Search

Abstract: The new ESA Venus Express orbiter is the first mission applying the probing technique of solar and stellar occultation to the atmosphere of Venus, with the SPICAV/SOIR instrument. SOIR is a new type of spectrometer used for solar occultations in the range 2.2–4.3 μm. Thanks to a high spectral resolving power (unprecedented in planetary space exploration), a new gaseous absorption band was soon detected in the atmospheric transmission spectra around 2982 cm−1, showing a structure resembling an unresolved Q branch and a number of isolated lines with a regular wave number pattern. This absorption could not be matched to any species contained in HITRAN or GEISA databases, but was found very similar to an absorption pattern observed by a US team in the spectrum of solar light reflected by the ground of Mars [Villanueva, G.L., Mumma, M.J., Novak, R.E., Hewagama, T., 2008. Icarus 195 (1), 34–44]. This team then suggested to us that the absorption was due to an uncatalogued transition of the 16O12C18O molecule. The possible existence of this band was soon confirmed from theoretical considerations by Perevalov and Tashkun. Some SOIR observations of the atmospheric transmission are presented around 2982 cm−1, and rough calculations of line strengths of the Q branch are produced, based on the isotopic ratio measured earlier in the lower atmosphere of Venus. This discovery emphasizes the role of isotopologues of CO2 (as well as H2O and HDO) as important greenhouse gases in the atmosphere of Venus. [Copyright &y& Elsevier]

Venus has thick clouds of H2SO4 aerosol particles extending from altitudes of 40 to 60 km. The 60–100 km region (the mesosphere) is a transition region between the 4 day retrograde superrotation at the top of the thick clouds and the solar–antisolar circulation in the thermosphere (above 100 km), which has upwelling over the subsolar point and transport to the nightside. The mesosphere has a light haze of variable optical thickness, with CO, SO2, HCl, HF, H2O and HDO as the most important minor gaseous constituents, but the vertical distribution of the haze and molecules is poorly known because previous descent probes began their measurements at or below 60 km. Here we report the detection of an extensive layer of warm air at altitudes 90–120 km on the night side that we interpret as the result of adiabatic heating during air subsidence. Such a strong temperature inversion was not expected, because the night side of Venus was otherwise so cold that it was named the ‘cryosphere’ above 100 km. We also measured the mesospheric distributions of HF, HCl, H2O and HDO. HCl is less abundant than reported 40 years ago. HDO/H2O is enhanced by a factor of ∼2.5 with respect to the lower atmosphere, and there is a general depletion of H2O around 80–90 km for which we have no explanation. [ABSTRACT FROM AUTHOR]

• The "science-softCon UV/Vis+ photochemistry database" contains EUV-VUV-UV–Vis-NIR spectral data. • The database also contains quantum yields and photolysis related data. • The database provides valuable information for astrophysics, earth and planetary sciences, medical and pharmaceutical sciences. The "science-softCon UV/Vis+ Photochemistry Database" (www.photochemistry.org) is a large and comprehensive collection of EUV-VUV-UV–Vis-NIR spectral data and other photochemical information assembled from published peer-reviewed papers. The database contains photochemical data including absorption, fluorescence, photoelectron, and circular and linear dichroism spectra, as well as quantum yields and photolysis related data that are critically needed in many scientific disciplines. This manuscript gives an outline regarding the structure and content of the "science-softCon UV/Vis+ Photochemistry Database". The accurate and reliable molecular level information provided in this database is fundamental in nature and helps in proceeding further to understand photon, electron and ion induced chemistry of molecules of interest not only in spectroscopy, astrochemistry, astrophysics, Earth and planetary sciences, environmental chemistry, plasma physics, combustion chemistry but also in applied fields such as medical diagnostics, pharmaceutical sciences, biochemistry, agriculture, and catalysis. In order to illustrate this, we illustrate the use of the UV/Vis+ Photochemistry Database in four different fields of scientific endeavor. [ABSTRACT FROM AUTHOR]

Formaldehyde (H2CO) is a critical precursor for the abiotic formation of biomolecules, including amino acids and sugars, which are the building blocks of proteins and RNA. Geomorphological and geochemical evidence on Mars indicates a temperate environment compatible with the existence of surface liquid water during its early history at 3.8–3.6 billion years ago (Ga), which was maintained by the warming effect of reducing gases, such as H2. However, it remains uncertain whether such a temperate and weakly reducing surface environment on early Mars was suitable for producing H2CO. In this study, we investigated the atmospheric production of H2CO on early Mars using a 1-D photochemical model assuming a thick CO2-dominated atmosphere with H2 and CO. Our results show that a continuous supply of atmospheric H2CO can be used to form various organic compounds, including amino acids and sugars. This could be a possible origin for the organic matter observed on the Martian surface. Given the previously reported conversion rate from H2CO into ribose, the calculated H2CO deposition flux suggests a continuous supply of bio-important sugars on early Mars, particularly during the Noachian and early Hesperian periods. [ABSTRACT FROM AUTHOR]

The VenSpec instrument suite consists of three channels: VenSpec-M, VenSpec-H and VenSpec-U. VenSpec-M will provide near-global compositional data on rock types, weathering, and crustal evolution by mapping the Venus surface in five atmospheric windows. The broadest window at 1·02 μm is mapped with two filters to obtain information on the shape of the window. Additional filters are used to remove clouds, water, and stray light. VenSpec-M will use the methodology pioneered by VIRTIS on Venus Express but with more and wider spectral bands, the VenSAR-derived DEM, and EnVision's circular orbit to deliver near-global multichannel spectroscopy with wider spectral coverage and an order of magnitude improvement in sensitivity. It will obtain repeated imagery of surface thermal emission, constraining current rates of volcanic activity following earlier observations from Venus Express. VenSpec-H will be dedicated to extremely high resolution atmospheric measurements. The main objective of the VenSpec-H instrument is to detect and quantify SO₂, H₂O and HDO in the lower atmosphere, to enable characterisation of volcanic plumes and other sources of gas exchange with the surface of Venus, complementing VenSAR and VenSpec-M surface and SRS subsurface observations. A nadir pointed high-resolution infrared spectrometer is the ideal instrument for these observations at the 1·0 μm, 1·7 μm, and 2·0 – 2·3 μm atmospheric windows that permit measurements of the lower atmosphere. Baseline observations will be performed on the night side but observations at all times of day are possible. VenSpec-U will monitor sulphured minor species (mainly SO and SO₂) and the as yet unknown UV absorber in Venusian upper clouds and just above. It will therefore complement the two other channels by investigating how the upper atmosphere interacts with the lower atmosphere, and especially characterise to which extent outgassing processes such as volcanic plumes are able to disturb the atmosphere through the thick Venusian clouds. A twin channel (0.2 nm in high-resolution, 2 nm in low-resolution) spectral imager in the 190-380 nm range able to operate in nadir would be especially suited to such a task. In combination, VenSpec will provide unprecedented insights into the current state of Venus and its past evolution. VenSpec will perform a comprehensive search for volcanic activity by targeting atmospheric signatures, thermal signatures and compositional signatures, as well as a global map of surface composition [ABSTRACT FROM AUTHOR]

The NOMAD ("Nadir and Occultation for MArs Discovery") spectrometer suite on board the ExoMars Trace Gas Orbiter has been designed to investigate the composition of Mars' atmosphere, with a particular focus on trace gases, clouds and dust. This will allow for a major leap in our knowledge and understanding of the Martian atmospheric composition and the related physical and chemical processes. NOMAD will conduct a spectroscopic survey of Mars' atmosphere in ultraviolet (UV), visible and infrared (IR) wavelengths covering large parts of the 0.2-4.3 µm spectral range [1,2]. NOMAD is composed of 3 spectrometers: a solar occultation dedicated spectrometer (SO – Solar Occultation) operating in the infrared (2.3-4.3 µm), a second infrared spectrometer (2.3-3.8 µm) capable of doing nadir, but also solar occultation and limb observations (LNO – Limb Nadir and solar Occultation) [3], and an ultraviolet/visible spectrometer (UVIS – UV visible, 200-650 nm) that can work in all three observation modes [4].Science phase started in April 2018. Since then NOMAD performed solar occultation and nadir observations using different options to test the instrument under various conditions. Several atmospheric species have been targeted, delivering profiles from solar occultation from 200 km down to the surface and integrated abundances from nadir measurements. Observations optimized for the detection of dust and clouds have also been performed. We will present first results from nadir and solar occultation observations, with an emphasis on the detection of the dust storm observed in June 2018 and afterwards, and its impact on the composition of the atmosphere in particular on water and D/H ratio. References[1] Vandaele, A.C., et al., 2015. Planet. Space Sci. 119, 233-249.[2] Vandaele et al., 2018. Space Sci. Rev., 214:80, doi.org/10.1007/s11214-11018-10517-11212.[3] Neefs et al., 2015. Applied Optics 54, 8494-8520.[4] Patel et al., 2017. Applied Optics 56, 2771-2782. [ABSTRACT FROM AUTHOR]

