Growth mechanisms and dune orientation on Titan

Compared to barchan dunes, the morphodynamics of linear dunes that elongate on a nonerodible bed have barely been investigated by means of laboratory experiments or numerical simulations. Using a cellular automaton model, we study the elongation of a solitary linear dune from a sand source and show that it can reach a steady state. This steady state is analyzed to understand the physical processes at work along the dune. Crest reversals together with avalanche processes control the shape of transverse sections. Dune width and height decrease almost linearly with distance downstream until the minimum size for dune is reached. This is associated with a constant sand loss along the dune, which eventually compensates for the sediment influx and sets the dune length. This sand budget is discussed to distinguish an elongating linear dune from a barchan dune and to explain the complexity of linear dune fields in nature. Plain Language SummaryGiven the seasonal cycle, multidirectional wind regimes are highly prevalent in modern sand seas where linear dunes are the most frequent dune type. They vary in shape and size, but they are all characterized by linear ridges extending over long distances. Here we study the morphodynamics of linear dunes that elongate from a sand source under the action of reversing winds. We show that the sand loss increases with the length of the elongating linear dune which eventually converges to a steady state. We relate the dimensions and the shape of the dune at equilibrium to the sediment influx and the wind properties. The characterization of this elementary dune type is an essential step toward a better understanding of the interplay between winds and complex dune fields in nature. This is important for Earth in a context of climate change, but also for Mars and Titan, Saturn's largest moon, where direct wind measurements are not available to date.

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“…Note that the obstacle is several times wider than the dune itself. With a typical drift potential of 30 m

^{2}

/year as measured in Niger (Lucas et al, ), the equivalent sediment influx would be of 275 m

^{3}

/year. A 100‐m‐wide obstacle could provide such an influx, catching a free sand flux of 2.75 m

^{2}

/year.…”

Section: Discussion and Concluding Remarksmentioning

confidence: 99%

“…An elongating linear dune, also described as a finger dune by Courrech du Pont et al (2014), can be seen as an elementary dune type. It is a simple, noncompound dune that forms on a nonerodible bed (Figure 1).…”

Section: Comparison Between Two Elementary Dune Types: Finger and Barmentioning

confidence: 99%

Elongation and Stability of a Linear Dune

Rozier

Narteau

Gadal

et al. 2019

Geophysical Research Letters

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“…Dune orientation and morphology are determined by wind patterns; given the difficulties in measuring Titan's winds remotely, the dunes provide some of our only constraints on the wind speed and direction at the surface. Although initial GCM wind predictions were inconsistent with the dune orientations [ Tokano , ], more recent works demonstrate that conditions around equinox, either a reversal of the prevailing winds (to strong westerly (eastward) winds) [ Tokano , ] or the generation of strong, westerly wind gusts by the observed equinoctial storms (because their momentum comes from higher altitudes where strong westerlies are present) [ Lucas et al , ; Charnay et al , ], control the dune elongation rather than the prevailing easterlies (westward). This interpretation is supported by experiments done using the Titan Wind Tunnel that show that threshold wind speeds are higher than previously predicted for Titan and faster than the prevailing easterlies [ Burr et al , ].…”

Section: Connection With the Surfacementioning

confidence: 99%

Titan's atmosphere and climate

2017

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Titan is the only moon with a substantial atmosphere, the only other thick N2 atmosphere besides Earth's, the site of extraordinarily complex atmospheric chemistry that far surpasses any other solar system atmosphere, and the only other solar system body with stable liquid currently on its surface. The connection between Titan's surface and atmosphere is also unique in our solar system; atmospheric chemistry produces materials that are deposited on the surface and subsequently altered by surface‐atmosphere interactions such as aeolian and fluvial processes resulting in the formation of extensive dune fields and expansive lakes and seas. Titan's atmosphere is favorable for organic haze formation, which combined with the presence of some oxygen‐bearing molecules indicates that Titan's atmosphere may produce molecules of prebiotic interest. The combination of organics and liquid, in the form of water in a subsurface ocean and methane/ethane in the surface lakes and seas, means that Titan may be the ideal place in the solar system to test ideas about habitability, prebiotic chemistry, and the ubiquity and diversity of life in the universe. The Cassini‐Huygens mission to the Saturn system has provided a wealth of new information allowing for study of Titan as a complex system. Here I review our current understanding of Titan's atmosphere and climate forged from the powerful combination of Earth‐based observations, remote sensing and in situ spacecraft measurements, laboratory experiments, and models. I conclude with some of our remaining unanswered questions as the incredible era of exploration with Cassini‐Huygens comes to an end.

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“…According to Fryberger and Dean [9], a very narrow bimodal wind regime with a wind direction variability of approximately 0.7-0.9 leads to bedforms aligned transversely to the wind direction. Barchans form if there is a relatively low sand supply [43][44][45][46][47] and are often asymmetric, with one limb extended downwind [1,2].…”

Section: Wind Regimes and Asymmetric Barchan Dunesmentioning

confidence: 99%

Migration and Morphology of Asymmetric Barchans in the Central Hexi Corridor of Northwest China

Zhang

Dong

et al. 2018

Geosciences

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Crescent-shaped barchan dunes often display an asymmetric shape, with one limb longer than the other. As shown in previous studies, asymmetric bimodal winds constitute one major cause of barchan asymmetry, but the heterogeneous conditions of sand availability or flux, as well as topographic influences, may be also important. Understanding the morphology and dynamics of asymmetric barchans may have an impact in a broad range of areas, particularly as these dunes may serve as a proxy for planetary wind regimes and soil conditions in extraterrestrial environments. However, in addition to the existing theories and numerical models that explain barchan asymmetry, direct measurements of migration rates and morphologic changes of real asymmetric barchans over a time span of several years would be beneficial. Therefore, here we report such measurements, which we have acquired by investigating asymmetric barchans in the Hexi Corridor, northwest of China. We have found that dune interactions and asymmetric influx conditions are the most important causes of barchan asymmetry in this field. Particle size distributions in the Hexi Corridor display strong variations over different parts of the asymmetric barchans, as well as over different dunes, with gravel particles being incorporated from the substrate as the dunes migrate. Our observations have shown that upwind sediment sources are important for dune formation in the Hexi Corridor, and that interdune interactions affect dune shape in different ways, depending on their offset. The asymmetric barchans in the Hexi Corridor are active, with an average migration rate (MR) between 8 and 53 m year −1 , in spite of the different asymmetric shapes. Our data for dune migration rates can be described well by a scaling of MR = A/(W + W0), where W is the barchan cross-wind width, A ≈ 2835 m 2 s −1 , and W0 ≈ 44 m. A similar scaling fits very well the migration rate as a function of dune along-wind width L, (i.e., MR = B/(L + L0), with B ≈ 1722 m 2 s −1 and L0 ≈ 13 m). Linear relations are also found between both dune widths and the average limb and windward side lengths, thus indicating that the morphometric relations that are predicted from models for steady-state, symmetric crescent-shaped dunes can be applied to different transitional morphologies of interacting, asymmetric barchans.

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Growth mechanisms and dune orientation on Titan

Cited by 57 publications

References 30 publications

Elongation and Stability of a Linear Dune

Elongation and Stability of a Linear Dune

Titan's atmosphere and climate

Migration and Morphology of Asymmetric Barchans in the Central Hexi Corridor of Northwest China

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