High-altitude CO2 Clouds On Mars: Comparison Of OMEGA And HRSC Observations And GCM Predictions

Abstract

We will present the recent observations of the high-altitude CO2 clouds on Mars by OMEGA and HRSC on Mars Express. The dataset contains nearly 3 Martian years of data from the two instruments adding up to a total of 60 cloud observations for OMEGA and 23 observations from HRSC with 11 simultaneous observations by the two instruments. The clouds are mainly observed at Ls=0-60° with activity ceasing before the summer solstice, and starts again after Ls=90°. After Ls=120° the clouds are nearly absent in our dataset. The spatial distribution of the clouds is limited in a latitude range of +/-20°N (with two exceptions) and a longitude range -120 - 30°E (3 exceptions). Most of the observed clouds are cirrus-type, filamented clouds, but in 9 OMEGA observations the clouds exhibit round, convective-type structures. It should anyhow be noted that when OMEGA observes near the pericentre, the image is quite narrow inhibiting clear identification of the cloud morphology. The HRSC observations enable accurate cloud altitude measurements, yielding values between 60 and 85 km (+/-2 km) for the near-equatorial clouds. The clouds speeds, related to the prevailing east-west winds, range from 15 to 107 m/s (+/-15 m/s). Two cloud shadow observations show that cloud opacities can go up to τ=0.6 and the effective radii are in the range reff=1-2 μm. A comparison with the LMD Mars Global Climate Model shows a good agreement between the model-predicted winds and those observed by the HRSC. The LMD-MGCM shows that the temperature has a strong diurnal variation at the cloud observation altitudes due to the strong diurnal tide. The coldest temperatures in the near-equator cloud altitude range (60-85 km) are observed towards the end of the afternoon, whereas the warmest temperatures are found in the early morning hours. AM wishes to acknowledge funding from CNES.