Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p53a1510d&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P53A-1510
Physics
[1825] Hydrology / Geomorphology: Fluvial, [5415] Planetary Sciences: Solid Surface Planets / Erosion And Weathering, [5419] Planetary Sciences: Solid Surface Planets / Hydrology And Fluvial Processes, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties
Scientific paper
Granular materials avalanche when a static angle of repose is exceeded and freeze at a dynamic angle of repose. Such avalanches occur subaerially on hillslopes and wind dunes and subaqueously at the lee side of deltas and dunes. Their properties are important for the inference of present morphodynamics and past climate from surface morphology on rocky planetary bodies. The objective of this work is to determine whether the angles of repose depend on gravity; that is, are different on planet Mars or the Moon compared to Earth. Until now it has been assumed that the angles of repose are independent of gravitational acceleration because both mobilising force and friction theoretically depend linearly on gravity. During 33 parabolic flights in a well-controlled research aircraft we imaged avalanching granular materials in rotating drums and Hele-Shaw cells at effective gravitational accelerations of 0.10 g and 0.38 g. Control measurements were done at 1 g in flight and on the ground. The 9 drums had a diameter of 0.2 m and rotated slow enough for discontinuous avalanching. The granular materials had particle sizes of about 0.2, 0.6 or 2 mm, were rounded or angular and had air or water as interstitial fluid. Angles were measured at 25 Hz by image analyses on the digital videos and acceleration of the aircraft was measured at 50 Hz in three directions. Hele-shaw cells were used to study auto-organization processes in discontinuous avalanching bi-dispersed granular materials and their respective individual grain species. In both setups the angular materials had time-averaged angles of about 40 deg and rounded materials about 25 deg for all g, except the finest glass beads in air, which is explained by static electricity. For all materials, the static angle of repose increases about 5 deg with decreasing g (from 1 to 0.1 g), whereas the dynamic angle decreases with about 10 deg. Consequently, the avalanche size increases with decreasing g. The interstitial fluid hardly affected the results: subaqueous avalanches were smaller than subaerial but the lubrication did not decrease the angles of repose. The particle size hardly made a difference, ruling out effects of particle momentum, fluid drag and groundwater dynamics. Consequently, granular avalanches and their strata are larger at reduced g. Stratification patterns of the bidisperse materials were reduced and in some cases disappeared completely at lower gravity. The lower dynamic angle of repose is expected to be preserved more often at planetary surfaces following threshold triggering events such as earthquakes and impacts. Furthermore, sediment processing methods on future European and American Mars landers that depend on the mobilization angle should account for the difference between angles of repose in reduced-gravity.
de Vet Simon J.
in't Veld A. C.
Kleinhans Maarten G.
Markies H.
Postema F. N.
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