The VenSpec-H instrument is part of the EnVision M5 mission payload which has beenselected by ESA to enter Phase A in January 2019 for a potential mission selection in 2021.EnVision is a medium class mission to determine the nature and current state ofgeological activity on Venus, and its relationship with the atmosphere, to understand howVenus and Earth could have evolved so differently. VenSpec-H is based on NOMAD(Nadir and Occultation for MArs Discovery), a suite of three spectrometers onExoMars Trace Gas Orbiter. Specifically, VenSpec-H is a redesign of the LNO (Limb,Nadir and Occultation) channel of NOMAD, retaining much heritage from theoriginal but with minor modifications to meet the science objectives of the EnVisionmission. VenSpec-H is part of the VenSpec suite, including also an IR mapper and a UVspectrometer suite. The science objectives of this suite is to (i) search for temporalvariations in surface temperatures and tropospheric concentrations of volcanicallyemitted gases, indicative of volcanic eruptions and (ii) study surface-atmosphereinteractions and weathering by mapping surface emissivity and tropospheric gasabundances. Recent and perhaps ongoing volcanic activity has been inferred in data from both VenusExpress and Magellan. Maintenance of the clouds requires a constant input of H2O andSO2. A large eruption would locally alter the composition in the following way: (i)increase H2O by several tens of percent; (ii) decrease D/H ratio by several tens ofpercent and (iii) increase SO2 by about a few percents. The latter effect is probablyunderestimated with respect to the others, since the Venusian interior is thought tobe much drier than Earth’s, so that the outgassed SO2/H2O ratio may be muchhigher on Venus. Observations of changes in lower atmospheric SO2 and H2Ovapour levels, cloud level H2SO4 droplet concentration, and mesospheric SO2, aretherefore required to link specific volcanic events with past and ongoing observationsof the variable and dynamic mesosphere, to understand both the importance ofvolatiles in volcanic activity on Venus and their effect on cloud maintenance anddynamics. To contribute to this investigation, VenSpec-H will be designed to (i) measure H2O andHDO contents in the first scale height of Venus’ atmosphere, using the 1.18μm spectralwindow and (ii) probe H2O, HDO, OCS, SO2 in the 30 to 40 km altitude range,using the 2.4μm spectral window. How spectral simulations and determination ofSignal-to-Noise ratios led to the design of the instrument will be presented in thiscommunication. [ABSTRACT FROM AUTHOR]

Ozone is a highly reactive species on Mars. In particular, it displays steep gradients across the terminator due to photolysis [4]. Odd hydrogen radicals play an important role in the destruction of ozone. This results in a strong anti-correlation between O3 and H2O [4].Here, we will present first retrievals of ozone vertical profiles obtained from NOMAD-UVIS solar occultations. NOMAD will help us improve our knowledge of the climatology of ozone and of its complex photochemistry.The NOMAD (Nadir and Occultation for MArs Discovery) – operating onboard the ExoMars 2016 Trace Gas Orbiter satellite – started to acquire the first scientific measurements on 21 April 2018.It is a spectrometer composed of 3 channels: 1) a solar occultation channel (SO) operating in the infrared (2.3-4.3 μm); 2) a second infrared channel LNO (2.3-3.8 μm) capable of doing nadir, as well as solar occultation and limb; and 3) an ultraviolet/visible channel UVIS (200-650 nm) that can work in the three observation modes [1,2]. The UVIS channel has a spectral resolution <1.5 nm. In the solar occultation mode it is mainly devoted to study the climatology of ozone and aerosols content [3]. We will present first retrievals of ozone vertical profiles. NOMAD-UVIS spectra are simulated using the line-by-line radiative transfer code ASIMUT-ALVL developed at IASB-BIRA [5]. In a preliminary study based on SPICAM-UV solar occultations (See [6]), ASIMUT was modified in order to take into account the atmospheric composition and structure at the day-night terminator. We followed the same method described in [7] to check that the spectra are correctly calibrated and accurately normalized to the solar spectrum. As input for ASIMUT, we used gradients predicted by the 3D GEM-Mars v4 Global Circulation Model (GCM) [8,9] and the UK version of the LMD GCM. UVIS ozone profiles will also be compared to SPICAM-UV retrievals.References:[1] Vandaele, A.C., et al., Planetary and Space Science, Vol. 119, pp. 233–249, 2015. [2] Neefs, E., et al., Applied Optics, Vol. 54 (28), pp. 8494-8520, 2015.[3] M.R. Patel et al., In: Appl. Opt. 56.10 (2017), pp. 2771–2782. DOI: 10.1364/AO.56.002771.[4] Lefèvre, F., et al., Aug. 2008. Nature 454, 971–975.[5] Vandaele, A.C., et al., JGR, 2008. 113 doi:10.1029/2008JE003140.[6] Piccialli, A., EPSC 2018.[7] Trompet, L., et al., 2016. Applied Optics 55, 9275-9281. [8] Neary, L., and F. Daerden (2018), Icarus, 300, 458–476, doi:10.1016/j.icarus.2017.09.028.[9] Daerden, F.; et al., GRL, 2015, 42, 18, 7319-7326. [ABSTRACT FROM AUTHOR]

NOMAD is a suite of spectrometers on the board of the ESA-Roscosmos Trace Gas Orbiter (TGO) spacecraft and is capable of investigating the Martian environment at very high spectral resolution in the ultraviolet–visible and infrared spectral ranges by means of three separate channels: UVIS (0.2–0.65 μm), LNO (2.2–3.8 μm), and SO (2.3–4.3 μm). Among all channels, LNO is the only one operating at infrared wavelengths in nadir-viewing geometry, providing information on the whole atmospheric column and on the surface. Unfortunately, the LNO data are characterized by an overall low level of signal-to-noise ratio (SNR), limiting their contribution to the scientific objectives of the TGO mission. In this study, we assess the possibility of enhancing LNO nadir data SNR by applying the Minimum Noise Fraction (MNF), a well-known algorithm based on the Principal Components technique that has the advantage of providing transform eigenvalues ordered with increasing noise. We set up a benchmark process on an ensemble of synthetic spectra in order to optimize the algorithm specifically for LNO datasets. We verify that this optimization is limited by the presence of spectral artifacts introduced by the MNF itself, and the maximum achievable SNR is dependent on the scientific purpose of the analysis. MNF application study cases are provided to LNO data subsets in the ranges 2.627–2.648 μm and 2.335–2.353 μm (spectral orders 168 and 189, respectively) covering absorption features of gaseous H2O and CO and CO2 ice, achieving a substantial enhancement in the quality of the observations, whose SNR increases up to a factor of 10. While such an enhancement is still not enough to enable the investigation of spectral features of faint trace gases (in any case featured in orders whose spectral calibration is not fully reliable, hence preventing the application of the MNF), interesting perspectives for improving retrieval of both atmospheric and surface features from LNO nadir data are implied. [ABSTRACT FROM AUTHOR]

A new IASI methane product developed at the Royal Belgian Institute for Space Aeronomy (BIRA-IASB) is presented. The retrievals are performed with the ASIMUTALVL software based on the optimal estimation method (OEM). This paper gives an overview of the forward model and retrieval concept. The usefulness of reconstructed principal component compressed (PCC) radiances is highlighted. The information content study carried out in this paper shows that most IASI pixels contain between 0.9 and 1.6 independent pieces of information about the vertical distribution of CH4, with a good sensitivity in the mid- to upper troposphere. A detailed error analysis was performed. The total uncertainty is estimated to be 3.73% for a CH4 partial column between 4 and 17 km. An extended validation with ground-based CH4 observations at 10 locations was carried out. IASI CH4 partial columns are found to correlate well with the ground-based data for 6 out of the 10 Fourier transform infrared (FTIR) stations with correlation coefficients between 0.60 and 0.84. Relative mean differences between IASI and FTIR CH4 range between -2.31 and 4.04% and are within the systematic uncertainty. For 6 out of the 10 stations the relative mean differences are smaller than ±1 %. The standard deviation of the difference lies between 1.76 and 2.97% for all the stations. [ABSTRACT FROM AUTHOR]

A new global IASI methane product developed at the Royal Belgian Institute for Space Aeronomy (BIRA-IASB) is presented. The retrievals are performed with the ASIMUT-ALVL software based on the Optimal Estimation Method (OEM). This paper gives an overview of the forward model and retrieval concept. The usefullness of reconstructed Principal Component Compressed (PCC) radiances is highlighted. The retrieval uncertainty of the CH4 profiles is less than 4 % below 100 hPa (~ 16 km). The information content study carried out in this paper shows that most IASI pixels contain between 0.9 and 1.6 independent pieces of information about the vertical distribution of CH4, with a good sensitivity in the mid to upper troposphere. An extended validation with ground-based CH4 observations at 10 locations was carried out. IASI CH4 partial columns are found to correlate well with the ground-based data for 7 out of the 10 Fourier Transform Infrared (FTIR) stations with correlation coefficients between 0.71 and 0.96. Mean differences between IASI and FTIR CH4 range between -1.93 and 4.40 % and are within the systematic uncertainty. For 7 out of the 10 stations absolute differences are less than 1 %. The standard deviation of the difference lies between 1.40 and 3.99 % for all the stations. [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]

