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SOIR is a high‐resolution spectrometer flying on board the ESA Venus Express mission. It performs solar occultations of the Venus high atmosphere, and so defines unique vertical profiles of many of the Venus key species. In this paper, we focus on the Venus main constituent, carbon dioxide. We explain how the temperature, the total density, and the total pressure are derived from the observed CO2 density vertical profiles. A striking permanent temperature minimum at 125 km is observed. The data set is processed in order to obtain a Venus Atmosphere from SOIR measurements at the Terminator (VAST) compilation for different latitude regions and extending from 70 up to 170 km in altitude. The results are compared to many literature results obtained from ground‐based observations, previous missions, and the Venus Express mission. The homopause altitude is also determined. Key Points Venus terminator mesosphere and thermosphere Carbon dioxide measurement Venus terminator model

Accurate modeling of the Venusian cloud structure remains challenging due to its complex microphysical properties. Condensation primarily determines the cloud particle size distribution within the various cloud layers. However, existing Venus microphysics models mainly use a full‐stationary bin scheme, which may be prone to numerical diffusion during condensation. To address this, we developed a new microphysics model, the Simulator of Particle Evolution, Composition, and Kinetics (SPECK), which incorporates a moving‐center bin scheme designed to minimize numerical diffusion. Furthermore, SPECK can accommodate any number of size distributions with multiple components, enabling versatile applications for more complex cloud systems. The 0‐D simulations demonstrated that this microphysics framework is a reliable tool for modeling cloud microphysics under Venusian atmospheric conditions, particularly in capturing condensation and evaporation processes. We further validated SPECK against recent Venus microphysics models in 1‐D simulations. The moving‐center scheme is shown to exhibit less numerical diffusion compared to an existing model based on a full‐stationary bin scheme, allowing for more accurate calculations of microphysical processes. Furthermore, SPECK reproduces the cloud structure observed by the Pioneer Venus Large Probe, using the same computational settings adopted in the latest microphysical model study. Thanks to the suppressed numerical diffusion, SPECK achieves high accuracy at half the typical resolution while reducing computational time sixfold, making it a promising tool for future 3‐D modeling. This microphysics framework will be useful for the upcoming EnVision mission and is applicable to other planetary atmospheres, including those of Mars, Titan, gas giants, and exoplanets. Plain Language Summary Understanding the cloud structure on Venus is challenging due to the complex interaction between cloud particles and the atmosphere. A major process shaping these clouds is condensation, which affects the size of particles in the clouds. However, existing models of the Venusian clouds often struggle with accuracy due to a method that can cause errors in the simulation of the condensation process. To address this, we developed a new model, the Simulator of Particle Evolution, Composition, and Kinetics (SPECK), which reduces these errors, allowing for more precise predictions of how particles change in size. SPECK also works with a wide range of particle types, making it adaptable for studying other complex cloud systems. In simple box simulations, SPECK proved effective for modeling cloud processes on Venus, especially in tracking how particles grow and evaporate. The model also successfully reproduces observations from the Pioneer Venus Large Probe, showing better accuracy than previous methods while requiring less computing power. This makes SPECK a valuable tool for future Venus studies, including the upcoming EnVision mission. Additionally, SPECK can be applied to study clouds on other planets, such as Mars, Titan, gas giants, and exoplanets. Key Points The moving‐center bin scheme is applied for the first time in a Venus cloud microphysics model The new model accurately simulates the Venusian clouds by minimizing numerical diffusion, successfully reproducing the observed structures Halving the resolution cut computation time sixfold, keeping accuracy intact, proving the model's potential for future 3‐D modeling

Still delivering ESA's Venus Express probe has been in orbit since April 2006. Eight research papers in this issue present new results from the mission, covering the atmosphere, polar features, interactions with the solar wind and the controversial matter of venusian lightning. Håkan Svedham et al . open the section with a review of the similarities and (mostly) differences between Venus and its 'twin', the Earth. Andrew Ingersoll considers the latest results, and also how the project teams plan to make the most of the probe's remaining six years of life. Venus' mesosphere is a transition region between the retrograde super rotation at the top of the thick clouds and the solar-antisolar circulation in the thermosphere. The mesospheric distributions of HF, HCl, H 2 O and HDO are reported, and an unexpected extensive layer of warm air at altitudes 90–120 km on the nightside is found. Venus has thick clouds of H 2 SO 4 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 1 , 2 . The mesosphere has a light haze of variable optical thickness, with CO, SO 2 , HCl, HF, H 2 O 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, H 2 O and HDO. HCl is less abundant than reported 40 years ago 3 . HDO/H 2 O is enhanced by a factor of ∼2.5 with respect to the lower atmosphere, and there is a general depletion of H 2 O around 80–90 km for which we have no explanation.

The growing demand for virtual design of composite materials necessitates the development of comprehensive models relating the microstructure of the constituents to the macroscopic mechanical behavior. In this spirit, a new model was recently introduced that enables the computation of polymer stiffness over the entire range of use temperature (C. A. Mahieux and K. L. Reifsnider, Polymer42(7) (2001) 3281). A preliminary study stated the feasibility of the approach and the apparent possibility of applying this model to all polymers. The present study investigated the possibility of applying the model to some commercial thermoplastics: PMMA, PEEK, PPS and AS4/PPS composite. Cryogenic DMA were performed and the properties (crystallinity and molecular weight) of the composites were systematically varied. The model was applied to the different materials. The influence of the chemical nature, the molecular weight and the crystallinity content on the model input was carefully studied. The molecular nature and molecular weight were found to have little influence on the statistical parameters; the statistical parameter associated with the glass transition was found to vary linearly with crystalline content for the semi-crystalline samples. The model was found to successfully represent the behavior of all of the polymer based systems considered in the present study.

