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Abstract:
The surfaces of Earth, Moon, Mars and many other celestial bodies of our solar system are composed mainly of particulate materials, such as regolith, sand and dust. One of the most important observables characterizing the physics of these materials is the angle of repose, i.e., the angle of maximum stability of inclined granular surfaces such as heaps, crater walls and dune slip faces. However, this angle is affected by still poorly understood grain-scale processes. Attractive particle-particle interactions become more effective than particle weight the smaller the grains are, but the stability of granular landscapes on planetary surfaces has remained elusive. Therefore, a theoretical model for predicting the angle of repose as a function of particle size and gravity would have broad impact in the planetary, environmental and engineering sciences. Here we present such a model, which we have derived from grain-scale simulations under consideration of contact and van der Waals interactions. Our model reproduces quantitatively experimental data of the angle of repose, from gravel and rock particles to dust-sized grains, as well as simulation predictions with gravity from 0.06 to 100 times that of Earth. Based on our model, we present a method for estimating the angle of repose on a given planetary surface by suitably adapting the particle size in the laboratory, without the need for generating extra-terrestrial gravity. We also show how the angle of repose can be used as a proxy for grain attributes and the solid fraction of particulate materials on the surface of celestial bodies.