Using the Nadir and Occultation for MArs Discovery instrument aboard Trace Gas Orbiter, we derived the CO/CO2 profiles between 75 and 105 km altitude with the equivalent width technique. The derived CO/CO2 profiles showed significant seasonal variations in the southern hemisphere with decreases near perihelion and increases near aphelion. The estimation of the CO/CO2 profiles with a one‐dimensional photochemical model shows that an altitude‐dependent eddy diffusion coefficient better reproduces the observed profiles than a vertically uniform one. Our estimation suggests that the eddy diffusion coefficient in Ls = 240–270 is uniformly larger by a factor of ∼2 than that in Ls = 90–120 in the southern hemisphere, while they are comparable in the northern hemisphere. This fact demonstrates that the eddy diffusion coefficient is variable with season and latitude. Plain Language Summary: The eddy diffusion coefficient is widely used to parameterize the efficiency of vertical diffusion in the planetary atmosphere, whose variation characterizes the transportation of trace gas species. Additionally, it could vary their vertical distributions in the middle and upper atmosphere, which might cause an impact on the species escaping to space. However, the variability of the eddy diffusion coefficient in those altitude regions have been poorly understood. In this study, we focus on the estimation of variation in the eddy diffusion coefficient by analyzing the CO and CO2 measurements made by the ExoMars Trace Gas Orbiter. The observed CO/CO2 ratio between altitudes of 75 and 105 km shows a significant seasonal variation in the southern hemisphere. The observed CO/CO2 profiles are compared with the simulated profiles obtained with a one‐dimensional photochemical model assigning several shapes and intensity of eddy diffusion coefficient. The comparison shows that the eddy diffusion coefficient is not constant but variable depending on altitude, season, and latitude, which suggests that the efficiency of the vertical diffusion varies with season and latitude. This fact is useful to other 1D photochemical models to reproduce the seasonal and latitudinal variation of atmospheric composition. Key Points: The CO/CO2 profiles from 75 to 105 km measured by NOMAD aboard TGO are used to investigate variations in the eddy diffusion coefficientThe estimated CO/CO2 profiles agree well with the observed profiles if altitude‐dependent eddy diffusion coefficients are consideredOur results demonstrate a substantial seasonal variation in the eddy diffusion coefficient in the southern hemisphere [ABSTRACT FROM AUTHOR]

We present observations of terminator CO2 ice clouds events in three groups: Equatorial dawn, Equatorial dusk (both between 20°S and 20°N) and Southern midlatitudes at dawn (45°S and 55°S east of Hellas Basin) with ESA ExoMars Trace Gas Orbiter's Nadir and Occultation for MArs Discovery instrument. CO2 ice abundance is retrieved simultaneously with water ice, dust, and particle sizes, and rotational temperature and CO2 column profiles in 16 of 26 cases. Small particles (<0.5 μm) prevail at dusk, while water ice likely provides most source nuclei at dawn. Clouds east of Hellas are found to be dominantly nucleated on surface‐lifted dust. CO2 ice is sometimes detected in unsaturated air together with dust nuclei at dawn, suggesting ongoing sublimation. Depending on latitude and local time, the interplay between particle precipitation and the lifetime of temperature minima (i.e., cold pockets) determines CO2 ice properties. Plain Language Summary: The upper atmosphere of Mars is characterized by the seasonal presence of CO2 ice clouds. Their properties have been long studied, as well as their formation mechanisms in relation to the thermal structure of the atmosphere and its variability. In this study, we present the first observations of these clouds at the terminator (dawn and dusk) by the NOMAD spectrometer onboard the Exomars Trace Gas Orbiter. CO2 ice is detected simultaneously with dust, water ice and the temperature profile. Our results agree with previous findings in terms of how clouds are spatially distributed and their temporal occurrence. However, we also explore the sources of condensation nuclei for CO2 ice particles, showing that water ice is a possible source at dawn near the Equator. We also identify surface‐lifted dust below the CO2 ice clouds observed east of Hellas Basin, suggesting that, at that location, dust could provide nuclei for CO2 ice. CO2 ice is also sometimes detected at temperatures higher than the CO2 freezing point, suggesting ongoing sublimation. In this work we explore for the first time the composition of CO2 ice clouds, which is critical to advance our understanding of how CO2 ice clouds form in the mesosphere at Mars. Key Points: Twenty Six mesospheric CO2 ice clouds were detected with NOMAD SO in Mars Year 35, simultaneously with water ice, dust, and CO2 saturation ratiosEquatorial CO2 clouds are observed at 50–80 km altitude at dusk, and 40–60 km at dawn, when water ice likely provides condensation nucleiSix CO2 ice clouds are found east of Hellas basin; their formation is likely sourced by surface‐lifted dust at 40–65 km [ABSTRACT FROM AUTHOR]

Purpose: This paper aims to describe the development of a knowledge management system (KMS) for the Nadir and Occultation for Mars Discovery (NOMAD) instrument on board the ESA/Roscosmos 2016 ExoMars Trace Gas Orbiter (TGO) spacecraft. The KMS collects knowledge acquired during the engineering process that involved over 30 project partners. In addition to the documentation and technical data (explicit knowledge), a dedicated effort was made to collect the gained experience (tacit knowledge) that is crucial for the operational phase of the TGO mission and also for future projects. The system is now in service and provides valuable information for the scientists and engineers working with NOMAD. Design/methodology/approach: The NOMAD KMS was built around six areas: official documentation, technical specifications and test results, lessons learned, management data (proposals, deliverables, progress reports and minutes of meetings), picture files and movie files. Today, the KMS contains 110 GB of data spread over 11,000 documents and more than 13,000 media files. A computer-aided design (CAD) library contains a model of the full instrument as well as exported sub-parts in different formats. A context search engine for both documents and media files was implemented. Findings: The conceived KMS design is basic, flexible and very robust. It can be adapted to future projects of a similar size. Practical implications: The paper provides practical guidelines on how to retain the knowledge from a larger aerospace project. The KMS tool presented here works offline, requires no maintenance and conforms to data protection standards. Originality/value: This paper shows how knowledge management requirements for space missions can be fulfilled. The paper demonstrates how to transform the large collection of project data into a useful tool and how to address usability aspects. [ABSTRACT FROM AUTHOR]

EnVision has been recently selected by ESA as a Cosmic Vision M5 finallist. This missionaims at characterizing the evolution and activity of our neighbouring planet Venus: is Venusstill active today? What is its internal structure? How did it evolve (habitability, weathering,resurfacing)? How are volatile species coupled between interior, surface and atmosphere? Inorder to answer these science objective, the payload onboard EnVision low-Venus orbitplatform consists of an interferometric radar (InSAR), a subsurface sounder (SRS) and aspectrometer suite known as VenSpec, consisting of three channels: VenSpec-M, VenSpec-Hand VenSpec-U. VenSpec-U is a dual UV spectral imager (190-380 nm @ 2 nm; 210-240 nm @ 0.2 nm)aiming at measuring SO2, SO and the yet unknown UV absorber on day side cloud top ofVenus. It is known from past observations, from orbiters such as Pioneer Venus and VenusExpress and Earth-based telescopes such as IRTF and HST that SO2 and SO are both highlyvariable species, spanning at least two orders of magnitude over timescales ranging from afew hours to several decades. The physical origin of this variability is poorly constrained(volcanic plumes and/or atmospheric oscillations), and VenSpec-U, with its unprecedentedaccuracy, spatial and temporal coverage, will be able to provide valuable answers. [ABSTRACT FROM AUTHOR]

Highlights • A complete spectral in-flight calibration of NOMAD is presented. • A model to compute spectral continuum is produced as a function of instrument properties. • NOMAD sensitivity to atmospheric CH 4 is evaluated with different aerosol loads. • NOMAD sensitivity to CH 4 in Solar Occultation is on sub-ppbv level. • NOMAD will be able to trace seasonal trends for CH 4 with nadir observations. Abstract The Nadir and Occultation for MArs Discovery instrument (NOMAD), onboard the ExoMars Trace Gas Orbiter (TGO) spacecraft was conceived to observe Mars in solar occultation, nadir, and limb geometries, and will be able to produce an outstanding amount of diverse data, mostly focused on properties of the atmosphere. The infrared channels of the instrument operate by combining an echelle grating spectrometer with an Acousto-Optical Tunable Filter (AOTF). Using in-flight data, we characterized the instrument performance and parameterized its calibration. In particular: an accurate frequency calibration was achieved, together with its variability due to thermal effects on the grating. The AOTF properties and transfer function were also quantified, and we developed and tested a realistic method to compute the spectral continuum transmitted through the coupled grating and AOTF system. The calibration results enabled unprecedented insights into the important problem of the sensitivity of NOMAD to methane abundances in the atmosphere. We also deeply characterized its performance under realistic conditions of varying aerosol abundances, diverse albedos and changing illumination conditions as foreseen over the nominal mission. The results show that, in low aerosol conditions, NOMAD single spectrum, 1σ sensitivity to CH 4 is around 0.33 ppbv at 20 km of altitude when performing solar occultations, and better than 1 ppbv below 30 km. In dusty conditions, we show that the sensitivity drops to 0 below 10 km. In Nadir geometry, results demonstrate that NOMAD will be able to produce seasonal maps of CH 4 with a sensitivity around 5 ppbv over most of planet's surface with spatial integration over 5 × 5° bins. Results show also that such numbers can be improved by a factor of ~10 to ~30 by data binning. Overall, our results quantify NOMAD's capability to address the variable aspects of Martian climate. [ABSTRACT FROM AUTHOR]

