Recent Widespread Deposition in the Martian North and South Polar Layered Deposits as Revealed by Multiband SHARAD Surface Reflectivity

Abstract

AbstractThe north and south polar layered deposits (PLD) on Mars are composed of stacks of layered ice and dust, but the SPLD is approximately twice as bright as the NPLD in 20‐MHz radar echoes. We use Shallow Radar (SHARAD) data in ∼4‐MHz bands centered on 17.5 MHz (“L”) and 22.5 MHz (“H”) to determine whether radar reflectivity variations are due to scattering effects related to closely spaced, near‐surface dielectric layering. We mapped the ratio of the surface echo power at the two frequencies (H/L) for both PLDs. The NPLD has large areas where H and L echo power differ, consistent with destructive interference in the H band within the uppermost ∼20 m. The SPLD is dominated by H ∼ L (unity), except for isolated regions in and near the residual CO2 cap and Australe Lingula. The H/L variations can be partly explained by near‐surface structure, where large variations in H/L match locations with numerous near‐surface reflecting interfaces, and locations where H ∼ L may contain few such reflectors. There is no obvious connection between H/L and surface morphology, but the distribution of non‐unity H/L resembles the extent of a widespread, recent accumulated package (WRAP) at both poles. The spatial association between H/L and WRAP and interference indicated by H/L suggests that large regions in the NPLD—and isolated areas in the SPLD—are characterized by shallow layer(s) of consistent thickness/separation potentially deposited within the past few tens of kyr as Mars emerged from the last obliquity‐driven ice age.