SPRING EVOLUTION OF MARTIAN SEASONAL CAPS FROM HIGH-RESOLUTION MRO OBSERVATIONS. A. Pommerol1, G. Portyankina1, N. Thomas1, K. -M. Aye1, T. Appéré2, C. J. Hansen3, M. Vincendon4, and Y. Langevin4, 1Physikalisches Institut, Universität Bern, Sidlerstrasse 5, CH-3012 Bern, Switzer-land., 2IPAG, Grenoble, France, 3PSI, Arizona, USA, 4IAS, 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 CO2 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 H2O and CO2 ices as well as a more precise determination of the dust/H2O ice/CO2 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 CO2 and H2O 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 CO2 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 CO2 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.pdfFifth Mars Polar Science Conference (2011)

During later spring (Ls = 200 – 250°), we observe from both HiRISE and CRISM a brightening of the volatiles layer accompanied by an increase of CO2 ice spectral signature and a decrease of H2O ice signature. The sublimation of the ice layer from its surface is responsible for these evolutions. Blue haloes around dark fans around Ls = 200° (figure 1 c) are caused by the large mineral grains sinking below an optically thin layer of ice at the time of the transition between bot-tom and top sublimation.

Figure 2: Illustration of the jet activity during early spring, triggered by meter-scale topography. The self-cleaning of the ice layer occurs where local slopes allow to collect the solar light more efficiently at high incidence angle.

After Ls=250°, both albedo and CO2 spectral signa-

tures decrease as the progressive sublimation of the volatiles layer results in a patchwork of frosted and defrosted areas until complete defrosting in summer.

Northern Hemisphere: The spring evolution of the northern seasonal cap differs in many ways from the southern cap. The general scenario recently pro-posed by [4] involves complex and dynamic evolutions of the stratigraphy between the CO2 and H2O ices. As for the southern seasonal cap, high spatial resolution is crucial to confirm and detail the processes suggested by global scale observations. Dynamic evolutions of ice on the northern dune fields have recently been doc-umented using HiRISE images [6].

Figure 3 shows an example of snapshot from a time series of CRISM visible images and near-IR composi-tional maps obtained for one of the nine studied re-gions. These images perfectly illustrates the complexi-ty of the temporal evolutions of the studied spectral parameters. In addition to the general evolution of H2O and CO2 band strengths described by [4], we observe

spatial heterogeneity at small spatial scales in most of the studied areas. The spatial variability is essentially due to the presence of dune fields or to the topography of the polar layered deposits. In addition, the compari-son between the two Martian Springs observed by MRO seems to indicate some significant interannual variability.

Figure 3: Snapshot from a time series of CRISM spectral maps at Ls=56.2° (Coordinates: 124°W, 84°N). This site was regularly observed by MRO to study avalanches on the scarps of the NPLD. VIS is a visible false color RGB image. H2O is a map of the strength of the 1.5-µm water ice band, and CO2 is a map of the strength of the 1.4-µm CO2 ice band. Brighter indicates higher band strength on these greyscale spectral maps. The raibow color-coded map is the ratio between the CO2 and H2O maps. The most interesting features in terms of spectral variability are the dunes fields and the basement of the NPLD scarps.

As we previously did for the southern seasonal cap, we merge the high resolution color observations of HiRISE into the CRISM spectral maps to get more insights into the dynamic processes occurring at small scale and interpret in terms of ice composition the col-or variations seen by HiRISE. This allows us to refine the scenarios of the volatile layer sublimation in the northern hemisphere. The comparison of this scenario with the one established for the southern hemisphere is particularly interesting, pointing to major differences, essentially related to the role of water.

References: [1] James P. B. (1979) JGR, 84, 8332–8334. [2] Kieffer H. H. et al. (2000) JGR, 105, 9653-9699. [3] Langevin Y. et al. (2007) JGR, 112, E08S12. [4] Appéré T. et al. (2011) JGR, 116, E05001. [5] Pommerol A. et al. (2011) JGR, in press. [6] Han-sen C. J. et al. (2011) Science, 331, 575-577. [7] Rus-sel P. et al. (2009) GRL, 25, L23204.

6013.pdfFifth Mars Polar Science Conference (2011)