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| The Little Twisters' Impact: Dust Devils on Mars | |
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Mercury, March/April 2000 Table of Contents
They're fun to watch play across a field, but little did we know the role dust devils play in dust trasport on our world. Now we can see them regularly on arid Mars, too. James R. Carr On Earth, wind is an agent of erosion, the importance of which is often overlooked in favor of fluvial processes (running water). Waves on oceans that break on shores giving rise to longshore transport are the direct result of the frictional drag of wind on water. In arid environments in which vegetation is sparse, water often originating from catastrophic thunder showers is the most potent agent of erosion. But, due to the aridity, wind is an important contributor to the erosion and sedimentation processes, especially contributing to airborne particulate. The global scale of wind transport is only recently recognized, as dust originating in the Sahara Desert of northern Africa is now determined to contribute to airborne particulate in Miami, Florida, and Brazil. A more localized example of dust entrainment and transport on Earth is that of the "dust devil." These relatively small, tornado-like phenomena are the result of daytime warming of surfaces when humidity is low. In arid to semiarid environments, these miniature cyclones reveal themselves when they entrain dust and debris, pumping material several tens, or occasionally hundreds, of meters skyward. If originating over vegetated surfaces, dust devils may not be visible because dust is not entrained, but they can still be felt, often sustaining winds in excess of 100 km/hr. Because of their high winds and transport of dust and debris, dust devils are often considered more of an irritant than an important geologic process. This notion is starting to change. In arid to semiarid regions of the United States, in particular the Great Basin, the Desert Southwest, and western Texas, dust devils are now considered to be a substantive reason that some communities are found to be noncompliant with U.S. Environmental Protection Agency mandated limits on PM10 and PM2.5 particulate. A single dust devil may be responsible for entraining and transporting several hundred kilograms of sand, dust, and debris. A particular local region may have several tens, or hundreds, of dust devils develop in a particular day. One hour's observation southeast of Ely, Nevada, in summer 1998, recorded ten dust devils. This is an example to suggest that up to 100 dust devils a day in any given environment conducive to dust devil formation is a reasonable estimate, given that dust devils in late spring and summer begin to develop between 9:00 a.m. and 10:00 a.m. and may continue to form until 6:00 p.m. This number of dust devils, and the amount of material entrained and transported by any one, illustrates the significant amount of erosion and transport caused by dust devils. Moreover, certain desert surfaces, desert pavement in particular, may be the direct result of dust devil activity. The formation of desert pavement-relatively flat surfaces covered by rocks with few fines-is a controversial subject. Most explanations involve the action of wind, but disagreement exists on how wind acts to remove only fines resulting in a surface covered by larger particles. Dust devils act like natural vacuum cleaners, easily sucking finer dust particles up into their vortices. These dust devils lack sufficient energy to entrain particles much larger than sand (4 mm in diameter or so), so they may be more efficient sorters of sediment size than prevailing winds. Superior sorting of sediment size by dust devils may be a superior model for the development of desert pavement. As of 1985, dust devils are known to exist on Mars, providing yet another example of the geologic similarity of that planet with Earth. As with the study of Earth, the importance of dust devils to present Martian geomorphological processes is probably understated. In defense of this claim, let's discuss the physics of dust devil formation. Dust Devil Formation Earth receives practically no direct heat (thermal electromagnetic energy) from the Sun. Most electromagnetic energy originating from the Sun and reaching Earth is in the form of intense, visible light. Earth materials (rock, soil, water) absorb visible light. This absorption brings about molecular excitation in these materials, and this excitation causes a release of the absorbed, electromagnetic energy. The released energy is in the thermal (heat) frequency range. Thus, Earth is warmed indirectly by the Sun, as light is absorbed, converted in frequency to heat, then re-emitted into the atmosphere effecting warming. Walking barefoot across pavement, rock, or soil exposed to intense sunlight is one way to appreciate this process. As such surfaces release heat, warming air directly above, the warm air rises because air higher above the surface is cooler. Differences, in this case of temperature, cause motion-air circulation in this example. Although heat is not visible to the human eye, thermal energy rising from a surface does distort light. This is the cause of mirages, especially evident during summer months.
