An arid-semiarid climate during the Noachian-Hesperian transition in the Huygens region, Mars: Evidence from morphological studies of valley networks

Fluvial activities play important roles in the geologic evolution of early Mars. As one of the most ancient fluvial features on Mars, the widely distributed valley networks (VNs) provide clues for understanding the early martian climate. Our study focuses on the morphological and hydrological features of VNs around the Huygens basin in the ancient southern highland. A total of 198 VNs are identified in the Huygens region with a total length of 23,991.6 km, among which 83 VNs are firstly identified. VNs possess drainage patterns of dendritic to parallel, and show average stream order of 2.45 with a maximum of 5, suggesting a simple branching morphology. Sinuosity indexes of mainstreams range from 1.02 to 1.98 with an average of 1.15, exhibiting a straight to winding form. Two types of VN channels are observed with distinct elevational distribution, headwater morphology, and longitudinal profile shape, and are considered to be sapping-related and surface runoff-related, respectively. Age estimations from crater size-frequency distribution measurements suggest that VN activities at the basin rim mainly started around the Noachian/Hesperian boundary at ~3.7 Ga, and decayed near the Early/Late Hesperian boundary. VNs at the basin floor show a younger age of ~3.5 Ga, which supports a potential short-lived lake inside the basin during the Noachian-Hesperian transition. A combined effect of rainfall and groundwater is considered as the dominant water sources of VNs in the Huygens region, with the climate condition characterized by transient warm and arid-semiarid periods under a possibly cold and arid environment. •Detailed mapping and morphological study of Huygens valley networks are conducted.•Regional valley network activities occur intermittently over ~3.7–~3.3 Ga.•A combined water source of limited rainfall and groundwater is favored.•Transient warm and arid-semiarid periods exist around Noachian/Hesperian boundary.