Winter and spring evolution of northern seasonal deposits on Mars from OMEGA on Mars Express

The OMEGA visible/near-infrared imaging spectrometer on Mars Express has observed the retreat of the northern seasonal deposits during Martian year 27-28 from the period of maximum extension, close to the northern winter solstice, to the end of the retreat at L_s 95°. We present the temporal and spatial distributions of both CO₂ and H₂O ices and propose a scenario that describes the winter and spring evolution of the northern seasonal deposits. During winter, the CO₂-rich condensates are initially transparent and could be in slab form. A water ice annulus surrounds the sublimating CO₂ ice, extending over 6° of latitude at L _s 320°, decreasing to 2° at L_s 350°, and gradually increasing to 4.5° at L_s 50°. This annulus first consists of thin frost as observed by the Viking Lander 2 and is then overlaid by H₂O grains trapped in the CO₂-rich ice layer and released during CO₂ sublimation. By L_s 50, H₂O ice spectrally dominates most of the deposits. In order to hide the still several tens of centimeters thick CO₂ ice layer in central areas of the cap we propose the buildup of an optically thick top layer of H₂O ice from ice grains previously embedded in the CO₂ ice and by cold trapping of water vapor from the sublimating water ice annulus. The CO ₂ ice signature locally reappears between L_s 50 and 70. What emerges from our observations is a very active surface-atmosphere water cycle. These data provide additional constraints to the general circulation models simulating the Martian climate.