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Abstract: The formation of CO2 ice clouds in the upper atmosphere of Mars has been suggested in the past on the basis of a few temperature profiles exhibiting portions colder than CO2 frost point. However, the corresponding clouds were never observed. In this paper, we discuss the detection of the highest clouds ever observed on Mars by the SPICAM ultraviolet spectrometer on board Mars Express spacecraft. Analyzing stellar occultations, we detected several mesospheric detached layers at about 100 km in the southern winter subtropical latitudes, and found that clouds formed where simultaneous temperature measurements indicated that CO2 was highly supersaturated and probably condensing. Further analysis of the spectra reveals a cloud opacity in the subvisible range and ice crystals smaller than 100 nm in radius. These layers are therefore similar in nature as the noctilucent clouds which appear on Earth in the polar mesosphere. We interpret these phenomena as CO2 ice clouds forming inside supersaturated pockets of air created by upward propagating thermal waves. This detection of clouds in such an ultrararefied and supercold atmosphere raises important questions about the martian middle-atmosphere dynamics and microphysics. In particular, the presence of condensates at such high altitudes begs the question of the origin of the condensation nuclei. [Copyright &y& Elsevier]

Mars northern polar latitudes are known to harbor an enhanced 3 μ m spectral signature when observed from orbit. This may indicate a greater amount of surface adsorbed or bound water, although it has not yet been possible to easily reconcile orbital observations with ground measurements by Phoenix. Here we re-analyzed OMEGA/Mars Express observations acquired during the Northern summer to further characterize this 3 μ m absorption band increase. We identify the presence of a new specific spectral signature composed of an additional narrow absorption feature centered at 3.03 μ m coupled with an absorption at λ ≥ 3. 8 μ m. This signature is homogeneously distributed over a high-albedo open ring surrounding the circumpolar low-albedo terrains between ∼ 68°N and 76°N and ∼ 0°E and 270°E. This location includes the Phoenix landing site. This feature shows no time variability and can be confidently attributed to a seasonally stable surface component. All together, the stability, spectral shape and absence of significant correlation with other signatures in the 1–2.5 μ m range discard interpretations relying on water ice or easily exchangeable adsorbed water. Sulfates, notably anhydrite, provide interesting comparisons to several sections of the spectrum. Analogies with Earth samples also show that the spectral signature could result from a latitudinal modification of the hydration state and/or grains size of salts contaminants. While the exact full spectral shape cannot be easily reproduced, plausible explanations to this observation seem to involve geologically recent water alteration at high northern latitudes. • Using OMEGA data, we have identified a new spectral signature composed of a narrow 3 μ m band associated with a 4 μ m absorption. • This signature is observed exclusively over an open-ring area surrounding the North polar dunes between 68°N and 76°N. • This observation may indicate a specific physical or chemical alteration potentially involving sulfates in a polar environment. [ABSTRACT FROM AUTHOR]

Abstract: We present the first results on solar occultations performed with the UV channel of SPICAM, on board Mars Express. From the dataset of over 900 occultations (performed between April 2004 and October 2011), about 640 atmospheric profiles of the martian atmosphere were derived. This dataset, spanning four martian years, allows characterization of the seasonal evolution and inter-annual comparisons of ozone and suspended particles. The dataset also includes observations of the Mars Year (MY) 28 global dust storm. In this paper the aforementioned data are analyzed with a focus on the aerosol profiles. We have mapped the seasonal behavior of the near-surface haze, revealing the typical behavior of the martian aerosol cycle, where the season most prone to develop dust storms (southern summer) shows aerosols lofted high in the atmosphere, whereas in the polar regions the aerosols are confined near the surface. More generally, aerosols seem to remain in the lower atmosphere at high latitudes and progressively penetrate to higher altitudes towards the tropics. This prevailing trend is probably related to enhanced atmospheric circulation at tropical regions due to high insolation and/or to higher cloud formation level in a warmer atmosphere. The dataset reveals frequent aerosol layers, found above or within the persistent near-surface haze. We have observed single and multiple layers (up to three layers in one profile) and we have mapped their properties. The highest layer altitudes observed during the global dust storm in the southern hemisphere, where thick layers form high above the abundant lower atmosphere dust haze. We present results on the analyzed Ångström coefficient α and its vertical variations. We also discuss the conversion of α into particle effective radius and present some examples of the effective radius vertical behavior. [Copyright &y& Elsevier]

