SPRING EVOLUTION OF MARTIAN SEASONAL CAPS FROM HIGH-RESOLUTION MRO OBSERVATIONS. A. Pommerol 1 , G. Portyankina 1 , N. Thomas 1 , K. -M. Aye 1 , T. Appéré 2 , C. J. Hansen 3 , M. Vincendon 4 , and Y. Langevin 4 , 1 Physikalisches Institut, Universität Bern, Sidlerstrasse 5, CH-3012 Bern, Switzer- land., 2 IPAG, Grenoble, France, 3 PSI, Arizona, USA, 4 IAS, Orsay, France. Introduction: The Spring evolution of the Martian seasonal caps has first been characterized in the visible and thermal infrared spectral ranges allowing the iden- tification of processes involving CO 2 ice and mineral dust [e.g. 1,2]. More recently, global time-resolved observations in the near-infrared spectral range ob- tained by the OMEGA instrument permitted a clear distinction between H 2 O and CO 2 ices as well as a more precise determination of the dust/H 2 O ice/CO 2 ice associations (stratigraphy, mixture…) [3,4]. Three instruments onboard the MRO spacecraft, the HiRISE and CTX cameras and the CRISM near- infrared imaging spectrometer, now provide invaluable complementary information on these processes by re- peatedly observing selected areas of the Northern and Southern polar areas during spring and summer sea- sons. Contrary to previous datasets, they only offer a very partial coverage but an extremely high spatial resolution (50 cm / pixel for HiRISE, 20 m / pixel for CRISM). We present time series of visible and spectral im- ages in both Northern and Southern hemispheres that we analyze in the context of global observations. We use the high spatial and temporal resolution observa- tions to test and refine the general scenarios that were derived from global lower resolution datasets. Methods: We have studied the evolution with time of 12 different regions of interest around the Martian south pole [5] and of 9 regions around the Martian North Pole. These regions were chosen for the availa- bility of repeated observations with good temporal resolution. In the South, we mostly use data from the first year of MRO operations while we use data from both first and second year of operation in the North to maximize the temporal coverage. From both HiRISE and CRISM data, we extract and follow the temporal evolution of albedo of selected terrains. In addition, we calculate from CRISM data, the strength of CO 2 and H 2 O spectral signatures that we follow in time as integrated time curves and/or time series of spectral maps. Time series of color images are produced from repeated HiRISE observations in a sim- ilar way. The local evolutions of terrains are analyzed and compared to large-scale evolutions previously obtained by other instruments. Scenarios are then proposed to account for the temporal evolutions of terrains appear- ance and composition. Southern Hemisphere: Figure 1 shows a typical example of the temporal evolution of south polar ter- rains during early spring. It displays many of the fea- tures, dark fans, elongated bright features, blue ha- loes…, described in more details by [5]. Figure 1: Temporal evolution of a fixed scene in the re- gion of Ithaca (178°E, 85°S) from HiRISE color observa- tions. We propose that the evolution of the appearance of south polar terrains during spring results from a com- petition between the sublimation of the CO 2 ice layer from its bottom (dominating at the beginning of spring) and from its top (dominating at the end of spring). Sublimation from the bottom of the ice layer is possible in early spring (Ls=170-200°) when the slab of CO 2 is transparent enough to let the solar radiation reach the mineral substratum [2]. The meter-scale to- pography triggers the on-set of a jet activity as illus- trated in figure 2. Dark fans are formed as the largest mineral grains emitted by the jet settle close to the vent source. Smaller grains are spread by winds over much larger areas and contribute to an early darkening of the polar area, particularly prominent in the so-called “cryptic region” [2]. As the surface of the ice layer is contami- nated by mineral dust, the transmission of solar energy to the substratum and hence the basal sublimation is inhibited, resulting in a rapid drop off of the jet activi- ty. 6013.pdf Fifth Mars Polar Science Conference (2011)