A pile of sand, gravel, or other granular material takes on a familiar conical shape, with the slope of the pile's walls coming to rest at the static angle of repose. If the material exceeds this angle, it will trigger an avalanche, tumbling down until it comes to rest at the dynamic angle of repose. Static angles of repose for coarse, angular materials tend to be around 40° from the horizontal, while smooth grains are stable at up to 20°. As largely a matter of geometry, grain properties, and internal friction, scientists have assumed these two angles of repose are fixed for a given substance. Observations of the angles of gully walls on Mars, found to be too shallow for the materials involved, have been used to argue that surface water must have played a part, either lubricating landslides or depositing the material directly. However, research by Kleinhans et al., using the parabolic flight of an airplane to test the effect of gravity on angles of repose, demonstrates that water need not have been present.