Insights on rock abrasion and ventifact formation from laboratory and field analog studies with applications to Mars

摘要

Wind tunnel studies are integrated with field observations to better understand the processes and rates of rock abrasion on Earth and Mars and how these factors affect ventifact morphology. The wind tunnel work consists of controlled experiments at terrestrial and Martian pressures in which known fluxes of sand are blown onto abradable targets of various geometric shapes. Mass loss and dimensional changes are measured and shape evolution observed as a function of total sand flux, wind speed, target shape, and target composition. To provide ground truth to these experiments, the same types of targets were placed in a field plot at a Mojave Desert ventifact locality for 6 months and measurements and observations like those in the wind tunnel were made. Weather data recorded by a co-located station provided wind speed and direction during this time. These data and results from the abraded field targets were compared to flute directions of local ventifacts. Initial results from this work are: (1) initial rock shape controls the rate of abrasion, with steeper faces abrading faster than shallower ones, (2) targets also abrade via slope retreat, with intermediate angled faces becoming shallower (flatter) at a greater rate than initially flat or steep faces, (3) the direction of maximum velocity winds exerts a greater control on ventifact flute orientations than the direction of average velocity winds, (4) irregular targets with pits or grooves abrade at greater rates than targets with smooth surfaces, with indentations generally enlarging and faces becoming rougher with time, and (5) there are many similarities between the experimental and terrestrial ventifacts, as well as rocks interpreted as ventifacts on Mars. The pitted and faceted appearance of many Martian rocks is easily attributable to aeolian abrasion. Many Martian rocks appear pitted or vesicular, characteristics which our laboratory experiments show enhance abrasion. Although measured Martian wind speeds are generally below those necessary to induce saltation, occasional gusts above threshold may be sufficient for some rock abrasion. Ventifact formation is potentially a common geomorphic process on Mars provided there are sufficient supplies of sand and high velocity winds needed for saltation.