Application of HRSC Data to North Polar Layered Deposit Stratigraphy

HRSC imaging data from Mars Express provide coverage of the north polar cap at resolutions suitable for regional analysis of the polar layered deposits (PLD) and trough systems. In order to analyze the interior structure of the PLD and address the role of troughs in layer formation and evolution, the orientation of the layers in three dimensions was examined using HRSC polar images (50 m/pxl) co-registered with MOLA gridded topography data. The stereo data from the HRSC camera was unavailable at the time of this study. As the polar MOLA data set is of comparable resolution to the data set that will be derived from HRSC stereo images (114 m/pxl and 100 m/pxl, respectively), utilizing the MOLA data was considered to be an excellent first step in this type of analysis. Layers exposed in local topographic variations such as a hill or depression within a trough were selected for analysis. To measure the orientation of an individual layer, the position of the layer was identified by locating at least eight specific points along the layer. The spatial information for each point in three dimensions was then extracted from the spacecraft data. Next, a trend plane was fitted to the points using a least-squares fit. Finally, the slope and aspect of the trend plane was calculated, providing the dip and strike of the layer. Measurement of 38 layers located around the cap and at different depths in the PLD stratigraphy reveals several trends in layer orientation. A majority of layers have strikes trending parallel to the orientation of the trough in which they are exposed; rather than following a circular pattern as might be expected of layers in a simple dome they follow the spiraling pattern of the troughs. A slight majority (21 out of 38) of layers dip away from the pole. The remaining (17 out of 38) layers dip toward the pole. The magnitude of the dip of an individual layer does not tend to reflect the surface slope of the portion of the cap surface immediately surrounding the trough within which the layer is exposed. These observations of PLD strike and dip are consistent with both 1) models of layer behavior in the presence of polar ice flow and 2) models of static ice accumulation in the presence of preexisting troughs. Further assessments of the potential existence and magnitude of flow of ice and ice-dust mixtures in the polar cap are required before we can successfully distinguish between these two possibilities or propose new interpretations that might involve combinations of these processes.