Prolonged Fluvial Activity Revealed by Mapping and Analyses of Valley Networks in the Northwestern Hellas Region, Mars.

Valley networks (VNs) on Mars are crucial for understanding the Martian hydrologic and climatic history. However, the limited resolution of remote sensing data hindered the complete identification of Martian VNs, affecting our understanding of the formation and duration of VNs as well as their climatic significance. In this study, we utilized high‐resolution imaging and topographic data to conduct detailed mapping and investigations of the VNs around the northwestern margin of the Hellas basin, the largest impact basin and major sedimentary sink in the Martian southern highlands. We identified a total of 911 VNs with a cumulative length of 32,086.3 km, more than twice that of previous mapping results. Additionally, we analyzed the morphological parameters of VNs, including stream order, sinuosity, junction angle, stream slope, etc., investigated their geomorphologic characteristics, and determined their formation ages. We propose that occasional precipitation and regional groundwater fostered the formation of well‐developed VNs and a "Hellas Ocean" in the Noachian Period. The main fluvial activity occurred during ∼3.9–3.2 Ga. Subsequently, the climate transitioned from warm and semiarid to cold and arid during the Noachian‐Hesperian transition, leading to the evaporation of the "Hellas Ocean." In the Amazonian, some small simple valleys formed during ∼2.1–1.0 Ga with the supply of meltwater associated with obliquity‐controlled glacial processes. These results reveal prolonged fluvial activity in the northwestern Hellas region with diverse water sources under changing climatic conditions, which make the region a very promising candidate for future in situ exploration missions. Plain Language Summary: Valley networks are widely distributed on the Martian surface and provide a significant record of climatic and geologic evolution. Hellas basin, the largest and oldest impact basin in the Martian southern highlands, has undergone extensive fluvial modification, making it an ideal place to conduct investigations on valley networks (VNs). In this study, we identified 911 VNs in the northwestern region of the Hellas basin, and conducted detailed analyses on their morphology and age using high‐resolution imaging and topographic data. The results indicate that the VNs were mainly active from ∼3.9 billion years ago to ∼3.2 billion years ago. Precipitation and groundwater might have been the sources of water carving the valleys. At this period, the climate was warm and semiarid, and subsequently changed to cold and arid. However, at later stages, small valleys can also be formed by meltwater released by obliquity changes. Based on these results, we can find that fluvial activity could have persisted for a long period in the northwestern Hellas region under different climatic conditions, leading to a high possibility for the existence of habitable environments, which makes this area an attractive place for searching for extraterrestrial life and establishing outposts on Mars. Key Points: We identified 911 valley networks in the northwestern Hellas region and conducted detailed geomorphologic and chronologic investigationsProlonged fluvial activity fed by diverse water sources existed in the study region from the Noachian to the AmazonianThe study region underwent a climate change from a warm and semiarid climate to a cold and arid climate during the Noachian‐Hesperian transition [ABSTRACT FROM AUTHOR]