Field measurements of horizontal forward motion velocities of terrestrial dust devils: towards a proxy for ambient winds on Mars and Earth

Dust devils – convective vortices made visible by the dust and debris they entrain – are common in arid environments and have been observed on Earth and Mars. Martian dust devils have been identified both in images taken at the surface and in remote sensing observations from orbiting spacecraft. Observations from landing craft and orbiting instruments have allowed the dust devil translational forward motion (ground velocity) to be calculated, but it is unclear how these velocities relate to the local ambient wind conditions, for (i) only model wind speeds are generally available for Mars, and (ii) on Earth only anecdotal evidence exists that compares dust devil ground velocity with ambient wind velocity. If dust devil ground velocity can be reliably correlated to the ambient wind regime, observations of dust devils could provide a proxy for wind speed and direction measurements on Mars. Hence, dust devil ground velocities could be used to probe the circulation of the martian boundary layer and help constrain climate models or assess the safety of future landing sites. We present results from a field study of terrestrial dust devils performed in the southwest USA in which we measured dust devil horizontal velocity as a function of ambient wind velocity. We acquired stereo images of more than a hundred active dust devils and recorded multiple size and position measurements for each dust devil. We used these data to calculate dust devil translational velocity. The dust devils were within a study area bounded by 10 m high meteorology towers such that dust devil speed and direction could be correlated with the local ambient wind speed and direction measurements. Daily (10:00 to 16:00 local time) and two-hour averaged dust devil ground speeds correlate well with ambient wind speeds averaged over the same period. Unsurprisingly, individual measurements of dust devil ground speed match instantaneous measurements of ambient wind speed more poorly; a 20-minute smoothing window applied to the ambient wind speed data improves the correlation. In general, dust devils travel 10-20% faster than ambient wind speed measured at 10 m height, suggesting that their ground speeds are representative of the boundary layer winds a few tens of meters above ground level. Dust devil ground motion direction closely matches the measured ambient wind direction. The link between ambient winds and dust devil ground velocity demonstrated here suggests that a similar one should apply on Mars. Determining the details of the martian relationship between dust devil ground velocity and ambient wind velocity might require new in-situ or modelling studies but, if completed successfully, would provide a quantitative means of measuring wind velocities on Mars that would otherwise be impossible to obtain

STRAIN INDUCED TRANSFORMATIONS AND PLASTICITY IN TRANSAGE Ti-11.6V-2Al-2Sn-6Zr (Tl134) AND Ti-11.5V - 2Al-2Sn-11.3Zr (Tl29) ALLOYS

An investigation is reported on two β quenched titanium Transage alloys, namely (i) T 134 which contains only orthorhombic α" martensite and (ii) T 129 which contains mainly retained β and a small amount of α" after quenching from the β phase. Strain induced shear, transformations of β to α" and the α" to hexagonal α' take place in the temperature range 77-373K and are associated with enhanced ductility

VARIATION IN STRUCTURE AND PROPERTIES IN AN Al-Li-Cu-Mg-Zr ALLOY PRODUCED BY EXTRUSION PROCESSING

The effects of extrusion processing variables on the structure and properties of an Al-Li-Cu-Mg-Zr alloy (8090 type) have been investigated. A combination of light and transmission electron microscopy have been used to characterise the as extruded microstructures and the precipitation reactions which take place on subsequent heat treatment. The corresponding mechanical properties have been determined by hardness, tensile and fracture toughness test methods. As extruded tensile properties are affected by the processing variables whilst within heat treated material the precipitation processes control the mechanical properties of the alloy. The effects of variation in heat treatment involving natural ageing and stretching on the fracture toughness are discussed in relation to the microstructural changes produced. By suitable process and heat treatment control, good combinations of strength, toughness and ductility can be obtained

THERMAL ANALYSIS STUDY OF THE PRECIPITATION REACTIONS IN Al-Li-Cu-Mg-Zr ALLOYS

Differential Scanning Calorimetry has been used to study the phase transformations in Al-Li-Cu-Mg-Zr alloys conforming to the commercial 8090 and 8091 specifications, together with selected binary and ternary alloys of similar solute concentrations. The evidence supports the existence of GP zones in the binary Al-Li system and demonstrates that their formation is affected by the presence of other solutes. Lithium slows down the precipitation of Al-Cu-Mg GP zones and of S phase