Anomalous Shallow Subsurface Radar Reflections Detected by MARSIS in the South Polar Layered Deposits

Analysis of Flash Memory [1] data acquired by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument aboard ESA’s Mars Express spacecraft, confirms the presence of additional strong subsurface reflections within the South Polar Layered Deposits, located near the northernmost extent of the previously identified subglacial water bodies [2,3].Figure 1 shows the ground track of orbit 10786 over the topography of the Martian South Pole, where anomalous subsurface reflections have been recorded, highlighted by the blue dots. Fig. 1. Topography Maps of the investigated area.The ground track of orbit 10786 (Figure 2, panel a) crosses the Martian south polar region where these anomalous reflections are detected at shallow depths, occurring approximately 5μs after the surface echoes. A comparison between the observed radar signals and electromagnetic simulations of surface returns (Figure 2, panels b and d) demonstrates that these features are authentic subsurface reflections rather than lateral clutter. The analysis of surface and subsurface echo power (Figure 2, panel e) reveals that, in several signals, the subsurface echoes are significantly stronger than the corresponding surface echoes, indicating a pronounced dielectric contrast variation, between the overlying medium and the subsurface target. Constraining the dielectric properties and the nature of the subsurface material, requires further investigation. This effort will be supported by future MARSIS observations planned for August 2027 and, in particular, April 2029, when the instrument will observe the region during the deep Martian night, thus minimizing ionospheric attenuation and distortion effects.Fig. 2. Science Investigation. a) Zoom of the topography map. b) Comparison between real and simulated data at echo level. c) Simulated Radargram. d) Real data. e) Trends of surface and subsurface echo power.References:[1] A. Cicchetti, et al., Observations of Phobos by the Mars Express radar MARSIS: Description of the detection techniques and preliminary results. Adv. Space Res. 60, 2289-2302 (2017).[2] Orosei R. et al., “Radar evidence of subglacial liquid water on Mars”, 2018, Science, 361[3] Lauro S.E. et al., “Multiple subglacial water bodies below the south pole of Mars unveiled by new MARSIS data”, 2022, Nature AstronomyThis work was supported by the Italian Space Agency (ASI) through contract 2024-40-HH.