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This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made., The NOMAD-UVIS instrument on board the ExoMars Trace Gas Orbiter has been investigating the Martian atmosphere with the occultation technique since April 2018. Here, we analyze almost two Mars Years of ozone vertical distributions acquired at the day-night terminator. The ozone retrievals proved more difficult than expected due to spurious detections of ozone caused by instrumental effects, high dust content, and very low values of ozone. This led us to compare the results from three different retrieval approaches: (a) an onion peeling method, (b) a full occultation Optimal Estimation Method, and (c) a direct onion peeling method. The three methods produce consistently similar results, especially where ozone densities are higher. The main challenge was to find reliable criteria to exclude spurious detections of O3, and we finally adopted two criteria for filtering: (a) a detection limit, and (b) the Δχ2 criterion. Both criteria exclude spurious O3 values especially near the perihelion (180° < Ls < 340°), where up to 98% of ozone detections are filtered out, in agreement with general circulation models, that expect very low values of ozone in this season. Our agrees well with published analysis of the NOMAD-UVIS data set, as we confirm the main features observed previously, that is, the high-altitude ozone peak around 40 km at high latitudes. The filtering approaches are in good agreement with those implemented for the SPICAM/MEx observations and underline the need to evaluate carefully the quality of ozone retrievals in occultations. © 2023 The Authors. Earth and Space Science published by Wiley Periodicals LLC on behalf of American Geophysical Union., The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), with Co-PI teams in the United Kingdom (Open University), Spain (IAA-CSIC) and Italy (INAF-IAPS). This project has received funding from the European Union's Horizon 2020 research and innovation programme under Grant agreement No 101004052. This work was supported by the NASA's Mars Program Office under WBS 604796, “Participation in the TGO/NOMAD Investigation of Trace Gases on Mars.” The material was based upon the work supported by the NASA under award No. 80GSFC21M0002. The authors acknowledge funding from the UK Space Agency through Grants ST/V002295/1, ST/V005332/1, and ST/S00145X/1. AM acknowledges the support from the French Space Agency CNES. JJLM acknowledges financial support from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709) and by Grant PGC2018-101836-B-100 funded by MCIU/AEI/10.13039/501100011033 and by “ERDF A way of making Europe.”, With funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (CEX2021-001131-S).

Validation of global climate models (GCMs) for planets in our solar system requires observational data, but observations from the orbit of Mars and its surface are limited in number and are constrained by their orbit or landing site. Ground-based observations of Mars can help by providing data across the entire Martian hemisphere, yet historically, ground-based observations at submillimeter wavelengths have been limited to disk-average, or at best, a few resolution elements across Mars. We used Atacama Large Millimeter/submillimeter Array (ALMA) observations of Mars to determine the spatial distribution of carbon monoxide in the Martian atmosphere, which can be related to the atmospheric temperature. ALMA’s comparably high spatial and spectral resolutions in the submillimeter wavelengths could allow the mapping of abundances and temperature profiles, and the comparison of these data to simulations generated by the Laboratoire de Météorologie Dynamique (LMD) Mars GCM. However, the long baselines associated with the high spatial resolution of ALMA introduced systematic errors that resulted in radiative transfer modeling degeneracies. We serve to provide insight to facilitate proposed ALMA observations of Mars in the future so that the systematic errors encountered within these observations might be avoided.

Mapping Mars' water history We know the water cycle on Earth is complex. Neither is it simple on Mars. Infrared maps of water isotopes made by Villanueva et al. show the distribution of H2O and "semiheavy" water (HDO: deuterated water containing a mixture of hydrogen isotopes) across Mars. HDO enrichment varies with time and location; for example, irregular isotopic signals associate with different terrain features. The measurements also allow seasonal sublimation levels of the northern ice cap to be estimated and thus could be used to reveal past climate behavior. Science, this issue p. 218 We measured maps of atmospheric water (H2O) and its deuterated form (HDO) across the martian globe, showing strong isotopic anomalies and a significant high deuterium/hydrogen (D/H) enrichment indicative of great water loss. The maps sample the evolution of sublimation from the north polar cap, revealing that the released water has a representative D/H value enriched by a factor of about 7 relative to Earth's ocean [Vienna standard mean ocean water (VSMOW)]. Certain basins and orographic depressions show even higher enrichment, whereas high-altitude regions show much lower values (1 to 3 VSMOW). Our atmospheric maps indicate that water ice in the polar reservoirs is enriched in deuterium to at least 8 VSMOW, which would mean that early Mars (4.5 billion years ago) had a global equivalent water layer at least 137 meters deep. Maps of atmospheric water distribution reveal seasonal and spatial variations in the enrichment of deuterium to hydrogen. Maps of atmospheric water distribution reveal seasonal and spatial variations in the enrichment of deuterium to hydrogen.