Channels are a common feature across the surface of Venus [Baker et al., 1992; Komatsu et al., 1993], and understanding their formation represents a challenging puzzle in Venus's geologic record. Venusian channels, open conduits believed to have transported fluid [Baker et al., 1992], are morphologically classified as simple, complex, and compound [Gulick et al., 1991; Baker et al., 1992]; a single channel may display all three morphologic classifications [Baker et al., 1997]. Despite morphologic differences, Venusian channels share some similar characteristics with terrestrial channels. For example, the sinuosity and meander properties of Venusian channels resemble terrestrial fluvial systems [Komatsu and Baker, 1994]. However, unlike terrestrial fluvial systems, many Venusian channels typically maintain constant widths of <1–3 km along lengths up to >1000 km [Baker et al., 1992] and lack tributaries, cutoff meanders, and extensive lateral flow deposits. In addition, Venusian channels are abundant in presumed volcanic regions [Komatsu et al., 1993] where sources and termini are generally indistinct [Baker et al., 1997] and channels maintain low gradients [Baker et al., 1992], reach depths up to tens of meters [Komatsu et al., 1992, 1993], and locally display what appear to be levees [Komatsu et al., 1992; Bussey et al., 1995]. Collectively, these characteristics make Venusian channels intriguing features on Venus's surface.
 Workers agree about channel characteristics, and they generally agree that fluids played a major role in sculpting channels [Baker et al., 1992; Komatsu et al., 1992, 1993; Komatsu and Baker, 1994; Gregg and Greeley, 1993; Kargel et al., 1994; Bussey et al., 1995; Williams-Jones et al., 1998; Jones and Pickering, 2003]. Channel association with presumed volcanic regions [Komatsu et al., 1993] and current surface conditions on Venus (∼450°C [Crisp and Titov, 1997], ∼100 bars [Fegley et al., 1997]) has led many workers, including us, to infer a volcanic origin for channels, although origins involving liquid water cannot be fully excluded [Baker et al., 1992; Jones and Pickering, 2003; Donahue et al., 1982, 1997]. In order to accommodate Venusian channel characteristics and surface conditions, a diversity of fluids and channel forming mechanisms have been proposed. However, all hypotheses of channel formation include the assumption that channels form at the atmosphere-surface interface. Hypotheses also include the assumption that channels form in a substrate of basalt [Baker et al., 1992; Komatsu et al., 1992, 1993; Komatsu and Baker, 1994; Gregg and Greeley, 1993; Kargel et al., 1994; Bussey et al., 1995; Williams-Jones et al., 1998], although Jones and Pickering  proposed a sediment substrate carved by water. Therefore, similar to Earth, the role of the chemical and physical properties of the surface environment, fluid, and substrate must each be considered when addressing processes of Venusian channel formation. On Earth, lava channels form from constructional processes as well as from mechanical, thermal, and thermomechanical erosion and it seems plausible to consider that the same processes of channel formation operate on Venus. Constructional processes involve the formation of levees to constrain fluid flow [Hulme, 1974]; levees will begin to form as lava cools. Mechanical erosion occurs by fluid physically plucking material from the substrate and is enhanced by turbulent fluid flow [e.g., Kargel et al., 1994; Williams-Jones et al., 1998]. Thermal erosion results from melting of the substrate [Hulme, 1973, 1982; Carr, 1974; Dawson, 1990; Komatsu et al., 1992; Bussey et al., 1995; Greeley et al., 1998; Fagents and Greeley, 2001; Kerr, 2001; Williams et al., 2004]. Thermomechanical erosion involves aspects of both mechanical and thermal erosion [Fagents and Greeley, 2001]. The efficiency of the proposed erosional channel-forming processes will depend largely on lava cooling rate, although physical properties of the lava and substrate also play a role [e.g., Williams et al., 1998; Kerr, 2001]. Consider, for example, a basalt flowing on a basaltic substrate. If the basalt flows in a turbulent regime, it will lose heat in an efficient manner [Sakimoto and Zuber, 1998] and not mechanically carve an extensive channel [Komatsu et al., 1992]. However, lava such as alkali-carbonatite or sulfur may possibly flow turbulently, and mechanically carve channels on a basaltic substrate, for distances up to thousands of kilometers [Kargel et al., 1994; Williams-Jones et al., 1998]. Furthermore, both thermal and thermomechanical erosion require sufficient time to supply enough energy to the substrate to induce melting. Hence the rate of lava heat loss will govern the amount of thermal and thermomechanical erosion that occurs [Hulme, 1973; Greeley et al., 1998; Fagents and Greeley, 2001; Kerr, 2001; Keszthelyi, 1995; Sakimoto and Zuber, 1998]. Thermal erosion will be more pronounced the greater the difference between the melting temperature of the substrate and the eruption temperature of the lava [Greeley et al., 1998; Williams et al., 1998; Fagents and Greeley, 2001]. For example, Fagents and Greeley  demonstrated that basalt can thermomechanically erode a basaltic substrate. However, erosion remained concentrated near the vent because the basalt lava cooled too quickly to sustain erosion of basalt substrate. Conversely, Williams et al.  demonstrated that komatiite lava can thermomechanically erode sediment substrate to create channels up to tens of kilometers long. Hence the cooling rate of lava on Earth contributes to the extent to which mechanical, thermal, and thermomechanical erosion may occur. Earth's atmosphere facilitates radiative cooling of lava flows [Snyder, 2002], which impedes the degree to which lava induced erosion occurs on Earth [e.g., Fagents et al., 2001]. Conversely, Venus's CO2-rich atmosphere acts as an insulating cover that impedes the loss of thermal energy from the surface, which allows lava flows to travel further on Venus than on Earth [Snyder, 2002]. The impeded cooling rate should enhance erosional processes compared to Earth. Clearly, many questions regarding the formation of Venusian channels still exist.