PLANETARY FLUVIAL GEOMORPHOLOGY STUDIES USING TERRESTRIAL ANALOGS

Although fluvial landforms require conditions that are unusual in our Solar System, fluvial analog studies continue to be instrumental in understanding the geologic evolution of other planetary bodies. Illustrative examples range from local to regional to global scale from three planetary bodies. (1) Earth: How do flowing rivers meander? Meanders require bank strength. On Earth, this strength is associated with vegetation, but extraterrestrial meanders impel re-examination of this issue. An analog study of the poorly vegetated Quinn River indicates mud banks, for which shear vane data give strength values similar to those of highly vegetated rivers. Thus, on silicate bodies, transport of clays may produce the bank cohesion necessary for meandering. (2) Mars: How are post-flow regional landscapes modified? Highly meandering flow features in the light-toned layered deposits of the Aeolis-Zephyria Plana region provide insight into post-flow landscape modification. These paleochannels and paleo-floodplains are preserved in inverted relief and collectively termed sinuous ridges (SRs). Through terrestrial analogy, these SRs are inferred to have resulted from cementation of fluvial sediments and subsequent aeolian abrasion of the surrounding formation. Topographic analysis shows that some SR profiles undulate (amplitudes > ~100m), demonstrating that the original (fluvial) slopes have been modified. Smaller undulations (amplitudes < SR relief) are inferred to be due to differential abrasion. Larger undulations (amplitudes > SR relief) are inferred to be a result of subsurface processes e.g., settling and/or volatile removal. These results suggest that observed slopes in this and possibly other light-toned layered landscapes do not represent paleoslope conditions. (3) Titan: How is fluvial flow influenced at the global scale? Fluvial networks on Titan are formed by the flow of liquid nitrogen/methane over icy bedrock. Network analysis shows that most Titan networks are rectangular, which on Earth connotes the influence of tectonic structures. Fluvial networks on Titan may be particularly responsive to tectonic constructs, as fluvial incision may only be effective along pre-existing weaknesses. These results suggest that Titan was tectonically active on the time scale of fluvial network formation.