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A global map of Titan showing the locations of the regions of interest: Shangri-La, Fensal and Belet sand seas. The basemap is an empirically corrected VIMS mosaic from [Rodriguez et al., 2014]. SAR swaths are overlain in transparency. The Hammer projection is centered at 0 • longitude with graticules every 30 • in both longitude and latitude.
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The texture, composition, and morphology of dunes observed in the equatorial regions of Titan may reflect present and/or past climatic conditions. Determining the physio‐chemical properties and the morphodynamics of Titan's dunes is therefore essential to understanding of the climatic and geological history of the largest moon of Saturn. We quantit...
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... sand seas present a dark signature in visible (ISS camera) and microwave (RADAR instrument) data, while they are "brown" in hyperspectral data (VIMS) (i.e. dark at 1.3, 2 and 5µm) (Figure 1). Geomorphic analyses show that most of the bedforms are 100-to-1000's km long linear dunes with a crest-to-crest distance around 3 km [ Lorenz et al., 2006Lorenz et al., , 2010Le Gall et al., 2011;Savage et al., 2014;Lucas et al., 2014a]. ...
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... this study, we aim to assess Titan's surface and subsurface properties within the different equatorial sand seas based on their Cassini RADAR SAR signature. By combining state-of-the-art SAR image despeckling, filtering and classification techniques, modeling of the microwave backscatter, and taking advantage of the large coverage in incidence angles over the sand seas now available, we seek to determine the bulk surface properties of the dunes, interdunes and inselbergs at three major sand seas: Belet, Shangri-La and Fensal ( Figure 1). We present the geomorphic classification and microwave backscatter extraction workflow for the SAR data in section 2. We describe our modeling approach and Bayesian inference in section 3. We discuss our results and their implications in section 4 and then present our conclusions. ...
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... split-spectrum map created from NLDSAR data greatly facili-tates classification, as shown in Figure 5. Finally, the backscattering cross-section values as a function of incidence angle are extracted from the NLDSAR data upon the split-spectrum segmentation. The complete data reduction workflow, including the main steps, is summarized on Figure 6 and has been applied to observations of Belet, Shangri-La and Fensal sand seas (Figure 1, Figure A.1, and Supplementary Information). ...
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... marginal posterior probabilities computed from our Bayesian analysis for each physical parameter are summarized in Figure 10. We obtain nearly Gaussian posterior probabilities for the dune units for each parameter ( , ξ/ζ, and a) of the three sand seas. ...
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... results suggest that the radar-bright areas in ShangriLa and Fensal are more heterogeneous. Note that both bound Xanadu (Figure 1). It is unclear at this stage how this proximity may affect our regions of interest by, for example, influencing sediment transport. ...
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... amplification of the diffuse part is in agreement with the findings for the radar-bright region of Xanadu from previous studies [Janssen et al., , 2016. Accounting for this effect by multiplying the subsurface term by a factor > 2 (i.e., 3) after Janssen et al. [2016], we go beyond the coherent backscattering effect and hence obtain the curve "ampV pp " in Figure 10. As shown, this correction remains to be explained by some mechanism, but it allows us to partly retrieve a Gaussian distribution of the labedo a. ...
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... Earth, dunes elongating over tens of kilometers are the prominent geomorphic features in the Ténéré desert in Niger and in the Kumtagh desert in China [Ping et al., 2017], where they form isolated linear ridges aligned with the resultant transport direction (Figure 11). These longitudinal dunes are regularly indented by transverse secondary bedforms, which may locally eject barchans. ...
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... the coexistence of the two dune growth mechanisms can naturally explain how finger-like structures elongate in a specific direction and generate superimposed bedforms with a different orientation as the bed instability develops on their flanks. The instability is not observed in the interdune areas because the bed surfaces exhibit armor layers composed of coarse-grained material and zibars ( Figure 11). The transport rate of uniform fine grains along the crest can then be derived from the local wind data to explain the elongation rate of dunes as well as the migration rate of secondary bedforms Ping et al., 2017]. ...
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... many (if not all) dune fields on Earth, such differences between dunes and interdunes is the result of granular sorting causing armoring effect in aeolian systems. A grain size ratio of 5 to 10 Figure 11. Examples of size segregation in terrestrial dune fields: (a) Ténéré desert (Niger); (b-d) Kumtagh desert (China). ...
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... credits: Y. Callot (Niger, 1976), G. Steinmetz and C. Narteau (China, 2011). and a surface roughness ratio of 10 between the interdune areas and the dunes are typically observed (Figure 11). Accounting for the above considerations and as we find a significantly differences in the RMS slopes, we propose that the sediment beds in the interdune areas have a mean grain-size larger than on the dune bodies. ...
