Mapping the mesospheric CO₂ clouds on Mars: MEx/OMEGA and MEx/HRSC observations and challenges for atmospheric models

This study presents the latest results on the mesospheric CO₂ clouds in the martian atmosphere based on observations by OMEGA and HRSC onboard Mars Express. We have mapped the mesospheric CO₂ clouds during nearly three martian years of OMEGA data yielding a cloud dataset of ∼60 occurrences. The global mapping shows that the equatorial clouds are mainly observed in a distinct longitudinal corridor, at seasons L_s=0-60° and again at and after L_s=90°. A recent observation shows that the equatorial CO₂ cloud season may start as early as at L_s=330°. Three cases of mesospheric midlatitude autumn clouds have been observed. Two cloud shadow observations enabled the mapping of the cloud optical depth (τ=0.01-0.6 with median values of 0.13-0.2 at λ=1μm) and the effective radii (mainly 1-3μm with median values of 2.0-2.3μm) of the cloud crystals. The HRSC dataset of 28 high-altitude cloud observations shows that the observed clouds reside mainly in the altitude range ∼60-85km and their east-west speeds range from 15 to 107m/s. Two clouds at southern midlatitudes were observed at an altitude range of 53-62km. The speed of one of these southern midlatitude clouds was measured, and it exhibited west-east oriented speeds between 5 and 42m/s. The seasonal and geographical distribution as well as the observed altitudes are mostly in line with previous work. The LMD Mars Global Climate Model shows that at the cloud altitude range (65-85km) the temperatures exhibit significant daily variability (caused by the thermal tides) with the coldest temperatures towards the end of the afternoon. The GCM predicts the coldest temperatures of this altitude range and the season L_s=0-30° in the longitudinal corridor where most of the cloud observations have been made. However, the model does not predict supersaturation, but the GCM-predicted winds are in fair agreement with the HRSC-measured cloud speeds. The clouds exhibit variable morphologies, but mainly cirrus-type, filamented clouds are observed (nearly all HRSC observations and most of OMEGA observations). In ∼15% of OMEGA observations, clumpy, round cloud structures are observed, but very few clouds in the HRSC dataset show similar morphology. These observations of clumpy, cumuliform-type clouds raise questions on the possibility of mesospheric convection on Mars, and we discuss this hypothesis based on Convective Available Potential Energy calculations.