Mars north polar spiral trough migration paths as revealed through 3D radar mapping.

Mars northern polar ice cap is mostly composed of the North Polar Layered Deposits (NPLD), a series of stratigraphically continuous, sub-horizontal layers (Phillips et al., 2008) of mostly pure water ice (Grima et al., 2009) with variable fractions of dust (Lalich et al., 2019). These layers record Mars climate variations due to orbital forcing (Hvidberg et al., 2012). The polar cap contains the notable spiral troughs, sub-kilometer sized depressions that reveal the upper fine-scale layers of the NPLD. Radar observations by the SHARAD instrument onboard Mars Reconnaissance Orbiter (MRO) reveal the deep subsurface to contain laterally continuous reflectors; however, in the upper ~50% of the NPLD, discontinuities in this subsurface layering denote bounding surfaces, also known as trough migration paths (TMPs), which indicate migration due to the mass balance of volatiles. These TMPs responded to climatic forces and therefore contain information about Mars climate variability (e.g., Howard et al., 1982, Smith et al., 2013). Bramson et al. (2019) recreated the slopes of two migration paths through initial modeling of ice accumulation and sublimation. We perform new mapping of the TMPs in the SHARAD 3D dataset (Foss et al., 2017). Using this dataset provides additional spatial context that can otherwise be lost/difficult to interpret in 2D radar tracks, namely for creating radar cross sections that are perpendicular to the trough strike, with the effect of minimizing distortion of the migration paths due to oblique viewing geometry. We find that most troughs have migrated during accumulation of ~500 meters, generally stratigraphically upwards and poleward. Variability occurs laterally across each trough, and some troughs show more dramatic variability in their 3D shape. Several troughs near the polar margin are not only of a younger generation than high latitude troughs, but have eroded in place to expose the underlying basal unit. Regional patterns in trough migration path behavior are emerging (e.g. troughs near 90 E initiate more recently). We corroborate many observations of Smith and Holt 2015, and present an updated survey based on our 3D mapping of trough migration and analysis of current surface morphology across the north polar cap. Figure: Planar view of mapped TMPs (grayscale) over MOLA topography basemap.