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Fluvial valley networks are a key record of the geomorphic effects of past liquid water on the surface of Mars. Most valley networks formed early in Mars history (>3.5 Ga) during a period of increased surface water availability, and these landforms may preserve information on the nature and spatial heterogeneity of the ancient martian hydroclimate. Here we present work to constrain patterns of valley network geometry (length, depth, and volume), and assess how these factors are related to topography and landscape position. We find that valley depth is most strongly controlled by regional slope and landscape relief, with deeper valleys on steeper/higher relief terrains, likely driven by a slope control on the rate of fluvial erosion. In contrast, total valley length and volume are most controlled by slope orientation, with larger values on north-draining slopes, which we suggest is related to more efficient fluvial integration (i.e., inter-connection of valleys) on such slopes. At lower elevations and towards terminal basins (northern plains and Hellas), valley depth increases, while total valley length and volume decrease, consistent with the geometry expected as a branching valley system converges downstream into fewer, larger valleys. Finally, we show that patterns of total valley length and eroded volume peak about a previously proposed paleo-latitude band on a pre-Tharsis-driven true polar wander Mars. Valley depth increases from paleo-south to paleo-north in this pre-true polar wander framework, consistent with valley network development in a paleo-north direction. This is potentially consistent with syn/pre-Tharsis growth and true polar wander valley network incision; however, this signal is not unique and may be amplified by patterns of post-valley network incision resurfacing. • We analyze spatial patterns of Mars fluvial valley geometry (length, depth, volume). • Depth increases with steeper slopes, likely due to increased fluvial incision rate. • Total valley length/volume larger on northward slopes due to landscape integration. • All aspects of valley geometry show signatures of downstream network growth. • Observed pattern potentially consistent with syn/pre-Tharsis growth valley incision. [ABSTRACT FROM AUTHOR]

• Overlapping crater populations constrain the total era of martian valley formation. • Martian valleys may have formed intermittently over hundreds of millions of years. • Long-term erosion rates would have been similar to the modern Atacama Desert. A number of studies have constrained the cessation of martian valley network activity to around the Noachian-Hesperian boundary, but the total time period over which these features formed remains poorly constrained. Most attempts to ascertain valley network formative timescales rely on an intermittency factor that varies widely for terrestrial rivers and is unknown for Mars. In this contribution, superposition relationships are used to identify crater populations that pre- and postdate valley network incision, resulting in maximum cumulative valley formation times of ∼108 years. These timescales correspond with long-term erosion rates of ∼0.25 m/Myr, similar to those calculated for the Middle to Late Noachian as well as the central Atacama Desert since the Late Miocene. Assuming sediment transport rates previously calculated for other martian valley networks, this implies a minimum fluvial intermittency of ∼10−5, several orders of magnitude lower than modern day Earth rivers but similar to values independently calculated for some martian fluvial deposits. This low intermittency suggests that martian valley incision may have been significantly hindered by erosion into existing bedrock or by boulder armoring of the valley floor. Alternatively, unlike river valleys on Earth, runoff events driving martian valley network activity may have been non-seasonal, Milankovitch-like cycles, with long eras of quiescence between periods of intensified fluvial activity. [ABSTRACT FROM AUTHOR]

Fan-shaped sedimentary landforms on Mars are important geomorphic markers of past water flow and characterizing the distribution and morphology of these features can yield insights into the planet's climatic evolution. We present a new database of 1501 martian fan-shaped sedimentary landforms that builds upon previous surveys and uses Context Camera images. This comprehensive global survey includes both alluvial fans and putative deltas, which we term scarp-fronted fan deposits (SFDs), across the entire martian surface. We classified each fan-shaped landforms based on their morphology as alluvial fans, channelized SFDs, smooth SFDs, and terraced SFDs. Martian alluvial fans are highly concentrated in the high southern tropical latitudes while SFDs are more common near the equator. Both alluvial fans and SFDs are found at lower elevations than Late Noachian to Early Hesperian valley networks, suggesting a climatic change in the locations of stable liquid water. Alluvial fans within impact craters are preferentially located on north, south, and east facing slopes, and are concentrated near the location of highest rim topographic relief; together these observations support orographically-influenced precipitation and snowmelt as a runoff source. For every fan in the database we collected morphologic data, which we compare with terrestrial alluvial fans. Relationships between alluvial fan catchment and fan morphology vary little across the martian surface, suggesting relatively similar processes. Although stratigraphic evidence of a deltaic deposition is lacking, previously conducted experiments along with the depositional basins of the channelized and terraced SFDs generally support deltaic formation. Crater counts indicate that many alluvial fans are within Hesperian and Amazonian impact craters, indicating that significant fluvial erosion occurred during Mars' relatively recent history. • Most martian alluvial fans are in craters, and many of these craters date to the Hesperian and Amazonian Periods. • Consistent morphology implies that alluvial fans across Mars formed under similar processes and environmental conditions. • The depositional setting of terraced and channelized scarp fronted deposits supports a deltaic formation. [ABSTRACT FROM AUTHOR]