Recent observations of the Martian hydrogen corona in the UV H Ly-alpha emission by the Hubble Space Telescope (HST) (Clarke et al., 2014) and the SPICAM UV spectrometer on Mars Express (Chaffin et al., 2014) reported its rapid change by an order of magnitude over a short few months period in 2007 (MY28), which is inconsistent with the existing models. One proposed explanation of the observed increase of the coronal emission is that during the global dust storm water vapor from the lower atmosphere can be transported to higher altitudes, where its photodissociation rate by near-UV sunlight increases, providing an additional source of hydrogen for the upper atmosphere. In this work we study the water vapor vertical distribution in the middle atmosphere of Mars during the 2007 global dust storm based on solar occultation measurements by the SPICAM IR spectrometer onboard the Mars-Express spacecraft. The vertical profiles of H 2 O density and mixing ratio have been obtained for solar longitudes Ls = 255°–300° in MY28. In the Northern hemisphere from Ls = 268° to Ls = 285° the H 2 O density at altitudes of 60–80 km increased by an order of magnitude. During the dust storm the profiles extended up to 80 km, with an H 2 O density exceeding 10 10 molecules/cm 3 (mixing ratio ≥200 ppm). Two maxima of the H 2 O density were detected. The largest H 2 O densities observed at latitudes higher than 60°N, over Ls = 269°–275°, do not directly correlate with the aerosol loading and likely relate to the downwelling branch of the meridional circulation that was intensified during the dust storm, and transported water from the Southern hemisphere to high northern latitudes. The second smaller maximum coincides with the high dust loading at middle northern latitudes. The comparison with geographically close observations in the quiet Mars year MY32, when the H 2 O content in the Northern hemisphere did not exceed 2 × 10 10 molecules/cm 3 and 50 ppm at 60 km, showed that the global dust storm was a unique event. The situation was different in the Southern hemisphere. During the dust storm the water density at 50–80 km increased by a factor of 4–5 with a mixing ratio >100 ppm, well correlated with the aerosol vertical extension. A somewhat weaker increase of the H 2 O density by a factor of 2–3 with a mixing ratio > 100 ppm was also observed during MY32 starting from Ls = 260°, suggesting a seasonal repeatability. The observed amount of water at high altitudes in both hemispheres can produce a large increase in the H escape rate on a timescale of weeks, as was shown in the photochemical modeling by Chaffin et al. (2017). Future modeling would be necessary to separate the seasonal and the dust storm contributions to the hydrogen escape. [ABSTRACT FROM AUTHOR]

Highlights: [•] The key radical OH is detected for the first time in the Mars atmosphere. [•] The OH emission appears as near-infrared night glow in the polar winter atmosphere. [•] The OH Meinel band structure constrains formation and de-excitation pathways. [•] The observations support current models of Mars atmospheric transport and chemistry. [Copyright &y& Elsevier]

Abstract: The two orders of magnitude drop between the measured atmospheric abundances of non-radiogenic argon, krypton and xenon in Earth versus Mars is striking. Here, in order to account for this difference, we explore the hypothesis that clathrate deposits incorporated into the current martian cryosphere have sequestered significant amounts of these noble gases assuming they were initially present in the paleoatmosphere in quantities similar to those measured on Earth (in mass of noble gas per unit mass of the planet). To do so, we use a statistical-thermodynamic model that predicts the clathrate composition formed from a carbon dioxide-dominated paleoatmosphere whose surface pressure ranges up to 3bars. The influence of the presence of atmospheric sulfur dioxide on clathrate composition is investigated and we find that it does not alter the trapping efficiencies of other minor species. Assuming nominal structural parameters for the clathrate cages, we find that a carbon dioxide equivalent pressure of 0.03 and 0.9bar is sufficient to trap masses of xenon and krypton, respectively, equivalent to those found on Earth in the clathrate deposits of the cryosphere. In this case, the amount of trapped argon is not sufficient to explain the measured Earth/Mars argon abundance ratio in the considered pressure range. In contrast, with a 2% contraction of the clathrate cages, masses of xenon, krypton and argon at least equivalent to those found on Earth can be incorporated into clathrates if one assumes the trapping of carbon dioxide at equivalent atmospheric pressures of ∼2.3bar. The proposed clathrate trapping mechanism could have then played an important role in the shaping of the current martian atmosphere. [Copyright &y& Elsevier]