Unveiling the Origin and Ice vs Lithic Composition of the Mars North Polar Basal Unit with Multiband Radar Analyses

The basal unit (BU) of Planum Boreum (PB) on Mars is an ice-rich sedimentary deposit between the Late Amazonian North Polar Layered Deposits (NPLD) and the Late Hesperian Vastitas Borealis interior unit. Its two subunits, rupēs and cavi, represent records of polar geologic and climatic processes across most of the Amazonian (~3.3 Ga). The cavi unit likely consists of alternating sand and ice sheet remnants of past polar caps, reflecting volatile–sedimentary interplay, while little is known about rupēs. Thanks to recent advances in radar data processing and dense coverage by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), it is now possible to reconstruct the stratigraphy and composition of the BU, and reveal the enigmatic nature of the rupēs unit.

We analyzed over 600 MARSIS profiles at 3, 4, and 5 MHz, leveraging optimized ionospheric corrections and deep penetration to map the full thickness of the BU and retrieve its frequency-dependent complex dielectric permittivity. We find that the rupēs unit spans the western half of PB and part of Olympia Planum as a continuous body beneath the cavi unit with a pole-facing upper unconformity, occupying ~191,000 km³ (~53% of BU volume). Dielectric inversions yield a real permittivity ε’ = 4.0±0.8 (consistent at all frequencies) and a frequency-dependent loss tangent tanδ = 0.017±0.006 (3 MHz) to 0.012±0.006 (5 MHz). Both components of the dielectric permittivity exhibit strong spatial heterogeneity, with values increasing toward Hyperborea Lingula (ε’ > 6, tanδ > 0.02).

These results indicate that the rupēs composition differs substantially from that of the cavi unit, with large loss tangent values indicating the presence of significant amounts of lithic materials despite the low real permittivity. Basalt alteration products with tanδ > 0.02 are required to explain the high loss tangent measurements, while their strong frequency-dependence matches the water ice imaginary permittivity behavior. We find a best match of real dielectric permittivity and loss tangent results using a mixture of 85-90% water ice and 10-15% basalt alteration products like hydrated sulfates (e.g., gypsum), clays, and ferric oxides, which are supported by spectroscopic detections at visible exposures. Rupēs lithic materials may have been transported from lower latitude sources, where aqueous alteration is more viable than at polar latitudes. However, the strong spatial heterogeneities suggest that significant localized alteration occurred in situ during the Amazonian period, perhaps facilitated by warmer high-obliquity periods predicted to occur during the last 3 Gyr. Regardless of their source, the volume of these materials corresponds to a 24 cm–thick global layer, indicating that the rupes unit constitutes a substantial sediment reservoir, not merely one of water ice. Finally, the high loss tangent measured in Hyperborea Lingula explains the lack of rupēs basal detections by SHARAD despite the relatively low thickness (i.e., 150-200 m) of the rupēs unit at that location.