Post by Dr. Maarten G. Kleinhans River and delta morphodynamics research group, University of Utrecht, The Netherlands.
Granular materials avalanche when a static angle of repose is exceeded and freeze at a dynamic angle of repose. Such avalanches occur subaerially on steep hillslopes (Image 2), aeolian dunes and subaqueously at the lee side of deltas (Image 1). The angle varies from 25° for smooth spherical particles to 45° for rough angular particles. Noncohesive granular materials are found in many contexts, from kitchen to industry and nature. The angle of repose is an empirical friction parameter that is essential in models of numerous phenomena involving granular material, most of them actually occur at slopes much lower than the angle of repose. The angle of repose is therefore relevant for many geomorphological phenomena.
Until now it has been assumed that the angles of repose are independent of gravitational acceleration and would therefore be the same on Earth, Mars, the Moon and asteroids. Recently the angles were measured for granular materials in rotating drums during parabolic flights where gravity was reduced. The static angle of repose increases about 5° with reduced gravity, whereas the dynamic angle decreases with about 10°. Consequently, the avalanche size increases with reduced gravity, and relatively low slopes of granular material on Mars may have formed by dry flows without a lubricating fluid. On asteroids even lower slopes are expected.
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