The Martian mesosphere and thermosphere, the region above about 60 km, is not the primary target of the ExoMars 2016 mission but its Trace Gas Orbiter (TGO) can explore it and address many interesting issues, either in-situ during the aerobraking period or remotely during the regular mission. In the aerobraking phase TGO peeks into thermospheric densities and temperatures, in a broad range of latitudes and during a long continuous period. TGO carries two instruments designed for the detection of trace species, NOMAD and ACS, which will use the solar occultation technique. Their regular sounding at the terminator up to very high altitudes in many different molecular bands will represent the first time that an extensive and precise dataset of densities and hopefully temperatures are obtained at those altitudes and local times on Mars. But there are additional capabilities in TGO for studying the upper atmosphere of Mars, and we review them briefly. Our simulations suggest that airglow emissions from the UV to the IR might be observed outside the terminator. If eventually confirmed from orbit, they would supply new information about atmospheric dynamics and variability. However, their optimal exploitation requires a special spacecraft pointing, currently not considered in the regular operations but feasible in our opinion. We discuss the synergy between the TGO instruments, specially the wide spectral range achieved by combining them. We also encourage coordinated operations with other Mars-observing missions capable of supplying simultaneous measurements of its upper atmosphere. [ABSTRACT FROM AUTHOR]

This paper deals with the composition and chemical processes occurring in the neutral atmosphere of Venus. Since the last synthesis, observers as well as modellers have emphasised the spatial and temporal variability of minor species, going beyond a static and uniform picture that may have prevailed in the past. The outline of this paper acknowledges this situation and follows closely the different dimensions along which variability in composition can be observed: vertical, latitudinal, longitudinal, temporal. The strong differences between the atmosphere below and above the cloud layers also dictate the structure of this paper. Observational constraints, obtained from both Earth and Venus Express, as well as 1D, 2D and 3D models results obtained since 1997 are also extensively referred and commented by the authors. An non-exhaustive list of topics included follows: modelled and observed latitudinal and vertical profiles of CO and OCS below the clouds of Venus; vertical profiles of CO and SO2 above the clouds as observed by solar occultation and modelled; temporal and spatial variability of sulphur oxides above the clouds. As a conclusion, open questions and topics of interest for further studies are discussed. [ABSTRACT FROM AUTHOR]

Nadir and Occultation for MArs Discovery (NOMAD) is a 3-channel spectrometer suite that is currently orbiting Mars onboard ESA's ExoMars Trace Gas Orbiter, measuring the composition of the Martian atmosphere in unprecedented detail. Of the three channels, two operate in the infrared: the Solar Occultation (SO) channel observes gas species in the 2.2–4.3 ​μm spectral region in solar occultation mode, while the Limb, Nadir and Occultation (LNO) channel observes in the 2.2–3.8 ​μm spectral region and can operate in limb-, nadir- and solar occultation-pointing modes. The Ultraviolet–VISible (UVIS) channel operates in the UV–visible region, from 200 to 650 ​nm. Both infrared channels have a spectral resolution typically an order of magnitude better than previous instruments orbiting Mars, to measure molecular absorption lines and therefore determine the abundances of constituents of the Martian atmosphere and the processes that govern their distribution and transport. To maximise the full potential of the instrument, a wide range of calibration measurements were made prior to launch and continue to be made in-flight. This work, part 1, addresses the aspects of the SO channel calibration that are not covered elsewhere, namely: the SO channel ground calibration setup, boresight pointing vector determination, detector characterisation, detector illumination pattern and saturation levels, and an investigation of the instrument line shape. An accompanying paper, part 2, addresses similar aspects for LNO, the other infrared channel in NOMAD (Thomas et al., 2021, this issue). [ABSTRACT FROM AUTHOR]

The Nadir and Occultation for MArs Discovery (NOMAD) instrument is a 3-channel spectrometer suite on the ESA ExoMars Trace Gas Orbiter. Since April 2018, when the nominal science mission began, it has been measuring the constituents of the Martian atmosphere. NOMAD contains three separate spectrometers, two of which operate in the infrared: the Solar Occultation (SO) channel makes only solar occultation observations, and therefore has the best resolving power (∼20,000) and a wider spectral region covering 2.2–4.3 ​μm. The Limb, Nadir and Occultation (LNO) channel covers the 2.2–3.8 ​μm spectral region and can operate in limb, nadir or solar occultation pointing modes. The Ultraviolet–VISible (UVIS) channel operates in the UV–visible region, from 200 to 650 ​nm, and can measure in limb, nadir or solar occultation modes like LNO. The LNO channel has a lower resolving power (∼10,000) than the SO channel, but is still typically an order of magnitude better than previous instruments orbiting Mars. The channel primarily operates in nadir-viewing mode, pointing directly down to the surface to measure the narrow atmospheric molecular absorption lines, clouds and surface features in the reflected sunlight. From the depth and position of the observed atmospheric absorption lines, the constituents of the Martian atmosphere and their column densities can be derived, leading to new insights into the processes that govern their distribution and transport. Surface properties can also be derived from nadir observations by observing the shape of the spectral continuum. Many calibration measurements were made prior to launch, on the voyage to Mars, and continue to be made in-flight during the science phase of the mission. This work, part 2, addresses the aspects of the LNO channel calibration that are not covered elsewhere, namely: the LNO ground calibration setup, the LNO occultation and nadir boresight pointing vectors, LNO detector characterisation and nadir/limb illumination pattern, instrument temperature effects, and finally the radiometric calibration of the LNO channel. An accompanying paper, part 1 (Thomas et al., 2021, this issue), addresses similar aspects for SO, the other infrared channel in NOMAD. A further accompanying paper (Cruz-Mermy et al., 2021, this issue) investigated the LNO radiometric calibration in more detail, approaching the work from a theoretical perspective. The two calibrations agree with each other to within 3%, validating each calibration method. • The LNO channel is 1 of 3 spectrometers in NOMAD on ExoMars Trace Gas Orbiter. • The LNO channel observes Mars in limb, nadir and solar occultation modes. • This article describes the operational modes and calibration of the LNO channel. • The boresight, detector illumination, slant, bad pixels and saturation were found. • The procedure for radiometrically calibrating nadir observations is described. [ABSTRACT FROM AUTHOR]

We present an in-flight straylight removal method for the Ultraviolet and Visible Spectrometer (UVIS) channel of the Nadir and Occultation for Mars Discovery (NOMAD) instrument aboard the ExoMars Trace Gas Orbiter (TGO). The presence of a 'red-leak' straylight signal in the UVIS instrument was discovered post-launch in ground calibration measurements of spectral lamps; UVIS observations of lamps with negligible UV light emission (RS12) showed a significant signal at UV wavelengths. Subsequent analyses of nadir observations of the martian atmosphere revealed that at UV wavelengths the red-leak straylight was in excess of 300% of the true UV signal, jeopardising the primary science observations of the instrument (retrievals of atmospheric ozone). By modifying the UVIS readout method to obtain a region of interest around the illuminated region on the Charge-Coupled Device (CCD) detector, instead of a binned one-dimensional spectrum, and utilising straylight profiles derived from measurements of the RS12 calibration lamp we show that the majority of the straylight at UV wavelengths can be successfully removed for the nadir channel in a self-consistent manner. The corrected UVIS radiances are compared to coincident Mars Color Imager (MARCI) instrument observations with residuals between the two instruments generally remaining within 15%. • Retaining the full CCD array can be a powerful tool for the removal of straylight. • Essential to measure straylight profiles using different light sources and filters. • Accurate removal of straylight and recovery of measured ultraviolet radiances. [ABSTRACT FROM AUTHOR]

