Fluvial features on Titan: Insights from morphology and modeling Devon M. Burr1,†, J. Taylor Perron2, Michael P. Lamb3, Rossman P. Irwin III4, Geoffrey C. Collins5, Alan D. Howard6, Leonard S. Sklar7, Jeffrey M. Moore8, Máté Ádámkovics9, Victor R. Baker10, Sarah A. Drummond1, and Benjamin A. Black2 1Earth and Planetary Sciences Department, University of Tennessee–Knoxville, 1412 Circle Drive, Knoxville, Tennessee 37996-1410, USA 2Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA 3Geological and Planetary Sciences, California Institute of Technology, 1200 E California Boulevard, MC 170-25, Pasadena, California 91125, USA 4Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, MRC 315, 6th Street at Independence Avenue SW, Washington, District of Columbia 20013, USA 5Physics and Astronomy Department, Wheaton College, 26 E. Main Street, Norton, Massachusetts 02766, USA 6Department of Environmental Sciences, University of Virginia, P.O. Box 400123, Charlottesville, Virginia 22904-4123, USA 7Department of Geosciences, San Francisco State University, 1600 Holloway Avenue, San Francisco, California 94132, USA 8Space Sciences Division, National Aeronautics and Space Administration Ames Research Center, MS 245-3, Moffett Field, California 94035, USA 9Astronomy Department, University of California, 601 Campbell Hall, Berkeley, California 94720-3411, USA 10Department of Hydrology and Water Resources, The University of Arizona, J.W. Harshbarger Building, Room 246, 1133 E. James E. Rogers Way, Tucson, Arizona 85721-0011, USA ABSTRACT leys rather than channels, and that uplands INTRODUCTION elsewhere on Titan may also have fi ne-scale Fluvial features on Titan have been iden- fl uvial dissection that is not resolved in SAR Observations of Titan from the Cassini- tified in synthetic aperture radar (SAR) data. Radar-bright terrain with crenulated Huygens mission to the Saturn system (Kerridge data taken during spacecraft fl ybys by the bright and dark bands is hypothesized here et al., 1992; Matson et al., 2002) revealed an Cassini Titan Radar Mapper (RADAR) and to be a signature of fi ne-scale fl uvial dissec- extraterrestrial world with an active volatile in Descent Imager/Spectral Radiometer tion. Fluvial deposition is inferred to occur cycle and resultant fl uvial landforms. Analysis (DISR) images taken during descent of the in braided channels, in (paleo)lake basins, of fl uvial landforms on Earth by the founders Huygens probe to the surface. Interpreta- and on SAR-dark plains, and DISR images of modern geology (e.g., Lyell, 1830) laid the tions using terrestrial analogs and process at the surface indicate the presence of fl uvial groundwork for a better understanding of terres- mechanics extend our perspective on fl uvial sediment. Flow suffi cient to move sediment is trial landscape evolution. The discovery of relict geomorphology to another world and offer inferred from observations and modeling of valley networks and outfl ow channels on Mars insight into their formative processes. At the atmospheric processes, which support the in- (Mars Channel Working Group, 1983) funda- landscape scale, the varied morphologies ference from surface morphology of precipi- mentally revised our view of that planet’s evolu- of Titan’s fl uvial networks imply a variety tation-fed fl uvial processes . With material tion and early history. Likewise, the revelation of mechanical controls, including structural properties appropriate for Titan, terrestrial of fl uvial features on Titan provides a powerful infl uence, on channelized fl ows. At the reach hydraulic equations are applicable to fl ow tool to better understand the geologic history of scale, the various morphologies of individual on Titan for fully turbulent fl ow and rough this largest satellite of Saturn. fl uvial features, implying a broad range of boundaries. For low-Reynolds-number fl ow On both Mars and Titan, fl uvial features have fl uvial processes, suggest that (paleo-)fl ows over smooth boundaries, however, knowl- been inferred through their planform similarity did not occupy the entire observed width edge of fl uid kinematic viscosity is necessary. to terrestrial fl uvial networks, a type of ana- of the features. DISR images provide a spa- Sediment movement and bed form develop- logical reasoning common in planetary geology tially limited view of uplands dissected by ment should occur at lower bed shear stress (Mutch, 1979) and all sciences (Hess, 1966). valley networks, also likely formed by over- on Titan than on Earth. Scaling bedrock ero- One pitfall with analogical reasoning is equi- land fl ows, which are not visible in lower- sion, however, is hampered by uncertainties fi nality, in which different causative processes resolution SAR data. This high-resolution regarding Titan material properties. Overall, yield similar effects (Mutch, 1979; Schumm, snapshot suggests that some fl uvial features observations of Titan point to a world per- 1991). An illustration from planetary science is observed in SAR data may be river val- vasively infl uenced by fl uvial