Radar-Bright Valley (Titan)
KeywordsRadar Backscatter Fluvial Erosion Radar Wavelength Interior Channel Fluvial Valley
A valley-like feature on Titan exhibiting high radar backscatter.
A type of valley-like feature (Titan)
Fluvial valleys on Titan have diverse morphologies with subclasses as defined by arid but cold climatic conditions. Dry valleys with lengths <300 km and width <8 km are observed. They appear radar-bright on Cassini SAR images because they are not filled with liquid but with sediments that are probably centimetric or even larger in size in the brightest valleys. They are similar to terrestrial desert washes (wadis), which suggests formation by sudden, episodic, and strong flow events in dry climatic conditions, followed by long dry periods. Outflow events may be induced by precipitation or sapping. Compared to valleys within integrated dendritic networks, Titan’s dry valleys are generally shorter and broader (Porco et al. 2005; Elachi et al. 2005; Lorenz et al. 2008; Jaumann et al. 2008, 2009; Le Gall et al. 2010; Langhans et al. 2012).
Although this landform has certainly been caused by fluvial flow, its morphology is characterized by a low branching level with only few tributaries. A system of putative dry valleys at southern mid-latitudes appears like broad and straight streams. Their course resembles laminar surface runoff or denudation, unlike the linear, channelized flow in valleys. The suspected flows are oriented nearly in parallel and appear darker on radar than their surroundings. Their overall morphology is largely similar to that of wadis in terrestrial deserts (Langhans et al. 2012).
Subtypes (by Morphology/Morphometry)
Broad and straight streams are suggestive of laminar surface runoff. They are sometimes braided but seldom meandering. Interior channels and valleys cannot be distinguished. Flat cross sections are expected (Langhans et al. 2012; Figs. 1 and 2).
Dry channel in floodplains. Radar-gray “floodplains” (interpreted as smooth sediment cover) with bright, meandering interior channels and without fluvial terminations. They are indicative of large variations in streamflow. The wide plains without continuation or mouth indicate high evaporation or infiltration rates over a short period, whereas the morphology of interior channel is consistent with steady discharge (Langhans et al. 2012; Fig. 4).
A dry valley is a valley that lacks a surface flow of liquid or of snow or ice cover, however was formed by fluvial or glacial erosion.
Occasional methane rains occurring near equinox over the arid low latitudes may reactivate dry river valleys. The observation of a large, arrow-shaped storm cloud and subsequent, short-term darkening of the surface in South Belet affected by the storm suggests that these valleys are carved by seasonal precipitation (Turtle et al. 2011).
Model ages for the surface of Titan range from a few million years up to 3.9 Ga, depending on the model chosen and its assumptions. Relative or comparative statements about the surface age of different surface units can only be made through the interpretation of stratigraphic relations. The incision of fluvial valleys, along with the accumulation of dunes, is certainly among the most recent processes on Titan’s geologic timescale (e.g., Jaumann et al. 2009; Lopes et al. 2010). Fluvial terrains are more or less free of craters and dunes since channels tend to even out these landforms given that fluvial erosion has been active for a substantial amount of time.
Titan’s valleys seem relatively young due to their pristine and undegraded morphology (Soderblom et al. 2007; Lunine and Atreya 2008). Fluvial features shape preexisting landforms, e.g., craters and mountains at many locations on Titan supporting the idea that fluvial erosion took place relatively recently.
They are found in mid-latitudes in plains (e.g., Elivagar Flumina) and homogeneous environments. Their occurrence is confined to the mid-latitudes of both hemispheres away from the mountain ranges. In contrast to dendritic fluvial systems, terrains covered by dry valleys are less extensive and distributed only over a small proportion of the total surface of Titan (Langhans et al. 2012).
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