Polarimetric radar evidence for middle–late Amazonian aqueous activity in Utopia Planitia, Mars

China’s Tianwen-1 mission successfully landed the Zhurong rover in southern Utopia Planitia on Mars on 15 May 2021, where it conducted in situ investigations of the shallow subsurface between Sol 11 and Sol 333 while traversing a total distance of 1.9 km. During this period, Zhurong deployed, for the first time on Mars, a high-frequency (450–2,150 MHz) quad-polarized ground-penetrating radar system. The high-frequency radar investigations were carried out by the Zhurong radar research team at the Institute of Geology and Geophysics, Chinese Academy of Sciences, in collaboration with the State Key Laboratory of Space Weather, National Space Science Center, State Key Laboratory of Remote Sensing Science, Aerospace Information Research Institute, Chinese Academy of Sciences.

The Martian subsurface preserves geological and climatic records that are largely inaccessible to surface observations alone. These surveys produced ultra-shallow subsurface images spanning depths from 0 to 7 m with a vertical resolution of up to ~5 cm across four polarization channels, enabling unprecedented characterization of the near-surface structure. The high-resolution radar images reveal centimeter-scale layered sediments, buried impact craters, three distinct shallow subsurface units, and regionally extensive northward-dipping structures in southern Utopia Planitia. In addition, the data provided the first measurements of polarization-dependent anisotropy in Martian subsurface materials, along with dielectric permittivity profiles derived from the HV and HH polarization modes. The stratigraphic architectures and geometries of these features are consistent with deposition and modification in aqueous environments during the middle to late Amazonian period （~750 Ma）. Together, these observations indicate that liquid-water-related processes persisted in this region later than previously recognized, extending constraints on the duration of middle-late Amazonian aqueous activity and providing new insights into the recent geological and climatic evolution of Mars.