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Abstract: Different missions have observed mesospheric clouds on Mars in the last years. The presence of these clouds implies, among other conditions, mesospheric temperatures below CO2 condensation temperature. We use a General Circulation Model to study the mesospheric temperatures and compare the observed distribution of the mesospheric clouds and the predicted climatology of mesospheric temperatures. Although the model does not usually predict temperatures below condensation for daytime conditions, in some regions the predicted temperatures are close enough to condensation that perturbations caused by small scale processes could produce local excursions below condensation. The location and time of the lowest temperatures predicted by the GCM correspond to a first order with the two observed populations of mesospheric clouds: equatorial clouds observed before and after the Northern summer solstice, and mid-latitude clouds observed around the Northern winter solstice. For the equatorial clouds season, the model predicts temperatures close to condensation at the longitude, latitude, altitude and local time where they have been observed. We find that the diurnal migrating thermal tide and non-migrating tides are at the root of the spatial confinement of the equatorial clouds. For the mid-latitude clouds season, the temperatures predicted by the model at the location of the observed clouds is too high. Stereo observations by two different instruments allow for the determination of the zonal speed of these clouds producing a rare dataset of mesospheric winds. We compare the mesospheric zonal winds predicted by the model with these observations, finding a good agreement, although in some cases the observed variability exceeds that predicted by the model. [Copyright &y& Elsevier]

The Cameron bands and the UV doublet are two of the most prominent emission systems in the UV in Mars dayside. Their altitude variation has been exploited in the past to derive thermospheric temperatures from measurements obtained by the Mariner 6, 7, and 9 missions, the SPICAM instrument on board Mars Express, and the IUVS instrument on board MAVEN. Here we identify and quantify possible biases in these temperature determinations. For this purpose, we use a global model able to simulate these two emission systems, and we compare the temperature derived from the simulated emission with that predicted by the model at the same location and time. We find that an exponential fit to the scale height of the UV doublet can be used to derive temperatures with an error less than 10 K at altitudes above about 170 km and for low and moderate values of the Solar Zenith Angle. The temperature derived from the Cameron bands is biased towards higher values due to the non-negligible contribution of CO to the emission. We find that, at 170 km, the difference between the temperature derived from the Cameron bands and the UV doublet can be related to the CO abundance. Our results have implications for previous temperature determinations from the Mariners, SPICAM/MEx and IUVS/MAVEN, some of them being biased by about 25 K. • Temperature derived from Cameron bands biased to high values due to CO contribution. • Temperature derived from UV doublet accurate in optically thin region, above ∼ 170 km. • The temperature difference between Cameron bands and UV doublet is linked to the CO abundance. [ABSTRACT FROM AUTHOR]

This work uses a subset of "quiet" MaRS ionospheric dayside observations (MaRS quiet , 2004–2017) and a 1-D photochemical model (IonA-2) to investigate the potential formation processes of the excess electron densities merged with the base of the main ionosphere (Mm). 42% of the investigated MaRS observations contain identified Mm, which occur in a large variety of shapes ranging from smoothly decreasing electron densities to peak structures below the base of M1. The Mm appear over the full range of accessible solar zenith angles (50° - 90°) and are found between approximately 70 and 110 km altitude. Their base is found on average deeper in the atmosphere than the base of the averaged undisturbed MaRS electron density profiles. This indicates a dependence of the Mm formation on energy sources that penetrate deep into the atmosphere. This is supported by a strong positive correlation with increasing solar activity when solar flares, coronal mass ejections, and enhanced short solar X-ray and Ly-α intensities are more common. No relationship is found between the Mm occurrence rate and the magnitude/inclination of the weak crustal crustal magnetic field in MaRS quiet. Investigations with the IonA-2 photochemical model for undisturbed and flare conditions show that the ionization of the local neutral atmosphere by solar X-ray radiation <2 nm provides a satisfying explanation for detected Mm features with smoothly decreasing electron densities below the M1 base in combination with moderate slopes of the lower Mm region α Mm and altitudes of the lower boundary h L , S. While sufficient ionization energy reaches the region of interest during flares, no Mm features with peaks below the M1 base occur in any of the model electron density profiles. This supports the conclusion that the subgroup of merged excess electron densities with peaks or intermediate features (Mi) below the M1 base must have an origin different from the sole variability of solar X-ray radiation during undisturbed and solar flare conditions. The size of the identified Mm makes an exclusive meteoric origin of the Mm peak structures unlikely. It is indicative from the IonA-2 model results that the general increase/decrease of solar X-ray <2 nm leads to a correlated response of the Mm region. The sporadic occurrence of the merged excess electron densities in the MaRS observations is therefore assumed to be a combination of observational (increased observation noise level compared to the available amount of X-ray radiation <2 nm, shift of the lower baseline by ionospheric deviations from radial symmetry) and environmental (e.g. variations in solar X-ray) factors. [ABSTRACT FROM AUTHOR]

We present a non-local thermodynamic equilibrium retrieval scheme for atmospheric composition and its application to Mars CO 2 infrared limb emissions as measured by the OMEGA instrument on board Mars Express (MEx). These emissions are caused by CO 2 fluorescence of solar radiation, and thus the retrieval scheme accounts for non-LTE processes. We analyzed the dayside limb observations from a selection of three OMEGA orbits or data qubes. Before the retrieval was applied, we performed a radiometric calibration, cleaned the spectra (including clustering techniques) and generated radiance vertical profiles for each dataset. We also present information on the inversion set up, results on the retrieved CO 2 density profiles, as well as the temperature profiles derived from the CO 2 densities by assuming hydrostatic equilibrium. An extensive sensitivity study of the retrieval scheme was carried out, including its application to the OMEGA spectra taken at different MEx orbital configurations, to conclude on its performance and to offer recommendations for its systematic use with MEx datasets. The uncertainty due to the instrumental Gain calibration and that caused by the retrieval noise error itself are of large importance for the inversion, but a comparable component of the total error comes from the uncertainties of the temperature provided by the GCM. We demonstrated that, between 120 and 160 km, CO 2 profiles can be derived with a precision around 30% and a vertical resolution of about 15 km. • OMEGA limb observations with high spatial resolution • Non-LTE model for CO 2 limb observations in the Mars thermosphere • Retrieval scheme applied to three interesting OMEGA data qubes • CO 2 profiles can be derived with a precision of around 20% between 120 and 160 km. • A vertical resolution of around 15 km is possible with the OMEGA spatial resolution. [ABSTRACT FROM AUTHOR]