Abstract
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.