SHARAD 3D Radargram Mapping of Unconformities within the Martian North Polar Layered Deposits

The Martian North Polar Layered Deposits (NPLDs) have been studied extensively since their discovery with Mariner 9 images. To radar sounding waves, water ice on Mars is often nearly transparent, allowing penetration deep into the polar caps and reflection from subsurface interfaces. Mars Reconnaissance Orbiter Shallow Radar (SHARAD) observations of the NPLDs illuminate >45 layers within their internal structure, and analysis of these radar signals suggest that stratigraphic layers are composed of mainly water ice and dust. Coupled with climate and constructional modeling, results indicate that the internal layers preserve periods of the Martian climatic record, providing clues on how the NPLDs evolved over time. Many studies using SHARAD 2D radargrams examine the internal structure regionally or zonally rather than cap-wide, with subsurface mapping limited to the available coverage density and restricted by the observation geometry due to interference from surface clutter over large portions of the cap. To date, there exists no extensive mapping of the entire set of layers within the NPLDs. Using the second version of the SHARAD Planum Boreum three-dimensional (PB3Dv2) radargram, we have embarked on mapping and a stratigraphic analysis of these layers, cap-wide and at high spatial resolution. Periods of extensive ablation present as unconformities (contacts between two distinct units representing a large gap in the geologic record) in the internal layering. Previous studies have identified three large-scale, nearly cap-wide unconformities that occur in the upper, middle, and lower depths of the NPLDs. Each unconformity represents the state of the NPLD prior to a period of net accumulation, and correlating the overlying layers to accumulation models provides critical constraints on their ages. Existing climate models of Mars predict three large-scale ablation periods ending at approximately ~3.2 Mya, ~1.9 Mya, ~0.7 Mya. Here, we present progress in our efforts to map out these unconformities, the results of which will inform new constructional modeling to better constrain the ages of the various layers within the cap, how the history of volatile transport to and from the NPLD has evolved over time, and how these fluxes are represented in the radar-derived stratigraphy.