Valley networks (Mars)

Valley networks are branching networks of valleys on Mars that superficially resemble terrestrial river drainage basins. They are found mainly incised into the terrain of the martian southern highlands, and are typically - though not always - of Noachian age (approximately four billion years old). The individual valleys are typically less than 5 kilometers wide, though they may extend for up to hundreds or even thousands of kilometers across the martian surface.

The form, distribution, and implied evolution of the valley networks are of great importance for what they may tell us about the history of liquid water on the martian surface, and hence Mars' climate history. Some authors have argued that the properties of the networks demand that a hydrological cycle must have been active on ancient Mars, though this remains contentious. Objections chiefly arise from repeated results from models of martian paleoclimate suggesting high enough temperatures and pressures to sustain liquid water on the surface have not ever been possible on Mars.

The advent of very high resolution images of the surface from the HiRISE, THEMIS and Context (CTX) satellite cameras as well as the Mars Orbital Laser Altimeter (MOLA) digital terrain models have drastically improved our understanding of the networks in the last decade.

The valleys of the networks are typically narrow (<0.5–4 km) and 50–200 m deep, with neither value changing consistently along their lengths. Their cross-sectional form tends to evolve from V-shaped in the headwaters to U-shaped in the lower reaches. The individual valleys form interconnected branching networks, typically less than 200 km long and draining into local topographic lows. The form of the tributary valleys is commonly described as "stubby" or a similar term, implying short lengths away from the trunk streams and amphitheater-like terminations at their heads. Many authors have described the drainage density of the networks as typically much lower than would be seen on Earth, though the extent to which this may an artifact of image resolution, landscape degradation or observer bias has also been raised in the literature.

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