. Carbon isotopes in CO 2 were measured precisely by the Sample Analysis at Mars (SAM) instrument suite on Mars Science Laboratory (MSL) and found to be 1.05 enriched in 13C compared to terrestrial Vienna Pee Dee Belemnite (VPDB Webster et al. Generally, atmospheric species on Mars are enriched in heavier isotopes and are attributed to the preferential loss of the lighter isotope to space (e.g. Isotopic ratios among volatile species on Mars are key parameters in the diagnosis of active atmospheric processes and the history of the planet. Key words: planets and satellites: atmospheres / techniques: spectroscopic / molecular data We conclude that chlorine in observed HCl likely originates from dust and is not involved in any long-term, surface-atmosphere cycle. This value agrees with available measurements of the surface materials on Mars. Unlike other volatiles in the Martian atmosphere, because it is enriched with heavier isotopes, the δ 37Cl is measured to be − 7 ± 20°, which is almost indistinguishable from the terrestrial ratio for chlorine. This work describes the observation, processing technique, and derived values for the chloride isotope ratio. Measurements by the Atmospheric Chemistry Suite mid-infrared channel (ACS MIR) on the ExoMars Trace Gas Orbiter allow us to measure the ratio of hydrogen chloride two stable isotopologues, H 35Cl and H 37Cl. Its connection with potential chlorine reservoirs and the related atmospheric chemistry is now of particular interest and actively studied. Hydrogen chloride gas was recently discovered in the atmosphere of Mars during southern summer seasons. ![]() Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS/CNRS), Space Research Institute (IKI) RAS Moscow,Į-mail: of Physics, University of Oxford,
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