The SOIR instrument, flying on board Venus Express, operates in the infrared spectral domain and uses the solar occultation technique to determine the vertical profiles of several key constituents of the Venus atmosphere. The retrieval algorithm is based on the optimal estimation method, and solves the problem simultaneously on all spectra belonging to one occultation sequence. Vertical profiles of H2O, CO, HCl, and HF, as well as some of their isotopologues, are routinely obtained for altitudes ranging typically from 70 to 120 km, depending on the species and the spectral region recorded. In the case of CO2, a vertical profile from 70 up to 150 km can be obtained by combining different spectral intervals. Rotational temperature is also retrieved directly from the CO2 signature in the spectra. The present paper describes the method used to derive the above mentioned atmospheric quantities and temperature profiles. The method is applied on some retrieval cases illustrating the capabilities of the technique. More examples of results will be presented and discussed in a following companion paper which will focus on the CO2 vertical profiles of the whole data set.

The Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus/Solar Occultation at Infrared (SPICAV/SOIR) suite of instruments onboard the Venus Express spacecraft comprises three spectrometers covering a wavelength range from ultraviolet to midinfrared and an altitude range from 70 to >100 km. However, it is only recently (more than 1 year after the beginning of the mission) that the three spectrometers can operate simultaneously in the solar occultation mode. These observations have enabled the study of the properties of the Venusian mesosphere over a broad spectral range. In this manuscript, we briefly describe the instrument characteristics and the method used to infer haze microphysical properties from a data set of three selected orbits. Discussion focuses on the wavelength dependence of the continuum, which is primarily shaped by the extinction caused by the aerosol particles of the upper haze. This wavelength dependence is directly related to the effective particle radius (cross section weighted mean radius) of the particles. Through independent analyses for the three channels, we demonstrate the potential to characterize the aerosols in the mesosphere of Venus. The classical assumption that the upper haze is only composed of submicron particles is not sufficient to explain the observations. We find that at high northern latitudes, two types of particles coexist in the upper haze of Venus: mode 1 of mean radius 0.1 ≤ rg ≤ 0.3 μm and mode 2 of 0.4 ≤ rg ≤ 1.0 μm. An additional population of micron‐sized aerosols seems, therefore, needed to reconcile the data of the three spectrometers. Moreover, we observe substantial temporal variations of aerosol extinction over a time scale of 24 h.

The detection of methane on Mars has been interpreted as indicating that geochemical or biotic activities could persist on Mars today. A number of different measurements of methane show evidence of transient, locally elevated methane concentrations and seasonal variations in background methane concentrations. These measurements, however, are difficult to reconcile with our current understanding of the chemistry and physics of the Martian atmosphere, which—given methane’s lifetime of several centuries—predicts an even, well mixed distribution of methane. Here we report highly sensitive measurements of the atmosphere of Mars in an attempt to detect methane, using the ACS and NOMAD instruments onboard the ESA-Roscosmos ExoMars Trace Gas Orbiter from April to August 2018. We did not detect any methane over a range of latitudes in both hemispheres, obtaining an upper limit for methane of about 0.05 parts per billion by volume, which is 10 to 100 times lower than previously reported positive detections. We suggest that reconciliation between the present findings and the background methane concentrations found in the Gale crater would require an unknown process that can rapidly remove or sequester methane from the lower atmosphere before it spreads globally.

The NOMAD (“Nadir and Occultation for MArs Discovery”) spectrometer suite on board the ExoMars Trace Gas Orbiter (TGO) has been designed to investigate the composition of Mars’ atmosphere, with a particular focus on trace gases, clouds and dust. The detection sensitivity for trace gases is considerably improved compared to previous Mars missions, compliant with the science objectives of the TGO mission. This will allow for a major leap in our knowledge and understanding of the Martian atmospheric composition and the related physical and chemical processes. The instrument is a combination of three spectrometers, covering a spectral range from the UV to the mid-IR, and can perform solar occultation, nadir and limb observations. In this paper, we present the science objectives of the instrument and explain the technical principles of the three spectrometers. We also discuss the expected performance of the instrument in terms of spatial and temporal coverage and detection sensitivity.

Global dust storms on Mars are rare but can affect the Martian atmosphere for several months. They can cause changes in atmospheric dynamics and inflation of the atmosphere , primarily owing to solar heating of the dust . In turn, changes in atmospheric dynamics can affect the distribution of atmospheric water vapour, with potential implications for the atmospheric photochemistry and climate on Mars . Recent observations of the water vapour abundance in the Martian atmosphere during dust storm conditions revealed a high-altitude increase in atmospheric water vapour that was more pronounced at high northern latitudes , as well as a decrease in the water column at low latitudes . Here we present concurrent, high-resolution measurements of dust, water and semiheavy water (HDO) at the onset of a global dust storm, obtained by the NOMAD and ACS instruments onboard the ExoMars Trace Gas Orbiter. We report the vertical distribution of the HDO/H O ratio (D/H) from the planetary boundary layer up to an altitude of 80 kilometres. Our findings suggest that before the onset of the dust storm, HDO abundances were reduced to levels below detectability at altitudes above 40 kilometres. This decrease in HDO coincided with the presence of water-ice clouds. During the storm, an increase in the abundance of H O and HDO was observed at altitudes between 40 and 80 kilometres. We propose that these increased abundances may be the result of warmer temperatures during the dust storm causing stronger atmospheric circulation and preventing ice cloud formation, which may confine water vapour to lower altitudes through gravitational fall and subsequent sublimation of ice crystals . The observed changes in H O and HDO abundance occurred within a few days during the development of the dust storm, suggesting a fast impact of dust storms on the Martian atmosphere.