James D. Iversen

Saltation threshold on Mars: The effect of interparticle force, surface roughness, and low atmospheric density

Abstract The effect of interparticle forces, as well as changes in surface roughness, particle diameter and density, and atmospheric density and viscosity are considered in new estimates of saltation thershold on Mars. These estimates result in somewhat lower minimum values of predicted threshold speed than by use of the “universal” A - B curve, with the minimum threshold speed occuring at smaller values of particle diameter. It is shown that the new predictions are much closer to the limited low atmospheric density data available than if the older method of estimation is used.

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.

Wind Tunnel Simulations of Light and Dark Streaks on Mars

Wind tunnel experiments have revealed a characteristic flow field pattern over raised-rim craters which causes distinctive zones of aeolian erosion and deposition. Comparisons of the results with Mariner 9 images of Mars show that some crater-associated dark zones result from wind erosion and that some crater-associated light streaks are depositional.

Estimates of the wind speeds required for particle motion on Mars

Abstract We have obtained estimates of the threshold wind speed Vgt near the top of the atmospheric boundary layer on Mars and of the rotation angle α between this wind velocity and the direction of the surface stress. this calculation has been accomplished by combining wind tunnel determinations of the friction velocity with semi-empirical theories of the Earths atmospheric boundary layer. Calculations have been performed for a variety of values of the surface pressure, ground temperature, roughness height, boundary layer height, atmospheric composition atmospheric stability, particle density, particle diameter, and strength of the cohesive force between the particles. The curve of threshold wind speed as a function of particle diameter monotonically decreases with decreasing particle diameter for a cohesionless soil but has the classical U shape for a soil with cohesion. Observational data indicate that the latter condition holds on Mars. Under “favorable” conditions minimum threshold wind speeds between about 50 and 100m/sec are required to cause particle motion. These minimum values lie close to the highest wind speeds predicted by general circulation models. Hence, particle motion should be an infrequent occurence and should be strongly correlated with nearness to small topographic features. The latter prediction is in accord with the correlation found between albedo markings and topographic obstacles such as craters. For equal wind speeds at the midpoint of the boundary layer, particle movement occurs more readily in general at night than during the day, more readily in the winter polar areas than the equatorial areas noon, and more readily for ice particles than for silicate particles. The boundary between saltating and suspendable particles is located at a particle diameter of about 100 μm. This value is close to the diameter at which the Vgt curve has its minimum. Hence, the wind can set directly into motion both saltating and larger-sized suspendable particles, but dust-storm-sized particles usually require impact by a saltating particle for motion to be initiated. Albedo changes occur most often in regions containing a mixture of dust-stoorm-sized particles and saltating particles. The threshold wind speed for surfaces containing large, nonerodible roughness elements can either be larger or smaller than the value for surfaces with only erodible material. The former condition for Vgt holds when the roughness height z0 is less than about 1 cm and may be illustrated by craters that have experienced less erosion than their environs. The latter condition for Vgt may be partly responsible for albedo changes detected on the elevated shield volcano, Pavonis Mons. Values of the angle α generally lie between 10 and 30°. These figures place a modest limitation on the utility of surface albedo streaks as wind direction indicators.

Applications of spaceborne radar laboratory data to the study of aeolian processes

Aerodynamic roughness (z0) is an important parameter in studies of sand and dust transport, as well as atmospheric circulation models. Aerodynamic roughness is a function of the size and spacing of surface roughness elements and is typically determined at point locations in the field from wind velocity profiles. Because field measurements require complex logistics, z0 values have been obtained for very few localities. If radar can be used to map z0, estimates can be obtained for large areas. In addition, because aerodynamic roughness can change in response to surface processes (e.g., flooding of alluvial surfaces), radar remote sensing could obtain new measurements on short timescales. Both z0 and the radar backscatter coefficient σ0 are dependent on topographic roughness at the submeter scale, and correlation between these two parameters was developed based on radar data obtained from aircraft (AIRSAR). The Spaceborne Radar Laboratory (SRL) afforded the opportunity to test the correlation for data obtained from orbit. SRL data for sites in Death Valley, California; Lunar Lake, Nevada; and Gobabeb, Namibia, were correlated with wind data and compared with previous radar z0 relations. Correlations between σ0 and z0 for L band (λ=24 cm) HV (H, vertically and V, vertically polarized modes) L band HH, and C band (λ=5.6 cm) HV compare favorably with previous studies. Based on these results, maps of z0 values were derived from SRL data for each site, demonstrating the potential to map z0 for large vegetation-free areas from orbit using radar systems.