The Ultraviolet and VIsible Spectrometer (UVIS), covering the 200–650 ​nm range, is one of three spectrometers that comprise the NOMAD instrument on the ExoMars 2016 Trace Gas Orbiter (TGO). UVIS can operate in solar occultation, nadir and limb viewing mode and was designed to monitor ozone and aerosols in the Martian atmosphere. Here, we describe the calibration procedure to convert the UVIS raw data into a calibrated data product ready for scientific exploitation. The calibration includes the CCD offset and dark current subtraction, the wavelength assignment, the noise identification and removal, the smearing removal, and the radiance or transmittance conversion. A straylight correction, critical for some parts of the UVIS spectral range, is also applied during the data reduction process, which is described in more detail in two companion papers [Mason et al., 2022; Depiesse et al., In prep] corresponding to two different and independent methods giving consistent results. The solar occultation observations are converted into transmittance and are therefore self-calibrating, while nadir and limb measurements require an absolute radiometric calibration. A comparison with coincident nadir MRO/MARCI measurements is provided as a final validation and generally shows a ±10% agreement on the radiances measured by both instruments. • Description of the calibration of UVIS measurements to produce level 1 data from raw data • CCD non-linearity correction, offset removal, dark current removal, Bad pixel removal, Smearing removal. • A straylight correction is also applied but described in companion papers. • Conversions: Pixel-wavelength assignment, Radiance (nadir) or Transmittance (occultation) conversion. • Radiometric comparison with coincident MRO/MARCI measurements, serving as validation for the radiance calibrated data. [ABSTRACT FROM AUTHOR]

Hubble Space Telescope Imaging Spectrograph (HST/STIS) UV observations of Venus’ upper cloud tops were obtained between 20N and 40S latitude on December 28, 2010; January 22, 2011 and January 27, 2011 in coordination with the Venus Express (VEx) mission. The high spectral (0.27 nm) and spatial (40–60 km/pixel) resolution HST/STIS data provide the first direct and simultaneous record of the latitude and local time distribution of Venus’ 70–80 km SO and SO 2 (SO x ) gas density on Venus’ morning quadrant. These data were obtained simultaneously with (a) VEx/SOIR occultation and/or ground-based James Clerk Maxwell Telescope sub-mm observations that record respectively, Venus’ near-terminator SO 2 and dayside SO x vertical profiles between ∼75 and 100 km; and (b) 0.36 μm VEx/VMC images of Venus’ cloud-tops. Updating the (Marcq, E. et al. [2011]. Icarus 211, 58–69) radiative transfer model SO 2 gas column densities of ∼2–10 μm-atm and ∼0.4–1.8 μm-atm are retrieved from the December 2010 and January 2011 HST observations, respectively on Venus’ dayside (i.e., at solar zenith angles (SZA) < 60°); SO gas column densities of 0.1–0.11 μm-atm, 0.03–0.31 μm-atm and 0.01–0.13 μm-atm are also retrieved from the respective December 28, 2010, January 22, 2011 and January 27, 2011 HST observations. A decline in the observed low-latitude 0.24 and 0.36 μm cloud top brightness paralleled the declining SO x gas densities. On December 28, 2010 SO 2 VMR values ∼280–290 ppb are retrieved between 74 and 81 km from the HST and SOIR data obtained near Venus’ morning terminator (at SZAs equal to 70° and 90°, respectively); these values are 10× higher than the HST-retrieved January 2011 near terminator values. Thus, the cloud top SO 2 gas abundance declined at all local times between the three HST observing dates. On all dates the average dayside SO 2 /SO ratio inferred from HST between 70 and 80 km is higher than that inferred from the sub-mm the JCMT data above 84 km confirming that SO x photolysis is more efficient at higher altitudes. The direct correlation of the SO x gases provides the first clear evidence that SO x photolysis is not the only source for Venus’ 70–80 km sulfur reservoir. The cloud top SO 2 gas density is dependent in part on the vertical transport of the gas from the lower atmosphere; and the 0.24 μm cloud top brightness levels are linked to the density of the sub-micron haze. Thus, the new results may suggest a correlation between Venus’ cloud-top sub-micron haze density and the vertical transport rate. These new results must be considered in models designed to simulate and explore the relationship between Venus’ sulfur chemistry cycle, H 2 SO 4 cloud formation rate and climate evolution. Additionally, we present the first photochemical model that uniquely tracks the transition of the SO 2 atmosphere from steady to non-steady state with increasing SZA, as function of altitude within Venus’ mesosphere, showing the photochemical and dynamical basis for the factor of ∼2 enhancements in the SO x gas densities observed by HST near the terminator above that observed at smaller SZA. These results must also be considered when modeling the long-term evolution of Venus’ atmospheric chemistry and dynamics. [ABSTRACT FROM AUTHOR]

Observations from Pioneer Venus and from SPICAV/SOIR aboard Venus Express (VEx) have shown the upper haze (UH) of Venus to be highly spatially and temporally variable, and populated by multiple particle size modes. Previous models of this system (e.g., Gao et al., 2014. Icarus 231, 83–98), using a typical temperature profile representative of the atmosphere (viz., equatorial VIRA profile), did not investigate the effect of temperature on the UH particle distributions. We show that the inclusion of latitude-dependent temperature profiles for both the morning and evening terminators of Venus helps to explain how the atmospheric aerosol distributions vary spatially. In this work we use temperature profiles obtained by two instruments onboard VEx, VeRa and SPICAV/SOIR, to represent the latitudinal temperature dependence. We find that there are no significant differences between results for the morning and evening terminators at any latitude and that the cloud base moves downwards as the latitude increases due to decreasing temperatures. The UH is not affected much by varying the temperature profiles; however, the haze does show some periodic differences, and is slightly thicker at the poles than at the equator. We also find that the sulphuric acid “rain” seen in previous models may be restricted to the equatorial regions of Venus, such that the particle size distribution is relatively stable at higher latitudes and at the poles. [ABSTRACT FROM AUTHOR]

We investigate water vapor saturation in the martian atmosphere in the presence or proximity of water ice clouds. We evaluate mixing ratio profiles of ambient water vapor derived from measurements by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and the Nadir and Occultation for MArs Discovery (NOMAD) instrument in comparison with saturated vapor mixing ratio profiles derived from co-located temperature measurements by the Mars Climate Sounder (MCS). We find that during the aphelion season the average saturation state of the atmosphere is close to saturation in the presence of clouds, with supersaturation ratios reaching values of no more than two to three towards the top of the cloud layer. During the perihelion season, subseasonally averaged water vapor is close to saturation or somewhat subsaturated in the presence of clouds, with some supersaturation at southern high latitudes towards the top of the clouds. Measurements at northern mid- to high latitudes during a Global Dust Storm suggest a shift of the cloud occurrence to higher altitudes but exhibit a similar saturation structure, with supersaturations not exceeding a ratio of five. The atmosphere above cloud layers is largely subsaturated, suggesting that cloud formation is fairly instantaneous upon the temperature dropping below the frost point and high levels of supersaturation are not required to form water ice clouds. We propose a schematic model of cloud evolution in which small cloud particles are formed rapidly in slightly supersaturated regions, and then fall and grow such that ice opacity is still observed even in the subsaturated regions below. [Display omitted] • Water vapor saturation close to clouds in the martian atmosphere is investigated. • The average state of the atmosphere is close to saturation. • The atmosphere above and below cloud layers appears largely subsaturated. • High levels of supersaturation are not required to form water ice clouds. [ABSTRACT FROM AUTHOR]

Abstract: This paper is the fourth of a series of papers reporting critically evaluated rotational–vibrational line positions, transition intensities, pressure dependences, and energy levels, with associated critically reviewed assignments and uncertainties, for all the main isotopologues of water. This paper presents energy level and transition data for the following doubly and triply substituted isotopologues of water: D2 16O, D2 17O, and D2 18O. The MARVEL (Measured Active Rotational–Vibrational Energy Levels) procedure is used to determine the levels, the lines, and their self-consistent uncertainties for the spectral regions 0–14016, 0–7969, and 0–9108cm−1 for D2 16O, D2 17O, and D2 18O, respectively. For D2 16O, D2 17O, and D2 18O, 53534, 600, and 12167 lines are considered, respectively, from spectra recorded in absorption at room temperature and in emission at elevated temperatures. The number of validated energy levels is 12269, 338, and 3351 for D2 16O, D2 17O, and D2 18O, respectively. The energy levels have been checked against the ones determined, with an average accuracy of about 0.03cm−1, from variational rovibrational computations employing exact kinetic energy operators and an accurate potential energy surface. Furthermore, the rovibrational labels of the energy levels have been validated by an analysis of the computed wavefunctions using the rigid-rotor decomposition (RRD) scheme. The extensive list of MARVEL lines and levels obtained is deposited in the Supplementary Material of this paper, in a distributed information system applied to water, W@DIS, and on the official MARVEL website, where they can easily be retrieved. [Copyright &y& Elsevier]

Highlights: [•] First determination of the CO2-broadening of lines in the ν 4 band of methane were realized. [•] CO2-broadening of lines in the ν 4 band of methane were measured accurately. [•] The line parameters were measured by high resolution diode-laser spectroscopy. [•] The line parameters were deduced from Voigt, Rautian–Sobel’man and Galatry fits. [•] Impact of the line parameters to CO2-rich planetary atmospheres were addressed. [Copyright &y& Elsevier]

