Coordination of WP2: S. Courrech du Pont (MSC)
Dunes are produced by winds and their shapes are recordings of present and past climatic conditions and sediment properties. In dry environment, these shapes can be related to the different wind regimes and sand availability (Bagnold, 1941; Pye and Tsoar, 1990). An interesting inverse problem is then to infer wind history and sediment properties from the dune observations. However, the coupling between the sediment transport laws, winds and granular medium properties is highly non-linear (Ho, et al. 2011; Claudin et al., 2011), which makes this inverse problem highly complex. Indeed, the observation of dune orientation alone cannot allow to unambiguously and finely characterize both the aeolian flow (number, orientation and strength of the winds) and the role of the “sand” mechanical properties. Jointly investigating the hierarchy -in size and orientation- of dune patterns, the reorganization of dunes around obstacles and the role of sediment cohesion may be of a great help in order to accurately address this problem. This is particularly relevant in the case of Titan and Mars where large obstacle imbedded in dune fields or dune fields in craters are observed and where climatic conditions make the sediment (possibly) cohesive.
The experimental setup at MSC laboratory : The physics of sand dunes relies on the coupling of sediment transport by a fluid and the modification of the flow by the bedform (Bagnold, 1941). As soon as there is sediment transport, this coupling makes any flat sand bed to destabilize and transform into dunes. The space lag between the sediment flux the wind can handle (saturated flux) and the effective sand flux controls the size of the most unstable wave wavelength, which sets the minimum size for a dune (Hersen et al., 2002). It scales like the turbulent drag length, which is proportional to the size of grains times the density ratio between the grains and the fluid. For sand grains in air, this minimum size is a few meter long, which makes laboratory studies of aeolian dunes challenging. However, it reduces by a factor of 1000 when considering water as the surrounding fluid. Dunes form at much smaller scale in water (typically one centimeter). Moreover, the characteristic time scale of pattern evolution being size dependent, they are also drastically reduced for underwater dunes. Sub-aqueous dunes are then perfect candidates to investigate dune morphogenesis but also long term dynamics in the laboratory. The experimental setup developed in the “Matière et Système Complexes” laboratory benefits of this physical minimum size scaling. The experimental principle consists of moving a plate covered with sand in a water tank. The plate is moved quickly in one direction to transport grains, and then brought back gently to its initial position in order to prevent any sand transport during this part of the motion. A rotation system of the plate makes possible to change the orientation of the sand bed towards the “wind” direction. It has been successfully used to reproduce and study barchans (Hersen, 2004), as well as transverse and longitudinal dunes (Reffet et al., 2010).
Effect of sediment cohesion on dune patterns
lead: S. Courrech du Pont (MSC)
coll.: A. Garcia (AIM,MSC), S. Douady (MSC)
Titan equatorial dunes are described as longitudinal dunes, which can be compared to the longitudinal dunes in the Namib desert on Earth. While this comparison suggests a bimodal wind regime, the current wind models propose a quasi mono-directional wind regime (Tokano, 2008), which is a priori incompatible with the observed dunes. In order to explain this discrepancy, it has been proposed that longitudinal dunes may arise from a monodirectional wind if the sediment is cohesive (Rubin and Hesp, 2009). Indeed, sediments on Titan, mainly composed of heavy hydrocarbons, may be very cohesive. Moreover Mars dunes are periodically stabilized by the frost. Parabolic dunes are also observed on Mars, with analogous morphology than the terrestrial “barchans” stabilized by the vegetation.
The objective of this task is then to test and quantify with our experimental setup the role of cohesion on dune morpholodynamics in order to better understand the patterns of Titan and Mars’ dunes, and “cohesive” dunes on Earth, and further constrain the wind regimes that have shaped them, in close collaboration with all the tasks of WP1. In order to study the role of cohesion on dune morphology, we intend to mix the sand with argil or to use very fine grains, on which cohesive Van der Walls forces are not negligible.
Impact of topography on dune patterns
lead: S. Courrech du Pont (MSC)
coll.: A. Garcia (AIM,MSC), S. Douady (MSC)
Linear dunes (transverse or longitudinal), which are formed by winds that blow successively in two directions, are the most represented aeolian landforms, not only on Earth but also on Mars and Titan (e.g. Malin et al. 1998; Lorenz et al., 2006). The transition between transverse and longitudinal dunes occurs for an angle between winds of about 90°, such that dunes maximize the sediment flux perpendicularly to their orientation (Rubin and Hunter, 1987; Rubin and Hesp, 2009). If the dune average orientation can thus give first order indications on the averaged direction of the air flow, it is not enough to conclude on the detail of the wind regime (number of winds, angle between winds, variability…). A particularly interesting way to quantify the wind detailed properties is to look at the modification of the dune orientation close to topographic obstacles.
Hills and small mountains are observed in longitudinal dune fields on Earth but also on Titan (see Figure 1). The topography (slope and orientation), of importance for this task, are not precisely known for obstacles observed on Titan (Lunine et al. 2008) and is part of study made by Partner 1 (Task 1.T). This study is also of prime interest for dunes on Mars, where dunes are observed in craters, where the sediment accumulates (topography of Martian dune fields will be extracted by Partner 1, Task 1.M.1). The impact of relief on dunes orientation will be experimentally studied by putting obstacles on the moving plate of our laboratory setup. Simple geometric shape will be first investigated but, thanks to the topography analysis of Tasks 1.M.1 and 1.T, it will be possible to incorporate in the experiments, at a smaller scale, the shape of the mountains and crater specific to Mars and Titan. An effort in a correct downscaling would then have to be made. Indeed, the characteristic length scale on which the flow is screened or modified by the obstacle depend on size and slope of the relief (which control for example the detachment of the boundary layer). Two main initial conditions will be tested: (1) an obstacle within a flat sand bed, and (2) an obstacle in an initially free sand field.
Specific experiments will be undergone (within the frame of both Tasks 2.1 and 2.2) in order to test the wind scenarii for Mars and Titan calculated from climate models (Task 3.1). This work at the “Matière et Systèmes Complexes” laboratory will be done in close collaboration with partners doing numerical simulations (Partner 3 - see WP3), which are very complementary to experiments, and will naturally benefit of the planetary dune database that this project aims to develop (see WP1). We scope to actively collaborate to its development with the experiments, in order to help the WP1 to define and extract the relevant physical parameters from the dunes observations.
Maj : 16/07/2014 (8)