Saltation and wind-flow interaction in a variable slope wind tunnel

Abstract In a 15 m long horizontal wind tunnel, we studied the internal boundary layer over fixed arrays with roughness (z0) similar to the roughness of saltation of a sand bed. Utilizing turbulence spires the logarithmic overlap-layer is at least 75 mm thick some 5 m downwind of the entry. This permits a precise determination of u∗ from wind profile data. Over a sand bed in a variable slope wind tunnel with a 6 m long working section we studied how slope, grain size, and friction speed influence bed roughness. Owen (1964) proposed that z 0 = C · u ∗ 2 /2 g where C is a constant, but we found that C depends in a complex manner on grain size and friction speed. C increases from very low values near the threshold to an apparently limiting value at large friction speeds. For an almost uniform 125 μm sand, the limiting value of C is 0.06. The limiting value decreases, however, with particle size: for a uniform 544 μm sand, C only approaches 0.03 for friction speeds as high as 1 m/s. With the measured values of friction speed corrected for the effects of slope, our results appear to show that the limiting values of C do not depend on the slope angle, even though the slope affects the individual shapes of particle trajectory. C is perhaps independent of slope angle because the roughness height, and, therefore, C, is more directly related to the amount of momentum extracted from the air by the particles (i.e. the shear stress) rather than to the manner of extraction (particle trajectory shapes).

Windblown sand on Venus: Preliminary results of laboratory simulations

Abstract Small particles and winds of sufficient strength to move them have been detected from Venera and Pioneer-Venus data and suggest the existence of aeolian processes on Venus. The Venus wind tunnel (VWT) was fabricated in order to investigate the behavior of windblown particles in a simulated Venusian environment. Preliminary results show that sand-size material is readily entrained at the wind speeds detected on Venus and that saltating grains achieve velocities closely matching those of the wind. Measurements of saltation threshold and particle flux for various particle sizes have been compared with theoretical models which were developed by extrapolation of findings from Martian and terrestial simulations. Results are in general agreement with theory, although certain discrepancies are apparent which may be attributed to experimental and/or theoretical-modeling procedures. Present findings enable a better understanding of Venusian surface processes and suggest that aeolian processes are important in the geological evolution of Venus.

Eolian erosion of the Martian surface, part 1: Erosion rate similitude

Abstract A similitude parameter is derived which is based on theoretical considerations of erosion due to sand in saltation. This parameter has been used to correlate wind tunnel experiments of particle flow over model craters. The characteristics of the flow field in the vicinity and downstream of a crater are discussed and it is shown that erosion is initiated in areas lying under a pair of trailing vortices. The erosion rate parameter is used to calculate erosion rates on Mars, reported in Part 2, to be published later.

Dust flux within dust devils: Preliminary laboratory simulations

Laboratory simulations using the Arizona State University Vortex Generator (ASUVG) were run to simulate dust flux in dust devils. These tests used particles 2 μm in diameter and 2600 kg m−3 in density, and the results were compared with data from natural dust devils on Earth and Mars. Typically, the cores of dust devils (regardless of planetary environment) have a pressure drop of ∼0.2–1.5 percent of ambient atmospheric pressure. Core pressure drops in our experiments ranged from ∼0.01 to 5.00 percent of ambient pressure (10 mbar Mars cases and 1000 mbar for Earth cases). Flux experiments were run at vortex tangential wind velocities of 1 to 42 m s−1; typically ∼35–50 percent above threshold values for the particles used. Dust flux was determined by time averaged measurements of mass loss for a given vortex size. Dust fluxes of ∼10−3 kg m−2 s−1 were obtained, similar to estimates for flux for dust devils on Earth and Mars, regardless of core size. Vortex strength appears to be closely related to the strength of the pressure drop in the core (ΔP) and is less determined by size of the vortex. This is critical in scaling the laboratory results to natural dust devils.

Assessment of aerodynamic roughness via airborne radar observations

The objective of this research is to assess the relationship among measurements of roughness parameters derived from radar backscatter, the wind, and topography on various natural surfaces and to understand the underlying physical causes for the relationship. This relationship will form the basis for developing a predictive equation to derive aerodynamic roughness (z0) from radar backscatter characteristics. Preliminary studies support the existence of such a relationship at the L-band (24 cm wavelength) direct polarization (HH) radar band frequencies. To increase the confidence in the preliminary correlation and to extend the application of the technique to future studies involving regional aeolian dynamics, the preliminary study has been expanded by: 1) defining the empirical relationship between radar backscatter and aerodynamic roughness of bare rocks and soils, 2) investigating the sensitivity of the relationship to microwave parameters using calibrated multiple wavelength, polarization, and incidence angle aircraft radar data, and 3) applying the results to models to gain an understanding of the physical properties which produce the relationship. The approach combines the measurement, analysis, and interpretation of radar data with field investigations of aeolian processes and topographic roughness.