The report and results of an IUPAC Task Group (TG) formed in 2004 on 'A Database of Water Transitions from Experiment and Theory' (Project No. 2004-035-1-100) are presented. Energy levels and recommended labels involving exact and approximate quantum numbers for the main isotopologues of water in the gas phase, H216O, H218O, H217O, HD16O, HD18O, HD17O, D216O, D218O, and D217O, are determined from measured transition frequencies. The transition frequencies and energy levels are validated using first-principles nuclear motion computations and the MARVEL (measured active rotational-vibrational energy levels) approach. The extensive data including lines and levels are required for analysis and synthesis of spectra, thermochemical applications, the construction of theoretical models, and the removal of spectral contamination by ubiquitous water lines. These datasets can also be used to assess where measurements are lacking for each isotopologue and to provide accurate frequencies for many yet-to-be measured transitions. The lack of high-quality frequency calibration standards in the near infrared is identified as an issue that has hindered the determination of high-accuracy energy levels at higher frequencies. The generation of spectra using the MARVEL energy levels combined with transition intensities computed using high accuracy ab initio dipole moment surfaces are discussed. A recommendation of the TG is for further work to identify a single, suitable model to represent pressure- (and temperature-) dependent line profiles more accurately than Voigt profiles. [ABSTRACT FROM AUTHOR]

Abstract: This is the third of a series of articles reporting critically evaluated rotational–vibrational line positions, transition intensities, and energy levels, with associated critically reviewed labels and uncertainties, for all the main isotopologues of water. This paper presents experimental line positions, experimental-quality energy levels, and validated labels for rotational–vibrational transitions of the most abundant isotopologue of water, H2 16O. The latest version of the MARVEL (Measured Active Rotational–Vibrational Energy Levels) line-inversion procedure is used to determine the rovibrational energy levels of the electronic ground state of H2 16O from experimentally measured lines, together with their self-consistent uncertainties, for the spectral region up to the first dissociation limit. The spectroscopic network of H2 16O containstwo components, an ortho (o) and a para (p) one. For o-H2 16O and p-H2 16O, experimentally measured, assigned, and labeled transitions were analyzed from more than 100 sources. The measured lines come from one-photon spectra recorded at room temperature in absorption, from hot samples with temperatures up to 3000K recorded in emission, and from multiresonance excitation spectra which sample levels up to dissociation. The total number of transitions considered is 184667 of which 182156 are validated: 68027 between para states and 114129 ortho ones. These transitions give rise to 18486 validated energy levels, of which 10446 and 8040 belong to o-H2 16O and p-H2 16O, respectively. The energy levels, including their labeling with approximate normal-mode and rigid-rotor quantum numbers, have been checked against ones determined from accurate variational nuclear motion computations employing exact kinetic energy operators as well as against previous compilations of energy levels. The extensive list of MARVEL lines and levels obtained are deposited in the supplementary data of this paper, as well as in a distributed information system applied to water, W@DIS, where they can easily be retrieved. [Copyright &y& Elsevier]

Abstract: High-resolution water vapor absorption spectra have been measured at room temperature in the 8800–11,600cm−1 spectral region. They were obtained using the mobile BRUKER IFS 120M Fourier transform spectrometer (FTS) from ULB-SCQP coupled to the 50m base long multiple reflection White type cell in GSMA laboratory. The absorption path was 600m and different H2O/HDO/D2O mixtures were used. Measurements of line positions, intensities and self-broadening coefficients were performed for the HD16O isotopologue. 6464 rovibrational assignment of the observed lines was made on the basis of global variational predictions and allowed the identification of new energy levels. 3ν 3, 2ν 1+ν 3, 3ν 1+ν 2, ν 1+2ν 3 and 2ν 2+2ν 3 are the five strongest bands. The present paper provides a complementary data set on water vapor for atmospheric and astrophysical applications. [Copyright &y& Elsevier]

Abstract: The variability of the aerosol loading in the mesosphere of Venus is investigated from a large data set obtained with SOIR, a channel of the SPICAV instrument suite onboard Venus Express. Vertical profiles of the extinction due to light absorption by aerosols are retrieved from a spectral window around 3.0μm recorded in many solar occultations (∼200) from September 2006 to September 2010. For this period, the continuum of light absorption is analyzed in terms of spatial and temporal variations of the upper haze of Venus. It is shown that there is a high short-term (a few Earth days) and a long-term (∼80 Earth days) variability of the extinction profiles within the data set. Latitudinal dependency of the aerosol loading is presented for the entire period considered and for shorter periods of time as well. [Copyright &y& Elsevier]

Abstract: Using the SPICAV-UV spectrometer aboard Venus Express in nadir mode, we were able to derive spectral radiance factors in the middle atmosphere of Venus in the 170–320nm range at a spectral resolution of R ≃200 during 2006 and 2007 in the northern hemisphere. By comparison with a radiative transfer model of the upper atmosphere of Venus, we could derive column abundance above the visible cloud top for SO2 using its spectral absorption bands near 280 and 220nm. SO2 column densities show large temporal and spatial variations on a horizontal scale of a few hundred kilometers. Typical SO2 column densities at low latitudes (up to 50°N) were found between 5 and 50μm-atm, whereas in the northern polar region SO2 content was usually below 5μm-atm. The observed latitudinal variations follow closely the cloud top altitude derived by SPICAV-IR and are thought to be of dynamical origin. Also, a sudden increase of SO2 column density in the whole northern hemisphere has been observed in early 2007, possibly related to a convective episode advecting some deep SO2 into the upper atmosphere. [ABSTRACT FROM AUTHOR]

Abstract: This is the second of a series of articles reporting critically evaluated rotational–vibrational line positions, transition intensities, pressure dependences, and energy levels, with associated critically reviewed assignments and uncertainties, for all the main isotopologues of water. This article presents energy levels and line positions of the following singly deuterated isotopologues of water: HD16O, HD17O, and HD18O. The MARVEL (measured active rotational–vibrational energy levels) procedure is used to determine the levels, the lines, and their self-consistent uncertainties for the spectral regions 0–22708, 0–1674, and 0–12105cm−1 for HD16O, HD17O, and HD18O, respectively. For HD16O, 54740 transitions were analyzed from 76 sources, the lines come from spectra recorded both at room temperature and from hot samples. These lines correspond to 36690 distinct assignments and 8818 energy levels. For HD17O, only 485 transitions could be analyzed from three sources; the lines correspond to 162 MARVEL energy levels. For HD18O, 8729 transitions were analyzed from 11 sources and these lines correspond to 1864 energy levels. The energy levels are checked against ones determined from accurate variational nuclear motion computations employing exact kinetic energy operators. This comparison shows that the measured transitions account for about 86% of the anticipated absorbance of HD16O at 296K and that the transitions predicted by the MARVEL energy levels account for essentially all the remaining absorbance. The extensive list of MARVEL lines and levels obtained are given in the Supplementary Material of this article, as well as in a distributed information system applied to water, W@DIS, where they can easily be retrieved. In addition, the transition and energy level information for H2 17O and H2 18O, given in the first paper of this series [Tennyson, et al. J Quant Spectr Rad Transfer 2009;110:573–96], has been updated. [Copyright &y& Elsevier]

Abstract: This is the first part of a series of articles reporting critically evaluated rotational–vibrational line positions, transition intensities, pressure dependence and energy levels, with associated critically reviewed assignments and uncertainties, for all the main isotopologues of water. The present article contains energy levels and data for line positions of the singly substituted isotopologues H2 17Oand H2 18O. The procedure and code MARVEL, standing for measured active rotational–vibrational energy levels, is used extensively in all stages of determining the validated levels and lines and their self-consistent uncertainties. The spectral regions covered for both isotopologues H2 17O and H2 18O are . The energy levels are checked against ones determined from accurate variational calculations. The number of critically evaluated and recommended levels and lines are, respectively, 2687 and 8614 for H2 17O, and 4839 and 29364 for H2 18O. The extensive lists of MARVEL lines and levels obtained are deposited in the Supplementary Material, as well as in a distributed information system applied to water, W@DIS, where they can easily be retrieved. A distinguishing feature of the present evaluation of water spectroscopic data is the systematic use of all available experimental data and validation by first-principles theoretical calculations. [Copyright &y& Elsevier]

Abstract: New high-resolution water vapor absorption spectra were obtained at room temperature in the 4200–6600cm−1 spectral region by combining Fourier transform spectrometers (FTS) with single and multiple reflection cells. With absorption paths from 0.3 to 1800 m in pure and air diluted water vapor, accurate measurements of about 10400 lines in an intensity range from 10−29 to 10−19 cm/molecule have been performed. Positions, intensities, self- and air-broadening coefficients and air-induced shifts were determined for the H2 16O, H2 17O, H2 18O and HDO isotopologues. The rovibrational assignment of the observed lines was performed with the use of global variational predictions and allowed the identification of several new energy levels. One major contribution of this work consists of the identification of 3280 new weak lines. A very close agreement between the new measured parameters and those listed in the database is reported as well as between the observations and the most recent variational calculations for the positions and the intensities. The present parameters provide an extended and homogeneous data set for water vapor, which is shown to significantly improve the databases for atmospheric applications, especially in the transmission windows on both sides of the band centered at 5400cm−1. [Copyright &y& Elsevier]

