A comparative planetological study of particle speed and concentration during aeolian saltation.

摘要

Aeolian gradation is important on any planetary surface possessing loose particulate material and winds of sufficient strength to initiate motion. The mode of aeolian transport with the greatest potential for geologic alteration is saltation, where sand-sized particles bounce along the surface. The goals of this dissertation are to: determine the relative contributions of particle concentration and particle speed to saltation flux for a wide range of environmental conditions, extrapolate the saltation flux predictions of previous investigators to non-terrestrial conditions, determine the range of environmental conditions over which those predictions remain valid, and assess the nature and consequences of aeolian activity on other planets.; Flux predictions developed for terrestrial saltation were found to be valid for a narrower range of wind speeds on Mars and Venus than in the terrestrial case. On Mars, low atmospheric density impairs the full development of saltation; on Venus, particle trajectories are so small that mid-air collisions readily impede, or choke, saltation. Actual saltation flux can be several orders of magnitude less than predicted, due to these and other environmental factors. The height of the zone of aeolian abrasion of surface materials is inversely proportional to the atmospheric density and the intensity of abrasion. The spectrum of particle speeds and the trajectory heights in the saltation cloud are controlled by surface conditions; particle size, shape, and density and wind speed. The particle speed distribution is more complex than previously supposed, particularly for particles that are relatively large and dense. Particle speed and flux distribution above the surface profoundly affects both the style and rate of small-scale aeolian erosion and bedform development. The onset of choking conditions in the saltation cloud can be considered to be a change in flow regimes, analogous to the change in flow regimes in water from that producing rippled bedforms to that producing plane beds.