Stratigraphic Anomalies in the Martian North Polar Layered Deposits

Introduction: We have identified numerous examples of anomalous trough stratigraphy within the North Polar Layered Deposits (NPLD). According to the standard model of NPLD stratigraphy [1], troughs can be divided into three main units: (1) Layered Terrain, which exhibit fine-scale laminae upon equatorward-facing (EWF) trough slopes; (2) Banded Terrain, more diffuse layers located on poleward-facing (PWF) trough walls; and (3) Smooth Terrain, which span the featureless regions between the troughs. PWF Layered Terrain: However, our preliminary survey of NPLD trough stratigraphy (Fig. 1) has revealed numerous exposures of Layered Terrain on PWF trough slopes. For example, the HiRISE image in Fig. 2 clearly shows layers that continuously wrap around a bowl-shaped depression located upon an NPLD trough wall. Our topographic analysis of this trough—which involves using lower-resolution MOC (Mars Orbiter Camera) and THEMIS (Thermal Emission Imaging System) images to place the HiRISE footprint upon MOLA (Mars Orbiter Laser Altimeter) topography—confirms that fine-scale layers are expressed on PWF slopes (Fig. 2). Such PWF Layered Terrain has important implications for the local mass balance of the trough, since deposition of a millimeterthick layer is more than sufficient to alter surface albedo and obscure the trough wall [2]. Hence, the presence of PWF layered terrain effectively precludes significant accumulation of water ice within a trough. As shown in Fig. 1, we have identified as many as twenty exposures of PWF Layered Terrain throughout the NPLD using HiRISE and MOC images, which strongly suggests that water ice deposition is not presently occurring within most NPLD troughs. In order to assess the frequency of these anomalous features, we decided to systematically inspect the 20 largest troughs in the NPLD (Fig. 1). We classified the troughs based on the number of internal ridges: “simple” troughs (outlined in yellow) have no such ridge, while “complex” troughs have either one (orange) or two (red) interior ridges. As seen in Fig. 1, almost all of the simple troughs do not exhibit PWF Layered Terrain, in accordance with the standard model. However, the majority of complex troughs (7 out of 11) contain at least one exposure of PWF Layered Terrain (as indicated by the green circles in Fig. 1). EWF Banded Terrain: Even more surprisingly, several complex troughs also exhibit EWF Banded Terrain (green triangles), which is also discordant with Figure 1: Anomalous Trough Stratigraphy. Exposures of PWF Layered Terrain and EWF Banded Terrain. Outlines correspond to twenty largest NPLD troughs (as defined by surfaces with contiguous poleward-facing slopes steeper than 1.5o), which are subdivided based on the number of internal ridges.