The Lateral Continuity and Vertical Arrangement of Dust Layers in the Martian North Polar Cap From SHARAD Multiband Data

Interpretation of radar sounder reflections to infer the structure and composition of the martian polar caps depends on whether bright returns correspond to single packed dust layers or a more finely layered structure. Reflections from multiple layers can create strong resonant scattering (interference) effects that impact analyses of radargram reflectors and inference of dielectric contrast. We identify resonant behavior for an areally extensive reflector in the north polar layered deposits from Shallow Radar data processed in two frequency bands. Echo strength varies by similar to 2 dB between subband reflections across a region similar to 400 km in extent, with the stronger echo shifting abruptly from the high- to low-frequency band outside the central region of Gemina Lingula. This behavior can arise from resonant scattering between two layers of dust (0.3-0.6 m thick) separated by 0.5-3 m of ice. Such layering requires there be little postdepositional aeolian activity to preserve layer thickness and spacing. Plain Language Summary The north polar cap of Mars is made of layers of ice and dust that are a record of the changing climate over the past few million years. These north polar layered deposits (NPLD) have been imaged with radar sounder data from the Shallow Radar (SHARAD) instrument. The radar data reveal reflectors caused by changes in the electrical properties of the layered ice and dust. However, as the radar beam is reflected off multiple layers it can interfere with itself, changing the appearance of the reflector. We identified resonance by splitting SHARAD data into two bands at different frequencies and looking for differences in the reflectors. We used two models to simulate interference and suggest plausible thicknesses for the ice and dust layers. Our simulations suggest a portion of the NPLD formed in less than 200 thousand years, at a time when the region was much less windy than it is currently.