Radar-Bright Valley (Titan)

  • Ralf JaumannEmail author
  • Mirjam Langhans
  • Alice Le Gall
Living reference work entry
DOI: https://doi.org/10.1007/978-1-4614-9213-9_123-1

Keywords

Radar Backscatter Fluvial Erosion Radar Wavelength Interior Channel Fluvial Valley 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Definition

A valley-like feature on Titan exhibiting high radar backscatter.

Category

Synonyms

Description

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).

Morphometry

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)

  1. (1)

    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).

     
  2. (2)

    Fanlike (delta). Alluvial fanlike features associated with SAR-bright sinuous and linear channels (Paganelli et al. 2005; Fig. 3).

     
  3. (3)

    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).

     
Fig. 1

Candidate dry channel system at southern mid-latitudes (Langhans et al. 2012) Cassini SAR observation T7. Image is centered at 7°W, 60°S. Scale bars 25 km (NASA/JPL-Caltech/ASI)

Fig. 2

Elivagar Flumina channel system east of Menrva crater interpreted to be large fluvial deposits. Channels converge into the area interpreted as a catchment basin. Cassini SAR observation T3 near 19°N, 77°W (NASA/JPL-Caltech/ASI)

Fig. 3

The two triangular radar-bright channels and the associated bright area are interpreted as alluvial units where icy coarse sediments were collected (Le Gall et al. 2010; Paganelli et al. 2005). Cassini SAR Ta observation image near 80°W, 54°N (NASA/JPL-Caltech/ASI)

Fig. 4

Interior channel surrounded by putative floodplains, derived from their radar reflectivity and morphology. Flow direction is from the right to the left. The floodplain surrounding the interior channel is radar-dark. Cassini SAR observation T41. Image is centered at 67°W, 27°S. Scale bar 50 km (NASA/JPL-Caltech/ASI)

Interpretation

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.

Their bright appearance indicates significant radar backscatter. For the brightest valleys (see Fig. 5), the most likely explanation at present is considered to be icy rounded rocks larger in size than the Cassini radar wavelength (2.2 cm) acting as natural radar retroreflectors (Le Gall et al. 2010). Pebbles of this size have been observed at the Huygens landing site (Tomasko et al. 2005). They are most probably debris, shaped, transported, and deposited by fluvial activity. Radar-bright features result from high return of backscattering or large amplitudes of the reflected radar signal indicating surface roughness comparable to the scale of the incident radar wavelength (see also radar features).
Fig. 5

Radar-bright valleys observed during the Cassini T44 radar swath (from Le Gall et al. 2010). The two radar-bright valleys are located southwest of the 2,500 km continental-scale feature Xanadu (near 17°N, 120°W). They are, respectively, 185 km and 140 km long and a few kilometers wide (1–8 km) (NASA/JPL-Caltech/ASI)

Formation

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).

Age

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.

Degradation

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.

Distribution

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).

See Also

References

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Copyright information

© Springer Science+Business Media New York 2014

Authors and Affiliations

  1. 1.DLRInstitut für PlanetenforschungBerlinGermany
  2. 2.Earth’s Magnetic FieldGFZ German Research Centre for GeosciencesPotsdamGermany
  3. 3.LATMOS (Laboratoire Atmosphères, Milieux, Observations Spatiales), UVSQ (Université Versailles Saint-Quentin)ParisFrance