Wind Work at Mars

->Wind is believed to be Mars geological and morphological changes most important present factor. Wind-blown and transported particles are responsible for erosion or sand dunes and ripples. Opportunity clearly saw at its landing crater rocky outcrop how wind worked about rocky layers as Spirit saw a lot of sand ripples and drifts along its way to Bonneville Crater or at crater's rim and bottom

->As far as martian particles' motion is concerned, three basic types undergo three different motions: dust, sand, and coarse sand. Dust has flour's consistency of flour, sand sugar's and coarse sand ball bearings'. Sand grains are main contributor to materials wind-generated motion: after having been taken up by wind sand grains are falling back downwind. Hitting back surface, they are sending dust up or they give coarse grains a little shove. Dust up is transported by wind. Coarse grains are creeping along surface. click picture for a view of particles motion processs (picture courtesy NASA/JPL/ASU)

->Dunes, sand ripples, and drifts are part of martian landscapes. Dunes have a somehow uniform distribution of particles and are shaped -and moved- by strong winds as ripples and drifts are due to gentle winds. Ripples display a disparity in particles' distribution: coarse grains are found on top of waves as fine-grained soil in troughs. Most of the time ripples and drifts have a surface-interior structure: surface is a crust made of coarse grains (about 0.4-0.8 inches in diameter (1-2 mm)) at some degree coated with Martian dust; interior is made of very fine sand grains (0.002 inches (50-60 micrometers)) -at Earth dunes grains are about 0.008 inches (200 micrometers)) and such fine grains like ones found at Mars would not accumulate but remain mere dust. To find such a material accumulating at Mars might mainly be due to Mars gravity being one-third Earth's only. Ripples and drifts coated surface is always lighter in color than inner material. This might be too a soil characteristic at most Martian regions: good examples were given by Spirit retracting airbags marks or wheels' tracks. By comparison soil at Meridiani Planum may display too such a texture but is too covered with hematite spherules. pictures top to bottom: drift near Spirit's landing platform (NASA/JPL/Cornell), drift surface and inner structure (NASA/JPL/Cornell/USGS), Spirit airbag retractation marks (NASA/JPL)

Another important wind-related Martian feature are dust devils. Dust devils, like their terrestrial counterparts are warm air columns, with circling winds, moving at ground surface. Martian dust devils are considerably larger than Earth's however. They may reach 500 m wide and several thousand meter high. Earth's dust devils are about 10-100 m wide (with winds circling warm air column at about 20-60 mph (32-96 kph)). Dust devils are an important erosion factor at Mars. They are transporting large quantities of dust. Dark track patterns are commonly found in many Mars regions and are changing from season to season. Interestingly a recent NASA study showed that Earth dust devils are producing large electric fields in excess of 4,000 volts/meter and magnetic fields. It is still unsure whether Martian dust devils have these same characteristics. Would they, this might be a hindrance to future Mars expeditions. Discharging and arcing would be produced in Mars low-pressure atmosphere. Electrification would increase dust adhesion at space suits and equipment as radio-communications would be blurred. Electric field at Earth's dust devils is due to small dust particles being negatively charged and located atop the air column as large being positively and located at column's bottom. Magnetic field is yielded by dust particles motion inside dust devil. Twin rovers are not equipped to measure possible electric charge of Martian dust devils and this will be up to future missions to assess it. The picture is showing the optical depth at Mars function of dust at both rovers' sites. the peak at Meridiani Planum is due to a regional Martian storm by year 2003's end (picture JPL/NASA)