Abstract: Line parameters for water vapour in natural abundance have recently been determined for the 9250–13000cm−1 region [M.-F. Mérienne, A. Jenouvrier, C. Hermans, A.C. Vandaele, M. Carleer, C. Clerbaux, P.-F. Coheur, R. Colin, S. Fally, M. Bach, J. Quant. Spectrosc. Radiat. Transfer 82 (2003) 99] and the 13000–26000cm−1 region [P.-F. Coheur, S. Fally, M. Carleer, C. Clerbaux, R. Colin, A. Jenouvrier, M.-F. Mérienne, C. Hermans, A.C. Vandaele, J. Quant. Spectrosc. Radiat. Transfer 74 (2002) 493] using a high-resolution Fourier-transform spectrometer with a long-path absorption cell. These spectra are analysed using several techniques including variational line lists and assignments made. In total, over 15000 lines were assigned to transitions involving more than 150 exited vibrational states of H2 16O. Twelve new vibrational band origins are determined and estimates for a further 16 are presented. [Copyright &y& Elsevier]

Abstract: This work presents new measurements of HDO line parameters in the near-infrared and visible regions (11500–23000cm−1). The measurements consist in high-resolution Fourier transform absorption spectra of H2O/HDO/D2O vapor mixtures, obtained using a long absorption path. Spectra with and without nitrogen as the buffer gas were recorded. Due to the simultaneous presence of the three isotopologues H2O, D2O, and HDO, the H2O lines removal and the D2O lines identification were two necessary preliminary steps to derive the HDO line parameters. The D2O contribution was small and confined to the well-known 4ν 1 + ν 3 band. An extensive listing of HDO spectroscopic parameters was obtained, for the first time, by fitting some 3256 observed lines to Voigt line profiles. The list contains calibrated line positions, absorption cross-sections and, for many of the lines, N2-broadening coefficients, as well as N2-induced frequency shifts. As a result of the low HDO vapor pressures, it was not possible to retrieve the self-broadening parameters. The list is available on the http://www.ulb.ac.be/cpm website. [Copyright &y& Elsevier]

The absorption of solar light by atmospheric water vapor in the visible spectral region is analyzed by means of ground-based absorption Fourier transform spectroscopy, performed at high resolution in Brussels during summer 2001. Several microwindows between 14,000 and 18,000 cm−1, in which water vapor lines are well isolated from solar lines and other atmospheric trace gases absorptions, are examined. They are demonstrated to be adequate for the retrieval of the total water vapor column. Based on the retrievals, a detailed analysis of the water vapor line parameters published in the HITRAN database and recently reinvestigated by different groups is performed. The analysis focuses on the one hand on the comparison of the retrieved water vapor columns with in situ measurements, performed at the same time as the spectroscopic measurements and at the same location, and on the other hand on the quality of the spectral fits. It is shown that the discrepancies between the line lists affect significantly the results. In particular it is shown that the weaker lines, not measured in earlier laboratory experiments, do contribute at large zenith angles and need to be taken into account in order to better simulate the atmospheric spectra. The importance of the pressure broadening parameters is also highlighted. [Copyright &y& Elsevier]

Considering the unique role that water in its vapor phase plays in atmospheric physical and chemical processes, there is a need for accurate spectroscopic parameters for this molecule. Long-pathlength Fourier transform spectra of water vapor with synthetic air as the perturbing gas were recorded and analyzed in the 26,000–13,000 cm−1 spectral region. New measurements of air-broadening and air-shifting parameters with associated uncertainties are presented for about 5000 lines. These data complement our existing database, providing a homogeneous and extensive dataset extending from 26,000 to 9250 cm−1. Comparisons with the two most frequently used HITRAN and ESA databases as well as with other literature data available are made. Agreements and discrepancies are underlined and briefly discussed. [Copyright &y& Elsevier]

Line parameters of water vapor have been obtained in the 13 000–9250 cm−1 spectral region. The measurements were performed with a high resolution Fourier transform spectrometer coupled to long-path multiple reflection cells. Emphasis was put on the determination of accurate parameters for the weak lines, most of which have not been previously reported in the literature. A total of 7061 lines have been studied. Their positions and intensities have been determined, and for many of analyzed lines, self- and air-broadening coefficients, as well as pressure-induced shifts were obtained. A preliminary analysis of the dependence of the broadenings and shifts, with vibrational and rotational quantum numbers is given. The discussion is focused on the comparison between our measured parameters and those listed in the HITRAN database. A very good agreement is reported for the positions and intensities. It is suggested that the construction of a comprehensive and complete dataset for water vapor in this spectral region requires the merging of independent measurements, and of calculated values of broadenings and shifts for weak and blended lines. [Copyright &y& Elsevier]

The radiative properties of water vapor play an important role in the physical and chemical processes occurring in the atmosphere. Accurate knowledge of the line parameters for this species is therefore needed. This work presents new measurements of water vapor line parameters in the 26 000–13 000 cm−1 spectral region. The measurements were obtained by combining a high-resolution Fourier transform spectrometer with a long-path absorption cell, thus allowing the observation of very weak, previously unobserved, lines. A total of more than 9000 lines have been identified and their position, integrated cross section and self-broadening parameter have been determined. The dependence of the line parameters on nitrogen buffer gas pressure (0–800 hPa) has also been studied. The complete line list presented here is primarily compared to the HITRAN spectroscopic database, most frequently used in atmospheric calculations. [Copyright &y& Elsevier]

More than a full Martian year of observations have now been made by the Nadir Occultation for MArs Discovery (NOMAD) instrument suite on-board the ExoMars Trace Gas Orbiter. Radiative transfer modeling of NOMAD observations taken in the nadir geometry enable the seasonal and global-scale variations of carbon monoxide gas in the Martian atmosphere to be characterized. These retrievals show the column-averaged volume mixing ratio of carbon monoxide to be about 800 ppmv, with significant variations at high latitudes caused by the condensation and sublimation of the background CO 2 gas. Near summer solstice in each hemisphere, the CO volume mixing ratio falls to 400 ppmv in the south and 600 ppmv in the north. At low latitudes, carbon monoxide volume mixing ratio inversely follows the annual cycle of surface pressure. Comparison of our retrieved CO volume mixing ratio against that computed by the GEM-Mars general circulation model reveals a good match in their respective seasonal and spatial trends, and can provide insight into the physical processes that control the distribution of CO gas in the current Martian atmosphere. • The global, seasonal distribution of CO gas is retrieved from NOMAD nadir observations. • Globally averaged CO abundance is ~800 ppm, but with strong seasonal variations. • GCM modeling can provide insight into physical processes behind retrieved abundances. [ABSTRACT FROM AUTHOR]

Because of its important role in the Martian carbon cycle, carbon monoxide (CO) has been the subject of many measurements from ground and from space. Daytime measurements have been mostly exploited to measure the CO abundance because of their good signal-to-noise ratio, but night-time observations have not been documented yet. We demonstrate here the possibility of using PFS (Planetary Fourier Spectrometer) night-time observations to measure the Martian CO abundance. More particularly in this paper, we show that measuring CO during night with PFS can be achieved by averaging a large number of spectra to reach sufficient signal-to-noise. Furthermore, we show that the number of averaged spectra is not the only driving parameter for the detection of CO. High surface temperatures and high thermal contrast (negative in our case) are the two other conditions required for the measurement of the night-time CO abundance. Because of this, the retrievals are especially successful in the Southern Hemisphere during spring and summer when and where these two conditions are met. For night-time spectra with a positive detection, CO vertical profiles are successfully retrieved using the Optimal Estimation method and are characterized in terms of vertical sensitivity. Successful retrievals imply the use of accurate temperature profiles, and in particular an appropriate representation of the thermal inversion in the lower atmosphere. The temperature was obtained by using the relevant information from the CO 2 ν 3 band (Bauduin et al., 2020, this issue). A complete error budget of the retrieved CO profiles is also performed and includes different sources of uncertainty. Although the retrieved profiles are not resolved vertically, we show in particular that night-time PFS observations carry information mostly on the CO abundance for the 0–10 ​km altitude region, and thus provide a stronger constraint on the near-surface CO abundance compared to daytime observations. [Display omitted] • CO night-time abundance is retrieved from averaged night-time nadir PFS/SWC spectra. • Averages of large ensemble of spectra required to detect CO from night-time spectra. • A detection method is developed to select spectra with detectable CO lines. • Detection of CO driven by the surface temperature and thermal contrast values. • Night-time CO is best measured in the Southern Hemisphere during spring and summer. [ABSTRACT FROM AUTHOR]

