Siliclastic aeolian dunes on Earth
lead : C. Narteau (IPGP)
coll.: Z. Dong (CAREERI)
Dune growth and pattern formation: First, the idea is to quantify (and model, see WP3) the instability that generates primary and secondary dune patterns by using an ongoing outdoor field scale experiment conducted by the Desert Laboratory of the CAREERI in the Tengger Desert (Shapotou). In this leading experiment, a dune field was completely flattened in 2004, and over the last 7 years, high frequency atmospheric measurements (one measure/minute) have been coupled with bi-annual topographic mapping. This unique dataset can be used (and compared to models, see WP3) to directly evaluate the characteristic length and time scale of the dune instability. Second, as this experiment is continuing, we will be able to describe the preliminary phases of coarsening which are extremely difficult to identify when initial conditions are unknown. It will be also possible to follow the formation mechanism of network dunes.
The role of cohesion: Cohesion is a critical ingredient for morphogenesis in natural landscape (river, coastline, dunes). In addition to the development of new numerical methods (see Tasks 3.2 and 3.3), we propose to perform field works and theoretical studies in order to document and quantify the mechanisms that control the cohesion of the sediment that form Mars and Titan’s dunes. In particular, we will investigate the possible role of inter-particular surface tension, as well as frost-melting cycles or cementation with evaporitic material (involving phase transition of a fluid in a porous granular compartment). Thus, we will investigate the origin and the role of cohesion on Mars and Titan (in collaboration with Tasks 1.M.1 and 1.T) with the help of the characterization of the Qaidam Basin dune fields, North of the Qinghai-Tibetan Plateau in China, where cohesive sand is observed (see Figure 1). Chemico-physical analysis of the Chinese sedimentary layer should bring invaluable information on the mechanical behavior of cohesive grains and will help to constrain the morphodynamics of similar features on Mars and Titan. All this work is based on the international collaboration between the IPGP and the CAREERI (Chinese Academy of Sciences). Note that drought and desertification are critical issues in China where millions of peoples are concerned.
Ice dunes on Earth
lead: O. Bourgeois (LPGN)
coll.: S. Le Mouélic (LPGN,) S. Rodriguez (AIM)
Ice dunes with wavelength of 2-5 km and amplitudes of 2-4 m have been discovered very recently in Antarctica and their internal structure has been observed by means of radar surveys (Frezzotti et al. 2002). These dunes are located in areas of strong katabatic winds. The first specificity of these ice dunes is that they grow and migrate upwind, which is at odd with the classical downwind sense of migration of aeolian dunes. The second specificity is that they are composed of cohesive material (ice or snow grains). The third specificity is that ice deposition and erosion in these ice dune systems are not solely caused by transportation of solid particles as they do in siliclastic and basaltic dunes; volatilization by sublimation and deposition by solid condensation probably also play a significant role in the dynamics of ice dunes. We aim at assessing the role of these three specificities on the development of dunes, from the study of an ice dune field located in the French Alps (Mont-Blanc Massif). To do so, we will use this dune field as a natural laboratory, in order to: (a) perform a temporal survey of the spatial migration of dunes over several tens of years, on the basis of archive satellite and aerial images, (b) monitor annual changes in the shape of the topographic surface of this dune field during 4 years, by means of repeated differential GPS surveys (2 surveys per year). This work will lead to the production of a series of maps showing the location of the dunes through time, from which will be computed maps of migration direction and velocity. These observational data will be used to constrain models of the formation of dunes composed of cohesive and volatile materials (Task 3.2 and 3.3) and help the understanding of Titan’s dunes morphology, which are possibly composed of highly cohesive “sand” (Task 1.T for the observations and 2.1 for the experiments). We will also measure infrared spectra of the surface of the Mont-Blanc Massif natural dune field with a field spectrometer, in order to characterize the spectral signature of ice in deposition zones (upwind of dunes) and in erosion zones (downwind of dunes). We will thus produce a library of infrared spectra for various kinds of natural ices (fresh snow, bounded snow grains, glazed ice, dry ice, wet ice, porous ice, compact ice), which will be used to constrain the hyperspectral analysis of Martian ice dunes (Task 1.M.2).