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

International audienceThe 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 CO 2 and H 2O ices and propose a scenario that describes the winter and spring evolution of the northern seasonal deposits. During winter, the CO 2-rich condensates are initially transparent and could be in slab form. A water ice annulus surrounds the sublimating CO 2 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 H 2O grains trapped in the CO 2-rich ice layer and released during CO 2 sublimation. By L s 50, H 2O ice spectrally dominates most of the deposits. In order to hide the still several tens of centimeters thick CO 2 ice layer in central areas of the cap we propose the buildup of an optically thick top layer of H 2O ice from ice grains previously embedded in the CO 2 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. Copyright 2011 by the American Geophysical Union