Buried palaeo-polygonal terrain detected underneath Utopia Planitia on Mars by the Zhurong radar

As the largest basin on Mars, Utopia Planitia has both experienced and recorded variations of the Martian palaeoclimate. Layered subsurface structures have been identified by ground-penetrating radar in southern Utopia Planitia but lateral variations of the subsurface, potentially linked to the Martian palaeoclimatic evolution, have not been investigated. Here we report the lateral frequency-variation patterns of Zhurong radar reflections and interpret them as buried polygonal terrain below a depth of 35 m. Sixteen polygonal wedges were identified within ∼1.2 km distance, suggesting a wide distribution of such terrain under Utopia Planitia. The contrast above and below ∼35 m depth represents a notable transformation of aqueous activity or thermal conditions in the Late Hesperian–Early Amazonian. The interpreted buried polygons, possibly generated by freeze–thaw cycles, imply that there was a strong palaeoclimatic variability at low-to-mid latitudes (∼25° N), potentially due to the high obliquity of ancient Mars. The Zhurong rover identified polygonal terrain at a depth of 35 m under Utopia Planitia on Mars. This finding suggests that the mid-latitudinal region experienced a cold and wet environment near the freezing point of water around the Hesperian–Amazonian transition, possibly induced by Mars’s high obliquity.