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... sand seas present a dark signature in visible (ISS camera) and microwave (RADAR instrument) data, while they are "brown" in hyperspectral data (VIMS) (i.e. dark at 1.3, 2 and 5µm) (Figure 1). Geomorphic analyses show that most of the bedforms are 100-to-1000's km long linear dunes with a crest-to-crest distance around 3 km [ Lorenz et al., 2006Lorenz et al., , 2010Le Gall et al., 2011;Savage et al., 2014;Lucas et al., 2014a]. ...
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... this study, we aim to assess Titan's surface and subsurface properties within the different equatorial sand seas based on their Cassini RADAR SAR signature. By combining state-of-the-art SAR image despeckling, filtering and classification techniques, modeling of the microwave backscatter, and taking advantage of the large coverage in incidence angles over the sand seas now available, we seek to determine the bulk surface properties of the dunes, interdunes and inselbergs at three major sand seas: Belet, Shangri-La and Fensal ( Figure 1). We present the geomorphic classification and microwave backscatter extraction workflow for the SAR data in section 2. We describe our modeling approach and Bayesian inference in section 3. We discuss our results and their implications in section 4 and then present our conclusions. ...
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... split-spectrum map created from NLDSAR data greatly facili-tates classification, as shown in Figure 5. Finally, the backscattering cross-section values as a function of incidence angle are extracted from the NLDSAR data upon the split-spectrum segmentation. The complete data reduction workflow, including the main steps, is summarized on Figure 6 and has been applied to observations of Belet, Shangri-La and Fensal sand seas (Figure 1, Figure A.1, and Supplementary Information). ...
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... marginal posterior probabilities computed from our Bayesian analysis for each physical parameter are summarized in Figure 10. We obtain nearly Gaussian posterior probabilities for the dune units for each parameter ( , ξ/ζ, and a) of the three sand seas. ...
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... results suggest that the radar-bright areas in ShangriLa and Fensal are more heterogeneous. Note that both bound Xanadu (Figure 1). It is unclear at this stage how this proximity may affect our regions of interest by, for example, influencing sediment transport. ...
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... amplification of the diffuse part is in agreement with the findings for the radar-bright region of Xanadu from previous studies [Janssen et al., , 2016. Accounting for this effect by multiplying the subsurface term by a factor > 2 (i.e., 3) after Janssen et al. [2016], we go beyond the coherent backscattering effect and hence obtain the curve "ampV pp " in Figure 10. As shown, this correction remains to be explained by some mechanism, but it allows us to partly retrieve a Gaussian distribution of the labedo a. ...
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... Earth, dunes elongating over tens of kilometers are the prominent geomorphic features in the Ténéré desert in Niger and in the Kumtagh desert in China [Ping et al., 2017], where they form isolated linear ridges aligned with the resultant transport direction (Figure 11). These longitudinal dunes are regularly indented by transverse secondary bedforms, which may locally eject barchans. ...
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... the coexistence of the two dune growth mechanisms can naturally explain how finger-like structures elongate in a specific direction and generate superimposed bedforms with a different orientation as the bed instability develops on their flanks. The instability is not observed in the interdune areas because the bed surfaces exhibit armor layers composed of coarse-grained material and zibars ( Figure 11). The transport rate of uniform fine grains along the crest can then be derived from the local wind data to explain the elongation rate of dunes as well as the migration rate of secondary bedforms Ping et al., 2017]. ...
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... many (if not all) dune fields on Earth, such differences between dunes and interdunes is the result of granular sorting causing armoring effect in aeolian systems. A grain size ratio of 5 to 10 Figure 11. Examples of size segregation in terrestrial dune fields: (a) Ténéré desert (Niger); (b-d) Kumtagh desert (China). ...
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... credits: Y. Callot (Niger, 1976), G. Steinmetz and C. Narteau (China, 2011). and a surface roughness ratio of 10 between the interdune areas and the dunes are typically observed (Figure 11). Accounting for the above considerations and as we find a significantly differences in the RMS slopes, we propose that the sediment beds in the interdune areas have a mean grain-size larger than on the dune bodies. ...
Citations
... Similarly, the evaporite hypothesis for the abundant 5.0 µm bright material on Titan's surface also requires soluble material(s) and identifying their composition would significantly improve the understanding of Titan's hydrology. Further, the ability to discriminate between hydrocarbons and water ice using the latter's 3.29 µm feature would help to determine what the bulk composition of the dunes isHayne et al., 2014;Rodriguez et al., 2014;Brossier et al., 2018;Solomonidou et al., 2018), and further constrain differences between the dunes and interdunes related to possible grain-size sorting or material propensity for triboelectrification(Bonnefoy et al., 2016;Méndez Harper et al., 2017;Lucas et al., 2019). With regards to aeolian processes, identifying areas of high methane or ethane content in concert with degraded dune fields could indicate that moisturerelated cohesion is responsible for the inactivity of these dunes(Ewing et al., 2015;McDonald et al., 2016). ...
