Dust devils on Earth and Mars: Extension of particle threshold laboratory simulations Conference Item How to cite: Neakrase, Lynn D. V.; Greeley, Ronald; Haan, Frederick L.; Sarkar, Partha; Iversen, James D.; Balme, Matthew R. and Eddlemon, Eric E. (2006). Dust devils on Earth and Mars: Extension of particle threshold laboratory simulations. In: 37th Lunar and Planetary Science Conference, 13-17 March 2006, Houston, Texas, USA.

Neakrase, L. D. V; Greeley, R.; Iversen, J. D.; Balme, M. R.; Foley, D. J. and Eddlemon, E. E. (2005).Dust devils on Mars: Effects of surface roughness on particle threshold. In: 36th Lunar and Planetary ScienceConference, 14-18 March 2006, Houston, Texas, USA.For guidance on citations see FAQs.c [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2005/Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copy-right owners. For more information on Open Research Online’s data policy on reuse of materials please consultthe policies page.

We describe for the first time the generation and measurement of capillary waves in a water surface in a wind tunnel running with air at pressures of 15–1000 mbar. These experiments suggest a stronger dependence of wave generation on atmospheric density than the simple proportionality that might be expected from energy transfer arguments. Additionally, airflow over a nonaqueous fluid (kerosene) was found to produce waves of higher amplitude than for water under the same conditions. These preliminary results may indicate different efficiencies of wave generation on other planets, for which empirical terrestrial relations therefore do not apply, and thus may have a bearing on the lack of strong shoreline features on Mars and the possibility of specular glints from hydrocarbon lakes on Titan.

The interaction between saltating sand grains and rock surfaces is assessed to gauge relative abrasion potential as a function of rock shape, wind speed, grain size, and planetary environment. Many kinetic energy height profiles for impacts exhibit a distinctive increase, or kink, a few centimeters above the surface, consistent with previous field, wind tunnel, and theoretical investigations. The height of the kink observed in natural and wind tunnel settings is greater than predictions by a factor of 2 or more, probably because of enhanced bouncing off hard ground surfaces. Rebounded grains increase the effective flux and relative kinetic energy for intermediate slope angles. Whether abrasion occurs, as opposed to simple grain impact with little or no mass lost from the rock, depends on whether the grain kinetic energy (EG) exceeds a critical value (EC), as well as the flux of grains with energies above EC. The magnitude of abrasion and the shape change of the rock over time depends on this flux and the value of EG > EC. Considering the potential range of particle sizes and wind speeds, the predicted kinetic energies of saltating sand hitting rocks overlap on Earth and Mars. However, when limited to the most likely grain sizes and threshold conditions, our results agree with previous work and show that kinetic energies are about an order of magnitude greater on Mars.

Dust devils have been proposed as effective mechanisms for lofting large quantities of dust into the martian atmosphere. Previous work showed that vortices lift dust more easily than simple boundary layer winds. The aim of this study is to determine experimentally the effects of non-erodable roughness elements on vortex particle threshold through laboratory simulations of natural surfaces. Additional information is included in the original extended abstract.

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.

The generation of waves by winds across Earth's water oceans is a topic of enduring fascination. However, the physics of the problem are rather forbidding and thus the relationships between real-world windspeed and sea state tend to be empirical. Such empirical relations are of limited utility in environments where the physical parameters are different, such as the surfaces of other planets. These environments have only recently come to oceanographers attention, with the discovery of ancient shorelines and lakes on Mars, and the prospects for and recent evidence of lakes and seas of liquid hydrocarbons on Saturn's moon Titan. We are aware of only one other published experimental wind-water tunnel study where the fluid parameters have been varied. This used artificially-generated mm-scale waves at 3.8-7.6 Hz in water, glycerol solutions (higher viscosity) and surfactant solutions (lower surface tension). Lower viscosity solutions had higher wave growth rates: surprisingly, higher surface tension led to more rapid wave growth. The liquid density was not appreciably varied, and 1 bar air was used throughout.We used the MARSWIT (Mars Wind Tunnel) operated by ASU at NASA Ames. A fiberglass tray (5 cm x 120 cm x 75 cm) was installed in the tunnel, with an approx. 1:5 ramp to prevent strong flow separation. The tray was filled to a depth of about 4 cm. Sensors were clamped to the tray itself or held by a steel and aluminium frame just above the water level. A towel was draped on the water surface at the downwind end of the tray to act as a damper to suppress wave reflection. Position-sensitive infrared (IR) reflection sensors (Sharp GP12D02) and ultrasonic rangers (Devantech DF-04) used in mobile robotics were used as water level sensors. The tray was observed with a video camera, whose output could be viewed on a monitor and recorded on VHS tape.

this estimate used the solubility for air, not pure oxygen; 2) The gases delivered to the ocean would be at 10MPa and 272 K, this is not a high enough pressure for oxygen hydrate stability, however if the pressure were to reach levels of 11-12MPa then such hydrates would be stable. Once delivered to the ocean, the fate of biologically useful gas hydrates will depend on the density, and hence salinity, of the europan ocean water. If the density of the hydrates exceeds that of the surrounding water and saturation has been reached, then the hydrates will precipitate to the seafloor and form a hydrate sediment. If the hydrates are buoyant in the europan ocean then an accretion layer of hydrates may form at the base of the ice shell. Indeed, such a layer may have implications for ice shell dynamics and evolution.