Wind tunnel studies of Martian aeolian processes

In order to determine the nature of Martian aeolian processes, an investigation is in progress which involves wind tunnel simulations, geologic field studies, theoretical model studies, and analyses of Mariner 9 imagery; this report presents the preliminary results. Threshold speed experiments were conducted for particles ranging in relative density from 1.3 to 11.35 and diameter from 10.2 to 1290 μm to verify and better define Bagnold’s (1941) expression for grain movement, particularly for low particle Reynolds numbers and to study the effects of aerodynamic lift and surface roughness. Wind tunnel simulations were conducted to determine the flow field over raised rim craters and associated zones of deposition and erosion. A horseshoe vortex forms around the crater, resulting in two axial velocity maxima in the lee of the crater which cause a zone of preferential erosion in the wake of the crater. Reverse flow direction occurs on the floor of the crater. The result is a distinct pattern of erosion and deposition which is similar to some Martian craters and which indicates that some dark zones around Martian craters are erosional and some light zones are depositional. Analyses of the erosional and depositional zones associated with a 6 m raised rim crater on an open field and a 1.2 km natural impact crater tentatively confirm the wind tunnel results. Application of the wind tunnel results to Mars indicates that for flat surfaces, free stream winds in excess of 400 km/h are required for grain movement. However, lower velocities would be required in regions of high surface roughness, e. g. cratered terrain, and it is proposed that such regions could be zones of origin for some Martian dust storms. Analysis of the Coriolis effect on surface stress shows that surface streaks would be deflected about 15° from the geostrophic wind direction at mid-latitudes.