From orbit, the visibility of Titan's surface is limited to a handful of narrow spectral windows in the near-infrared (near-IR), primarily from the absorption of methane gas. This has limited the ability to identify specific compounds on the surface -- to date Titan's bulk surface composition remains unknown. Further, understanding of the surface composition would provide insight into geologic processes, photochemical production and evolution, and the biological potential of Titan's surface. One approach to obtain wider spectral coverage with which to study Titan's surface is by decreasing the integrated column of absorbers (primarily methane) and scatterers between the observer and the surface. This is only possible if future missions operate at lower altitudes in Titan's atmosphere. Herein, we use a radiative transfer model to measure in detail the absorption through Titan's atmosphere from different mission altitudes, and consider the impacts this would have for interpreting reflectance measurements of Titan's surface. Over our modeled spectral range of 0.4 - 10 micron, we find that increases in the width of the transmission windows as large as 317% can be obtained for missions performing remote observations at the surface. However, any appreciable widening of the windows requires onboard illumination. Further, we make note of possible surface compounds that are not currently observable from orbit, but could be identified using the wider windows at low altitudes. These range from simple nitriles such as cyanoacetylene, to building blocks of amino acids such as urea. Finally, we discuss the implications that the identifications of these compounds would have for Titan science.
... Despite the difficulties in documenting the linear regime in natural environments, more sites should be systematically tested. This is the object of recent studies, that uses the theoretical framework described in this chapter to quantitatively interpret field data (Lucas et al. 2019, Iacono et al. 2020, Delorme et al. 2020). ...
Aeolian dunes result from the erosion and deposition of sand grains transported by the wind. In turn, the flow adapts to the shape of the sedimentary bed. This coupling between topography, hydrodynamics and transport controls the dune size, orientation and morphology, particularly during the early stage of their growth. In this thesis, we highlight the influence of certain environmental parameters, such as wind regime, sand availability, and dune field boundary conditions on dune emergence. For this, we use a theoretical approach, coupled with laboratory experiments, numerical simulations and field studies.First, we validate the description of dune emergence in areas of high sediment availability as a linear instability. These dunes form as surface waves, whose amplitude increases temporally or spatially, depending on whether they form in the middle of the sedimentary bed, or on its upstream edge. Their orientation, wavelength and spatial/temporal growth rate are then mainly controlled by the angular distribution of sand fluxes.On a non-erodible ground, dunes develop from various sources of sediments (other dunes, river and lakes deposits, etc.). Under specific wind regimes, they elongate to form linear ridges. We show that the spatial organization of these dunes is then controlled by the distribution of sedimentary sources, and not by a length scale intrinsic to the elongation mechanism. Thus, they form periodic dune fields at the downstream edge of sediment beds, or remain isolated structures in zones of localized deposition. In the latter case, they can reach a stable equilibrium state, whose morphology is controlled by the wind reorientation period.Finally, we carry out a large-scale analysis of the Namib Sand Sea based on the theoretical framework of sediment transport and dune morphodynamics, coupled to wind data from climate reanalyses. We then compare field observations with our predictions on sand fluxes and dunes (orientations, morphologies, growth rates). These results allow us to discuss the formation and evolution of sand seas on time scales covering several climatic cycles. More generally, this methodology also makes it possible to infer information on winds or sedimentary material when these are not directly measurable, as well as to gain insight into the evolution and resilience of dune systems subject to climate or anthropogenic changes.
... A multi-phase alcanological * cycle is present, active across the surface of the body, which includes hydrocarbon lakes and seas primarily clustered around Titan's northern pole [8,9], as seen in Fig. 1. This makes Titan the only location in the Solar System other than Earth to have bodies of liquid on the surface and when considered alongside the presence of organically-rich dunes [10], aeolian activity [11], fluvial features [5] and cryovolcanic activity [12], Titan can be seen as analogous to the early Earth, and so is naturally of interest to abiogenetic studies. ...
