Martian polygonal terrain and its hints of hydrothermal conditions on ancient Mars

Martian polygonal terrain, widely distributed on the surface, provides valuable geological insights into the environmental conditions during their formation, including aspects related to thermal conditions and the possibility of past aqueous activity of Mars. First, we provide an overview of the polygonal terrain on Mars, including their distribution, size, and possible formation mechanisms. Second, in order to quantitatively analyze the geometric features of polygonal terrain and their relationship with the ancient climate on Mars, we process images from the High-Resolution Imaging Science Experiment (HiRISE) of a polar region and recognize the boundaries of polygons. Subsequently, we calculate the polygonal terrain’s area, orientation, and wedge density to build up new constraints on the formation mechanism of the polygonal terrains. Finally, we report special frequency-variation patterns of Zhurong radar reflections and interpret them as buried polygonal terrain beneath the landing site. Sixteen polygonal wedges deeper than 35 m have been identified within ∼1.2 km traveling distance, indicating a potentially widespread distribution of such terrain under Utopia Planitia on Mars. Based on constraints of the geometric features of the polygons and the geological background of the landing site, the identified buried polygons are interpreted as having been generated by freeze-thaw cycles. The contrast above and below ∼35 m depth represents a notable transformation of aqueous activity or thermal conditions in the Late Hesperian–Early Amazonian. This finding is remarkable as it indicates 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. These studies on Martian polygonal terrain in terms of polygons’ geometric characteristics and the formation mechanism provide significant constraints for understanding the climate and environment of ancient Mars.