Martian Mesospheric CO2 Clouds: a Review of a Decade of Studies
Résumé
The mesospheric CO2 ice clouds were among the major discoveries of the ESA Mars Express (MEx) mission: they were detected by OMEGA [Montmessin et al., 2007, JGR 112] after clear hints from SPICAM and PFS [Montmessin et al., 2006. Icarus 183; Formisano et al., 2006. Icarus 182]. We present here a review of OMEGA and HRSC observations, compared to other datasets, and the most recent results from modeling. The cloud climatology that can be compiled from all published datasets covers nine Martian Years. It shows a clear confinement in the tropics (+/-20°N) with a distinct longitudinal distribution, and a group of midlatitude clouds observed at local autumn in both hemispheres. The first tropical clouds form around the spring equinox (Ls=330°-30°), and have a clear formation peak at the beginning of the year until just before the summer solstice. A second formation period starts some time after the solstitial pause. No equatorial cloud has been observed in the Ls=150°-330° timeframe. The equatorial daytime clouds form mostly at 60-80 km, with a particle effective radius reff of 1-3 μm and an opacity τ<0.5. As Clancy and Sandor 1998 [GRL 25] suggested, the mesospheric clouds form in supersaturated pockets created by the coincident effects of thermal tides and gravity waves: Gonzalez-Galindo et al. 2011 [Icarus 216] showed that the tides could explain the latitudinal and longitudinal distribution of the clouds, and their different formation altitudes at night and during the day. Recently Listowski et al. 2014 [Icarus 237] used their microphysical 1D model to study in detail the properties of the mesospheric clouds, and showed that the vertical extent and lifetime of the clouds are closely linked to the properties of the cold pockets created by gravity waves, and that using dust lifted from the surface as condensation nuclei can not explain the observed cloud opacities: an exogenic source, likely meteoric dust, is therefore required. We will also address our current projects focusing on enlarging the observational datasets with previously unexploited observations and on our ambition of modeling the CO2 ice clouds in a global climate simulation context.