Characteristics of the Basal Interface of the Martian South Polar Layered Deposits

Abstract

Introduction: The South Polar Layered Deposits (SPLD) are several kilometer-thick stacks of layered H2O ice deposits extending outward from the martian south pole. The layers within the SPLD are thought to be caused by variations in H2O ice and dust content potentially linked to changes in Mars’ obliquity and orbital eccentricity [1]. Thus, the SPLD are a unique landform which may hold millions of years of Mars’ recent climatic history within its layers. [2] used subsurface radar sounding data from the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) to map the basal interface of the SPLD, measure the thickness and volume of the SPLD, and characterize the electrical properties of these deposits. Here we build on this previous work by making use of the wealth of data acquired by MARSIS since 2005 over the south pole to improve the derived characteristics of the SPLD basal interface. Methods: MARSIS operates simultaneously at 2 of 4 frequency bands (1.8, 3.0, 4.0 and 5.0 MHz) with a 1 MHz bandwidth [3]. In this work, we use a compiled south polar data set, consisting of over 2000 orbits, taken in the 3 higher frequency bands (3, 4 and 5 MHz) to generate a 3D radar imaging volume [4, 5]. Key features of this 3D radar imaging volume are: (1) voxel (volume pixel) dimensions 1.5 km x 1.5 km (horizontal) x 50 m (depth), (2) depth correction is applied in the subsurface using a wave speed in pure water ice (real dielectric constant ε = 3.1), (3) overlapping echo frames from different orbits are averaged, (4) empty voxels are filled with horizontally applied nearest neighbor interpolation, and (5) slices are extracted for all vertical and horizontal planes in each volume for individual study and animations [4, 5]. By assuming that the wave speed is equal to that in pure water ice, reflectors are repositioned in an approximately correct geometry to facilitate identification of interfaces. In areas known to contain lenses of CO2 ice [6], some distortions in reflector position can occur. However, this effect is evident in only a small fraction of the 3D volume. We annotated subsurface (basal) interface reflectors for all slices where it was discernible in the volume. The thickness of the basal interface can vary slightly, but all annotations were made manually and are typically 2 3 pixels thick (equivalent to 100 – 150 m uncertainty in depth). In some cases, multiple subsurface interfaces appear to be present, which were distinguished based on their elevation, context and relative brightness in each slice. The elevation of each detected subsurface interface relative to the MOLA reference ellipsoid was extracted, using the water ice depth correction. Then, the elevation of the MOLA surface overlying each subsurface interface was extracted to find the thickness of the SPLD at each point. Results and Discussion: About 44, 000 points representing the SPLD basal interface were obtained (Fig. 1), representing a 25x improvement over previous work [2]. The majority of the basal interface detections lie at elevations 3-4 km above the reference ellipsoid. However, unusually low elevation detections are present in Ultimi Lingula (which also contains high elevation detections), and within some craters. Note that we have omitted the unusually deep depressions in the near-polar region previously mapped by [2] because their regional context suggests that they represent a distinct unit that lies below the base of the SPLD.