ALBERT

Description: The National Aeronautics and Space Administration's (NASA) Mars Exploration Program is focused on understanding the past or present habitability of Mars (MEPAG, 2010). Central to this aim is determining the history of liquid water on Mars. Some of the most salient evidence for liquid water is giant outflow channels that debouch into Chryse Planitia, hypothesized to be part of an ancient Martian ocean (Parker et al., 1993; Head et al., 1999; Perron et al., 2007; Di Achille and Hynek, 2010). An ocean should be a fertile nursery for Martian life, based on terrestrial evidence (e.g., Walter et al., 1980; Van Kranendonk et al., 2003). Thus, understanding whether the streamlined forms around Chryse Planitia were created in the outflow channels or within the hypothesized ocean contributes to identifying habitable zones on Mars...

Description: Fluvial features on Titan have been identified in synthetic aperture radar (SAR) data taken during spacecraft flybys by the Cassini Titan Radar Mapper (RADAR) and in Descent Imager/Spectral Radiometer (DISR) images taken during descent of the Huygens probe to the surface. Interpretations using terrestrial analogs and process mechanics extend our perspective on fluvial geomorphology to another world and offer insight into their formative processes. At the landscape scale, the varied morphologies of Titan’s fluvial networks imply a variety of mechanical controls, including structural influence, on channelized flows. At the reach scale, the various morphologies of individual fluvial features, implying a broad range of fluvial processes, suggest that (paleo-)flows did not occupy the entire observed width of the features. DISR images provide a spatially limited view of uplands dissected by valley networks, also likely formed by overland flows, which are not visible in lower-resolution SAR data. This high-resolution snapshot suggests that some fluvial features observed in SAR data may be river valleys rather than channels, and that uplands elsewhere on Titan may also have fine-scale fluvial dissection that is not resolved in SAR data. Radar-bright terrain with crenulated bright and dark bands is hypothesized here to be a signature of fine-scale fluvial dissection. Fluvial deposition is inferred to occur in braided channels, in (paleo)lake basins, and on SAR-dark plains, and DISR images at the surface indicate the presence of fluvial sediment. Flow sufficient to move sediment is inferred from observations and modeling of atmospheric processes, which support the inference from surface morphology of precipitation-fed fluvial processes. With material properties appropriate for Titan, terrestrial hydraulic equations are applicable to flow on Titan for fully turbulent flow and rough boundaries. For low-Reynolds-number flow over smooth boundaries, however, knowledge of fluid kinematic viscosity is necessary. Sediment movement and bed form development should occur at lower bed shear stress on Titan than on Earth. Scaling bedrock erosion, however, is hampered by uncertainties regarding Titan material properties. Overall, observations of Titan point to a world pervasively influenced by fluvial processes, for which appropriate terrestrial analogs and formulations may provide insight.