Sediment transport by liquid surficial flow: Application to TitanIntroductionEntrainmentTransportOverland flow depths and possible enhancement of sediment transportMinimum flow velocity and unit discharge required to transport sedimentConclusionsAcknowledgmentsReferencesIcarus 181 (2006) 235–242www.elsevier.com/locate/icarusSediment transport by liquid surficial flow: Application to TitanDevon M. Burra,∗, Joshua P. Emeryb, Ralph D. Lorenzc, Geoffrey C. Collinsd, Paul A. CarlingeaSETI Institute, 515 N. Whisman Rd., Mt. View, CA 94043, USAbNASA Ames Research Center/SETI Institute, Mail Stop 245-6, Moffett Field, CA 94035, USAcLunar and Planetary Lab, University of Arizona, 1629 E. University Blvd., Tucson, AZ 85721, USAdPhysics and Astronomy Department, Wheaton College, Norton, MA 02766, USAeSchool of Geography, Highfield University of Southampton, Southampton, SO17 1BJ, UKReceived 17 June 2005; revised 12 November 2005Available online 17 January 2006AbstractSediment transport by surficial flow likely occurs on Titan. Titan is thought to have a volatile cycle, such as on Earth and likely in the paston Mars, w hich would entail surficial liquid flow. And surficial flow is implied in interpretations of Cassini–Hyugens data as showing fluvialchannels, which would require sediment transport by surficial flow to form the observable features. We present calculations from basic hydraulicformulae of sediment entrainment and transport by surficial flow. First, w e describe the conditions for (non-cohesive) sediment entrainmentby grain size through use of the Shields’ threshold curve. We then calculate settling velocities by grain size to describe the type of sedimenttransport—washload, suspended load, or bedload—that would follow entrainment. These calculations allow derivation of required flow depthsfor sediment transport by grain size over a given slope. A technique to estimate r equired flow velocities and unit discharges is also presented. Weshow the r esults of these calculations for organic and water ice sediment movement by liquid methane flow under Titan gravity. For comparativepurposes, plots for movement of quartz sediment by water on Earth and basalt sediment by water on Mars are also included. These results indicatethat (non-cohesive) material would move more easily on Titan than on Earth or Mars. Terrestrial field observations suggest that coarse graintransport is enhanced by hyperconcentration of fine-grained sediment; and the apparent availability of organic (fine grained) sediment on Titan, inconjunction with the possibility of convection-driven rainstorms, may lead to hyperconcentrated flows. Thus, significant sediment transport mayoccur on Titan during individual overland flow events. 2005 Elsevier Inc. All rights reserved.Keywords: Titan; Mars, surface; Surfaces, planets; Satellites of Saturn; Surfaces, satellite1. IntroductionEarth, Mars, and Titan are the three bodies in the Solar Sys-tem currently inferred to show evidence for surficial liquid flowduring volatile cycling. On Earth, this evidence appears as riverchannels and many other features in which liquid flow is ac-tive today. On Mars, similar evidence is visible as valley net-works, intercrater deltas, and other possible aqueous flow fea-tures indicative of a past volatile cycle (e.g., Baker, 2001). OnTitan, recent Cassini–Huygens results show dark, dendritic, flu-viatile networks on bright, elevated surfaces (Porco et al., 2005;*Corresponding author.E-mail address: [email protected] (D.M. Burr).Elachi et al., 2005; Soderblom et al., 2005; Tomasko et al.,2005).Volatile cycling and associated surficial flow on Earth orMars involve or likely involved water. A volatile cycle on Titanlikely involves hydrocarbons. At Titan’s surface temperature(94 K) and pressure (1.44 bar), methane and ethane would beliquid (summarized in Lorenz et al., 2003 ). This fact has led tosuggestions of hydrocarbon rains, lakes or seas, and possiblyfluvial systems (Lunine et al., 1983; Ori et al., 1998). Ethanemay be present in surficial bodies of water, but it is likely tooinvolatile to be rapidly recycled by evaporation into Titan’s at-mosphere. Consequently, possible rainfall and rivers would bedue to cycling of methane only (Lorenz and Lunine, 2005).Methane was first detected remotely in Titan’s troposphere byKuiper (1944). Samuelson et al. (1997) analyzed Voyager spec-0019-1035/$ – see front matter  2005 Elsevier Inc. All rights reserved.doi:10.1016/j.icarus.2005.11.012236 D.M. Burr et al. / Icarus 181 (2006) 235–242tra, indicating a near-surface methane mole fraction of ∼6% atlow latitude, falling near-symmetrically to 2–3% at 60◦latitude.Lemmon et al. (2002) suggested a low-latitude near-surfacemole fraction of 3.8% using spatially-resolved near-infraredspectra acquired by the Hubble Space Telescope in 1997. TheHuygens probe measured near-surface methane abundance atits low-latitude landing site of ∼5% by mass spectrometry(Niemann et al., 2005).The expected liquid hydrocarbon seas are not apparent in theCassini–Huygens optical or radar images acquired to date ofTitan’s surface (Turtle et al., 2005), but some features in theseimages are suggestive of fluvial systems (Elachi et al., 2005;Porco et al., 2005; Tomasko et al., 2005). Sediment entrain-ment, transport, and deposition by fluvial systems may havecaused surficial modification over Titan’s history, possibly ac-counting in part for the near absence of impact craters on Ti-tan’s surface (Porco et al., 2005). Fluvial systems on Earthentail overland flow in the interfluve areas and then channel-ized flow in the channels. Sediment entrained and transportedby surficial flow on Earth and Mars is silicate material de-rived from the planets’ crusts. Sediment on Titan is likely tobe of two other types of material. One type would be waterice, derived from Titan’s water ice outer crust (Stevenson, 1992;Lorenz and Lunine, 1996; Griffith et al., 2003). A second typewould be organic material, which settles from the atmosphereafter formation by photochemical reactions of hydrocarbons(Khare et al., 1978, 1984; Tran et al., 2003; Lorenz and Lu-nine, 2005).Here we present calculations of sediment entrainment andtransport by surficial flow. Hydraulic formulae are applied to es-timate entrainment thresholds and settling velocities as a func-tion of sediment grain size. The settling velocity estimates arethen used to determine threshold frictional shear