Key questions surrounding the origin and evolution of Titan and the Saturnian system in which it resides remain following the Cassini-Huygens mission. In-situ measurements performed at key locations on the body are a highly effective way to address these questions, and the aerial-aquatic platform proposed in this report serves to deliver unprecedented access to Titan's northern surface lakes, allowing an understanding of the hydrocarbon cycle, the potential for habitability in the environment and the chemical processes that occur at the surface. The proposed heavier-than-air flight and plunge-diving aquatic landing spacecraft, ASTrAEUS, is supported by the modelling of the conditions which can be expected on Titan's surface lakes using multiphysics fluid-structure interaction (FSI) CFD simulations with a coupled meshfree smoothed-particle hydrodynamics (SPH) and finite element method (FEM) approach in LS-DYNA.
... A multi-phase alcanological * cycle is present, active across the surface of the body, which includes hydrocarbon lakes and seas primarily clustered around Titan's northern pole [8,9], as seen in Fig. 1. This makes Titan the only location in the Solar System other than Earth to have bodies of liquid on the surface and when considered alongside the presence of organically-rich dunes [10], aeolian activity [11], fluvial features [5] and cryovolcanic activity [12], Titan can be seen as analogous to the early Earth, and so is naturally of interest to abiogenetic studies. ...
Key questions surrounding the origin and evolution of Titan and the Saturnian system in which it resides remain following the Cassini-Huygens mission. In-situ measurements performed at key locations on the body are a highly effective way to address these questions, and the aerial-aquatic platform proposed in this report serves to deliver unprecedented access to Titan's northern surface lakes, allowing an understanding of the hydrocarbon cycle, the potential for habitability in the environment and the chemical processes that occur at the surface. The proposed heavier-than-air flight and plunge-diving aquatic landing spacecraft, ASTrAEUS, is supported by the modelling of the conditions which can be expected on Titan's surface lakes using multiphysics fluid-structure interaction (FSI) CFD simulations with a coupled meshfree smoothed-particle hydrodynamics (SPH) and finite element method (FEM) approach in LS-DYNA.
The transport of fine particles in suspension in rivers plays a key role in the erosion of continents and in the dynamics of landscapes. It is therefore important to understand the dynamics of this transport. River discharge is the main driver of sediment transport. Yet the relation between the suspended load concentration and the discharge is not unequivocal, and both clockwise and counter-clockwise loops, or hysteresis, are commonly observed in many rivers. To explain this phenomenon, we propose a phenomenological model for the suspended transport that accounts for the exchange of fine particles between the riverbed and the water column. This model involves three parameters: (1) a threshold water level, which characterizes the shear stress, above which the flow begins to entrain fine particles from the bed, (2) an erosion rate that characterizes the intensity of sediment entrainment, and (3) a settling velocity accounting for sediment deposition, which is related to the size of the transported grains. A theoretical investigation, based on synthetic floods, allows us to estimate the sensitivity of the model output to the input parameters. We show that the model accounts for counter-clockwise hysteresis of the concentration-discharge relation like those observed in the time series of many rivers. We then test the validity of the model against data collected in the Capesterre and Bras-David catchments, two small catchments (about 15 km2), located in Guadeloupe, monitored by the Observatoire de l’Eau et de l’éRosion aux Antilles (ObsERA). Although the model is based on simplifying assumptions, it consistently reproduces our field data and successfully captures the hysteretic counter-clockwise behavior of the concentration-discharge relation. Using inversion methods, we evaluate the model parameters on a few isolated floods and then on longer time series, of duration about two weeks. We also show that the characteristic size of transported grains can be derived from turbidity measurements, even in the absence of a concentration-turbidity relation. The model also demonstrates its effectiveness in quantitatively reproducing fine particle transport in a small river, at the flood scale and over longer periods, typically two weeks. Yet the model fails to represent some flood events, characterized by a clock-wise hysteresic loop in the concentration-discharge relation. We suspect that transport during these events is dominated by the input of fine particles originating from hillslopes, a process that is not accounted for by our model.
Titan's rich and dense atmosphere, composed mainly of methane and nitrogen, maintains a methane cycle that shapes its surface, like the water cycle on Earth. Methane precipitations erodes Titan's surface and forms complex river networks observed at all latitudes by the Cassini-Huygens mission. However, precipitation rates are poorly constrained and, in the absence of in situ measurements, understanding Titan’s global climate and meteorology is done primarily through climate models. Here, we apply a physics-based theory of river morphogenesis to provide new constraints on methane precipitation rates. In particular, we estimate the river discharges and precipitation rates required to shape two methane rivers located at the equator and south pole, which incise the surface of Titan. Our results show that the use of river morphology is relevant for inferring methane precipitation rates at various latitudes. Our estimates reduce the uncertainties from the climate models. In addition, our results reflect the climate variability between the equator and the south pole. Finally, this work sheds the light to unknown river on Earth: the giant gravel-bed rivers.