Nadir remote sensing of the night side of Mars is challenging, mainly due to the low signal-to-noise ratio of such observations. We show in a companion paper that the abundance of carbon monoxide (CO) during night can be retrieved from the observations of the Planetary Fourier Spectrometer (PFS). This requires, however, an accurate knowledge of the temperature profile, and especially of the night-time thermal inversions, to properly model the atmospheric emission. While the temperature profile is usually retrieved from the ν 2 band of CO 2 (centered at 667 ​cm−1), this work shows that, for averaged night-time PFS observations built from a large ensemble of spectra, the temperature profile can be retrieved from the more saturated ν 3 band of CO 2 (centered at 2349 ​cm−1). We show especially that, due to IFOV (instantaneous field-of-view) size differences and boresight offset between the longwave and shortwave channels of PFS, the temperature profile retrieved from the ν 3 band is more consistent with the emission observed in the 1-0 band of CO (centered at 2143 ​cm−1), which is used in the second part paper. We provide a complete characterization of the retrieved temperature profiles in terms of error and vertical sensitivity. Using this, we show that using the ν 3 CO 2 band allows to properly constrain and characterize the thermal inversions encountered near the surface for most night-time observations. The resulting set of temperature profiles is essential for the retrieval of the night-time CO abundance that is presented in the companion paper. Beyond their usefulness for the night-time CO retrieval, we suggest with a last example that temperature profiles retrieved from the ν 3 band of CO 2 could be use more generally to study surface thermal inversions encountered at night. [Display omitted] • Retrieval of temperature profiles from averaged night-time nadir PFS/SWC spectra. • The wings of the saturated CO 2 ν 3 band can be exploited for the temperature retrieval. • Full characterization of the retrieved temperature profiles. • The night-time thermal inversions are well constrained when using the CO 2 ν 3 band. [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]

• Fully blind machine learning approach to produce a quicklook and discover new lines on large spectroscopic datasets. • Validation on ESA/ExoMars TGO NOMAD-SO dataset. • Absence of methane in 100–500 ppt from this analysis. • Exhibition of unexpected lines. One of the main difficulties to analyze modern spectroscopic datasets is due to the large amount of data. For example, in atmospheric transmittance spectroscopy, the solar occultation channel (SO) of the NOMAD instrument onboard the ESA ExoMars2016 satellite called Trace Gas Orbiter (TGO) had produced ~ 10 millions of spectra in ~ 20000 acquisition sequences since the beginning of the mission in April 2018 until 15 January 2020. Other datasets are even larger with ~ billions of spectra for OMEGA onboard Mars Express or CRISM onboard Mars Reconnaissance Orbiter. Usually, new lines are discovered after a long iterative process of model fitting and manual residual analysis. Here we propose a new method based on unsupervised machine learning, to automatically detect new minor species. Although precise quantification is out of scope, this tool can also be used to quickly summarize the dataset, by giving few endmembers ("source") and their abundances. The methodology is the following: we proposed a way to approximate the dataset non-linearity by a linear mixture of abundance and source spectra (endmembers). We used unsupervised source separation in form of non-negative matrix factorization to estimate those quantities. Several methods are tested on synthetic and simulation data. Our approach is dedicated to detect minor species spectra rather than precisely quantifying them. On synthetic example, this approach is able to detect chemical compounds present in form of 100 hidden spectra out of 104, at 1.5 times the noise level. Results on simulated spectra of NOMAD-SO targeting CH 4 show that detection limits goes in the range of 100–500 ppt in favorable conditions. Results on real martian data from NOMAD-SO show that CO 2 and H 2 O are present, as expected, but CH 4 is absent. Nevertheless, we confirm a set of new unexpected lines in the database, attributed by ACS instrument Team to the CO 2 magnetic dipole. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The Solar Occultation in the InfraRed (SOIR) instrument onboard Venus Express was designed to measure the Venusian atmospheric transmission at high altitudes (65–220 km) in the infrared range (2.2–4.3 μm) with a high spectral resolution. In this work, we investigate the optical properties of Venus's haze layer above 90 km using SOIR solar occultation observations. Vertical and latitudinal profiles of the extinction coefficient, optical thickness, and mixing ratio of aerosols are retrieved. One of the most remarkable results is that the aerosol mixing ratio tends to increase with altitude above 90 km at both high and low latitudes. We speculate how aerosols could be produced at such high altitudes. [ABSTRACT FROM AUTHOR]

• CO total columns obtained from constrained profile retrievals on nadir measurements. • Retrieved profiles fully characterized in terms of error and vertical sensitivity. • Potential use of single PFS spectra to monitor CO with good accuracy demonstrated. The knowledge of the carbon monoxide (CO) abundance on Mars is essential in order to assess the processes driving the carbon cycle on the planet. Solar occultation measurements provide vertically-resolved measurements of CO from a few kilometers to higher altitudes and can be complemented by nadir measurements to enhance the spatial coverage of observations and the monitoring of the near-surface layer. Up to now, in the shortwave, CO retrievals from nadir observations have, however, mostly been performed on mean spectra and only total column abundances have been obtained. In this work we explore the possibility of exploiting nadir measurements from the Planetary Fourier Spectrometer (PFS) in the 1–0 band of CO (centered at 2143 cm − 1) to retrieve vertical profiles of that species on individual measurement. The retrievals are performed for a set of 16 selected PFS spectra with reasonable signal-to-noise ratios by applying the Optimal Estimation Method (OEM) with appropriate constraints, built from model simulations of the Martian atmosphere. The retrieved profiles are characterized in terms of vertical sensitivity and errors. We demonstrate, in particular, that the PFS nadir measurements carry information mostly on the CO column below 15 km, with a maximum sensitivity to the near-surface atmosphere. These measurements allow to substantially reduce the prior uncertainty on the CO abundance in this altitude range, with an estimated total retrieval error on the column-averaged volume mixing ratio (VMR) around 10%. We show that the set of retrieved VMRs are in the range of values reported from other instruments. The retrieved VMRs also capture well the known spatial and seasonal CO variability, which is promising in the perspective of better exploiting the exceptional set of PFS observations on Mars. [ABSTRACT FROM AUTHOR]

Eventual past forms of life on Mars and future human missions have been and will be strongly affected by radiation. Here we present the estimation of the radiation environment (Galactic Cosmic Rays, GCRs, and Solar Energetic Particles, SEPS) and corresponding doses in proxies for biological targets at Oxia Planum, chosen as the landing site for the next ExoMars 2020 mission [1], and Mawrth Vallis, previously a candidate landing site. The study is performed through detailed Monte Carlo particle transport calculations using the dMEREM code [2] based on the Geant4 toolkit [3], for the period of the Halloween storm (October 2003).The results [4] show that both elevation (and thus atmospheric depth) and mineralogical content influence the doses received at the surface of the two sites. In particular, we find that the spectra of downward GCRs are slightly influenced by differences in the atmospheric depth, while SEPs, being more stopped in the upper part of the atmosphere, have a downward spectrum relatively similar for the two locations. The different behavior of the two types of radiation is also analyzed in their interaction with the regolith. The influence of diurnal variations of the atmosphere on the radiation environment are also studied, and compared with the observations from Curiosity [5]. For the two sites mentioned above and also for Gale Crater, an underestimation of the radiobiological doses can be noted. We speculate that the Monte Carlo particle transport approach partially understimate the radiation damage at lower energies, as also demonstrated by some few previous works, and we present our current efforts in better understanding how to model the effects of radiation across different energy/lengths scales [6].This work is initiated in the context of the EU H2020 project ESC2RAD "Enabling Smart Computations to study space RADiation Effects", Grant 776410 [6].[1] Jorge L. Vago, Frances Westall, Pasteur Instrument Team, Landing Site Selection Working Group, and Other Contributors: Habitability on Early Mars and the Search for Biosignatures with the ExoMars Rover, Astrobiology Vol. 17, pp. 471-501, 2017[2] McKenna-Lawlor S., Gonçalves P., A. Keating A., Morgado B., Heynderickx D., P. Nieminen P., Santin G., Truscott P., Lei F., Foing B., Balaz J.: Characterization of the particle radiation environment at three potential landing sites on Mars using ESA's MEREM models, Icarus Vol. 218, pp. 723–734, 2012[3] https://geant4.web.cern.ch/[4] F. Da Pieve et al, in preparation[5] Hassler, D., Cary Zeitlin M., F. Wimmer-Schweingruber R., Ehresmann B., Rafkin S., L. Eigenbrode J., E. Brinza D., Weigle G., Böttcher S., Böhm E., Burmeister S., Guo J., Köhler J., Martin S, Reitz G., A. Cucinotta F., Kim M.-H., Grinspoon D., A. Bullock M., Posner A., Gómez-Elvira J., Vasavada A., Grotzinger J.P., MSL Science Team (2014). Mars' surface radiation environment measured with the Mars Science laboratory's curiosity rover. Science 343, 1244797[6] www.esc2rad.eu [ABSTRACT FROM AUTHOR]