processes, for the past debate over the volcanic or impact ori- which appropriate terrestrial analogs and gin of lunar craters. This directionality, whereby †E-mail: [email protected] formulations may provide insight. landscapes are fi rst encountered remotely at low GSA Bulletin; March/April 2013; v. 125; no. 3/4; p. 299–321; doi: 10.1130/B30612.1; 7 fi gures; 1 table. For permission to copy, contact [email protected] 299 © 2013 Geological Society of America Burr et al. resolution and only later (if at all) at outcrop 2007a), and the Cassini Titan Radar Mapper in SAR data are equally consistent with inter- scale, is the inverse of the traditional direction- (RADAR; Elachi et al., 2004, 2005) (Fig. 1). Of pretations as river valleys as they are with the ality for terrestrial geology. It makes planetary these three data sets, fl uvial features have been more commonly published interpretation as geology particularly susceptible to the effects most extensively observed in synthetic aperture channels. We discuss interpretations of individ- of equifi nality because the low resolution limits radar (SAR) data (examples in Fig. 2) from the ual fl uvial landforms in the section Individual discernment of key characteristics. As corrob- RADAR instrument. These images have the best Fluvial Landforms Observed in SAR Data. orative input to geologic analogs, mathematical combination of map resolution (best map reso- The fi ne-scale fl uvial networks observed in models are strong analogs in which attributes lution ~300 m/pixel) and coverage (~40% of the the DISR image mosaic (Fig. 3) are not dis- and physical laws are expressed in a simplifi ed surface to date) for observing fl uvial networks, cernible in SAR data (Soderblom et al., 2007b; formulation that can be compared to the land- and they were collected at a wavelength simi- Jaumann et al., 2009), raising a second question scape under study. Models also provide versatil- lar to the size of fl uvial cobbles observed on the as to the extent of fi ne-scale fl uvial dissection on ity in attribute parameters, which is critical for surface (Tomasko et al., 2005). Although fl uvial Titan. Based on mapping using SAR data, ~1% extraterrestrial investigations. features were inferred from early VIMS data of Titan’s surface was inferred to be covered by The largest contribution of the Mars Channel (Barnes et al., 2007b; Jaumann et al., 2008), fl uvial landforms (Lorenz et al., 2008a; Lopes Working Group (1983) to understanding Mars more recent work has not shown a strong global et al., 2010) with endogenic and other exogenic developed a decade after the fi rst Mariner 9 correlation between fl uvial features mapped processes shaping the remaining surface. How- images were returned from that planet, where from SAR and from VIMS data (Langhans ever, an alternative interpretation of Titan’s fl uvial features were formed by fl owing water et al., 2012). Fluvial networks have also been global landscape is formation entirely by exo- eroding and transporting silicate rock and sedi- mapped in higher-resolution visible light im- genic processes (Moore and Pappalardo, 2011). ment, as on Earth. Almost a decade has now ages (~20–90 m/pixel) taken by the Descent Some regions on Titan show a distinctive tex- passed since the Cassini-Huygens mission fi rst Imager/Spectral Radiometer (DISR; Tomasko ture in SAR images of alternating brighter and returned images from Titan, where fl uvial fea- et al., 2005) (Fig. 3) on the Huygens probe dur- darker kinked, wrinkled, or crenulated banding tures are formed by fl owing liquid alkanes (pri- ing its descent to the Titan surface. Jaumann (e.g., Figs. 2A, 2B, 2F, and 2G). Although fl u- marily methane) acting on materials derived et al. (2009) provide a detailed overview of the vial networks are typically not discernible in this from an icy crust and organic-rich atmosphere. characteristics of the Cassini-Huygens surface- crenulated terrain, they are often adjacent to or As with Mars, understanding of fl uvial features sensing data sets for Titan. connected with it, suggesting a possible genetic on Titan is advanced through analogy to ter- Because of the difference in data set resolu- relationship. We discuss our hypothesis that the restrial features and application of mechanical tions, the mapped fl uvial features give contrast- crenulated terrain on Titan is fl uvial dissected principles. The distinct materials and very dif- ing impressions regarding formation as river in the section Crenulated Terrain: Evidence of ferent conditions on Titan highlight the need channels versus river valleys. The individual Fine-Scale Fluvial Dissection. for morphologic analogies to be supported by features observed in the SAR data are of order These discussions are presented by decreas- physical theory. 0.5–1 km in width, with maximum reported ing scale, beginning with networks observed in This paper presents an analysis of fl uvial widths of ~3 km to ~10 km (Lorenz et al., SAR data, followed by individual features ob- processes on Titan through both qualitative inter- 2008a).