Titan's equatorial regions are covered by vast fields of longitudinal dunes. Several observations point to solid hydrocarbons as the most likely candidate for the dune particle composition. Together with the polar lakes and seas, dune regions are thus the main reservoir of organic deposits on Titan. A refined estimate of the dune material volume and distribution is essential to constrain Titan total organic inventory and therefore to understand the carbon cycle on Titan. Using Cassini SAR observations we find that Titan's dune fields are generally hosted by basins and may cover ~12.5% of Titan's surface, which corresponds to an area of ~10 million km2 (roughly the area of the United States). Polarized radiometry observations indicate that dune particles are mainly composed of organic solids, consistent with spectroscopic measurements. This would imply that the dune particles were dominantly created by atmospheric photochemical production rather than fluvial erosion. However, it is not clear whether the aggregation occurred primarily during aerosol sedimentation from the stratosphere to the surface, or by subsequent sticking and growth during fluvial or eolian transport. Assuming that, everywhere, the dunes are 100m-high and that the interdunes spaces are clear of dune material and of equal area than the dunes, the volume of sand-sized sediments should approach ~250 000 km3, which is an order of magnitude higher than the current estimate of the volume of liquid hydrocarbons on Titan (Lorenz et al., 2008). However, the combined radar and radiometry measurements indicate regional variations among the dunes. In this paper we will show that differences in the microwave backscatter and emission of the dune regions can be well explained by various degrees of exposure of the icy bedrock of Titan in the interdunal corridors. In some regions, a thick sheet of sand-sized material covers the interdunes. In other places, the original substrate is peeking through. These variations need to be taken into account in order to estimate the volume of sand-sized sediments. Investigating them also bring new insights on the distribution of the available sand-sized sediments supply, which vary across Titan probably owing to differences in the ground humidity and wind patterns.< Réduire