Rectangular drainage patterns, characterized by 90 degree junction branching angles and bends, are documented across the solar system. These landforms are considered the mark of structurally controlled landscape evolution, since in terrestrial contexts, their geometry commonly aligns with pre-existing orthogonal fracture networks. Rectangular drainages appear on Mars, however, despite their implication for the structural and geomorphic processes that control the evolution of the Martian surface, their formation mechanism has only minimally explored so far. Here, we focus on two such rectangular valley systems located in the northern part of the largest outflow channel on Mars, Kasei Valles. These Kasei rectangular valley systems are situated on elevated terrain approximately 2500 meters above the base of the outflow channel. In addition to their rectangularity, they exhibit exceptionally straight valley trajectories, elongated sinkholes along and adjacent to the valleys, and along-valley "bridges"—sections of higher topography, sometimes at the original elevation of the surfaces that host these valley systems.
Our study was guided by a potential terrestrial analogue at Wadi Pratzim in the Dead Sea Basin, Israel. Wadi Pratzim shares many of the morphological properties of the Kasei rectangular valley systems on Mars, including a rectangular pattern, straight valley banks, elongated sinkholes, and "bridges" that act as hydrological barriers at the surface. A previous study of the Wadi Pratzim system revealed the role of subsurface flow and erosion along pre-existing fractures in dictating the rectangular geometry of the system, and suggested that subsurface erosion created pipe caves, which subsequently developed into sinkholes and new canyon tributaries following pipe cave roof collapse. The bridges were seen as remnant roofs of pipe caves.
We used morphometric analyses to investigate whether subsurface erosion, similar to that proposed to Wadi Pratzim, could be a dominant mechanism controlling the formation of the Kasei rectangular valley system in Mars. As part of this study, we also aimed to distinguish between hydraulically shaped rectangular channel systems and other Martian valley types, such as lava channels and volcanic pit chains. Using high-resolution satellite imagery (~5 m/pixel) and topographic data (~200 m/pixel) from Mars, we show comparable morphometric characteristics between the terrestrial analogue and the Kasei rectangular valley systems, potentially indicating the key role of subsurface erosion by flowing water in their formation.
Determining the potential and extent of subsurface erosion near Kasei Valles could redefine how we interpret the paleo-environmental history of Mars. Moreover, underground voids associated with subsurface erosion may represent key targets for future manned missions to Mars, as they offer natural shelters.