WHAT CAUSES RADAR REFLECTIONS INSIDE MARTIAN POLAR DEPOSITS?

When probed with subsurface sounding radar, the polar layered deposits of Mars commonly show continuous reflecting horizons, consistent with the appearance in optical imagery of well-organized strata. The banded nature of the layered deposits in images has been ascribed to variations in dust content, but numerous observations suggest that weathering style and surficial deposits are more likely the controls on apparent albedo [e.g., Herkenhoff and Murray, 1990, JGR 95 14511; Fishbaugh et al., 2009, Icarus, in press]. The aim of this study is to tie the positions of radar reflectors to outcrops of layered deposits visible in high resolution images, such as HiRISE from Mars Reconnaissance Orbiter (MRO). The morphologic and topographic characteristics of the reflecting layers can then be used to constrain the mechanism that causes a radar reflection at a particular place in the stratigraphy.

We examine an area of the North Polar Layered Deposits (NPLD) that was observed numerous times by the SHAllow RADar (SHARAD) on MRO, and for which an adjacent trough was imaged in stereo by HiRISE and studied in detail for stratigraphic information [Fishbaugh et al., 2009]. These datasets allow absolute 3D positioning to be obtained for reflecting horizons in the radar data and for the associated nearby outcrop. While the radar reflectors cannot be traced all the way to the sloping exposure, an extrapolation of the flat lying layers of several km does not introduce large errors in elevation position. The upper ~500 m of layers are well-imaged by SHARAD, consisting of individual reflecting horizons, groups of closely spaced reflectors, and regions of low return between reflectors. Preliminary results indicate that radar reflectors may be correlated with so-called “marker beds”, layers with a hummocky, protruding appearance in images of sloping exposures. This suggests that the compositional and/or textural character of these layers provides both a dielectric contrast with adjacent layers, and a mechanical contrast that results in a distinctive weathering style.