Fluvial features on Titan: Insights from morphology and modeling

Fluvial features on Titan have been identified in synthetic aperture radar (SAR) data taken during spacecraft fl ybys 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 fl uvial geomorphology to another world and offer insight into their formative processes. At the landscape scale, the varied morphologies of Titan’s fl uvial networks imply a variety of mechanical controls, including structural infl uence, on channelized fl ows. At the reach scale, the various morphologies of individual fl uvial features, implying a broad range of fl uvial processes, suggest that (paleo-)fl ows 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 fl ows, which are not visible in lowerresolution SAR data. This high-resolution snapshot suggests that some fl uvial features observed in SAR data may be river valleys rather than channels, and that uplands elsewhere on Titan may also have fi ne-scale fl uvial 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 fi ne-scale fl uvial 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 fl uvial sediment. Flow suffi cient to move sediment is inferred from observations and modeling of atmospheric processes, which support the inference from surface morphology of precipitation-fed fl uvial processes . With material properties appropriate for Titan, terrestrial hydraulic equations are applicable to fl ow on Titan for fully turbulent fl ow and rough boundaries. For low-Reynolds-number fl ow over smooth boundaries, however, knowledge of fl uid 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 infl uenced by fl uvial processes, for which appropriate terrestrial analogs and formulations may provide insight. INTRODUCTION Observations of Titan from the CassiniHuygens mission to the Saturn system (Kerridge et al., 1992; Matson et al., 2002) revealed an extraterrestrial world with an active volatile cycle and resultant fl uvial landforms. Analysis of fl uvial landforms on Earth by the founders of modern geology (e.g., Lyell, 1830) laid the groundwork for a better understanding of terrestrial landscape evolution. The discovery of relict valley networks and outfl ow channels on Mars (Mars Channel Working Group, 1983) fundamentally revised our view of that planet’s evolution and early history. Likewise, the revelation of fl uvial features on Titan provides a powerful tool to better understand the geologic history of this largest satellite of Saturn. On both Mars and Titan, fl uvial features have been inferred through their planform similarity to terrestrial fl uvial networks, a type of analogical reasoning common in planetary geology (Mutch, 1979) and all sciences (Hess, 1966). One pitfall with analogical reasoning is equifi nality, in which different causative processes yield similar effects (Mutch, 1979; Schumm, 1991). An illustration from planetary science is the past debate over the volcanic or impact origin of lunar craters. This directionality, whereby landscapes are fi rst encountered remotely at low For permission to copy, contact [email protected] © 2013 Geological Society of America 299 GSA Bulletin; March/April 2013; v. 125; no. 3/4; p. 299–321; doi: 10.1130/B30612.1; 7 fi gures; 1 table. E-mail: [email protected]