Dust devil imaged southeast of Ely, NV, in late June, 1998. Image courtesy of the author. Warmer air rises through cooler surrounding air much like gas bubbles rise through an opened bottle of soda water. As the warmer air rises, a localized cell of low pressure is created. Air under higher pressure surrounding the local cell rushes in, forcing the warmer air farther upward (upwelling). Coincidentally, the rising air rotates, counterclockwise if in the northern Hemisphere, forming a vortex. If the rise of air is rapid, this rotation will likewise be rapid. In this event, a dust devil is born. Ideal weather conditions for dust devil formation are a relatively clear day (letting sufficient sunlight fall on ground surfaces) with associated low humidity. Lower humidity results in cooler nighttime air temperatures. After dawn, as surfaces are warmed, a higher thermal gradient develops between warm surface air and cooler air above. The larger this gradient, the faster the heat from the surface will rise. The summer season is most ideal, because sunlight will fall on ground surfaces more directly.
A large Terran dust devil formed in Australia. Note the distinctive vortex shpae and the sheer quantity of dust entrained by this single dust devil. Image (c)2000 by John Roenfeldt Inflow Images. Finally, the ideal geographic environment is one that is arid to semiarid, wherein ground surfaces are sparsely vegetated. Soil and rock absorb sunlight, releasing it as heat more efficiently than does vegetation; in fact, vegetation may act to cool a surface. Valleys in the North American Great Basin are an example of such an ideal environment. From late spring through early autumn, nighttime temperature in many of these valleys drops below 12 degrees Celsius. As day breaks, the ground surface is quickly warmed by intense sunlight. Dust devils commonly develop between 9:00 a.m. and 10:00 a.m., local time, after sufficient warming occurs and thermal plumes of rising air start to develop. Martian Dust Devils Of course, the ideal climatic and geographic conditions mentioned for dust devil formation on Earth are closely matched on Mars. There, atmospheric pressure approximately 1/100 that of Earth, little cloud cover, very low humidity (especially away from the polar ice caps), and ground surfaces perfectly barren of vegetation allow sunlight to efficiently heat ground surfaces. Because of the very low atmospheric pressure, the thermal gradient in the air above these surfaces is large. During one experiment, the Mars Pathfinder lander recorded a temperature of 16 degrees Celsius, 30 cm above the ground surface, whereas a temperature of -7 degrees Celsius was recorded 150 cm above the ground surface. Given the lack of humidity, light from the Sun having an intensity less than that reaching Earth but with sufficient energy to effect warming, barren ground surfaces, and very cold air above these surfaces, dust devils should theoretically be as common, or more so, on Mars as they are on Earth. Early Explorations Mars was first imaged by the Mariner spacecraft-Mariner 4 in 1964, Mariners 6 and 7 in 1969, and a detailed mapping from orbit by Mariner 9, launched in 1971. None of the Mariner missions, though, provided the resolution necessary to observe a relatively small dust devil from space. Such resolution was available with the next Mars mission, Viking, consisting of two lander/orbiter combinations, both of which were launched and reached Mars in 1976. The Viking I and II orbiters acquired images of the planet over two full Martian years, enough time and seasonal variation to image dust devils if they occur more than rarely on that planet. Although the Viking spacecraft acquired images in the Earth period, 1976-1980, it was five years before dust devils were identified in their images. This marked the first substantiation of dust devil features on Mars.
Five dust devils imaged by the Viking orbiter in 1976. Image courtesy of NASA. Viking represented a two-component system. One component remained in orbit, imaging Mars over the period, 1976-1980. The other component soft landed, via parachute, on the surface. The Viking landers conducted several experiments, most notably soil analysis to detect evidence for life, results from which were inconclusive. Additionally, each lander was equipped with a camera that acquired digital images from the Martian surface. The manner in which these cameras operated, especially their long exposure times, favored the imaging of stationary objects on the Martian surface. Dynamic surface processes, such as dust devils, could not be imaged with sufficient resolution by the Viking landers. Mars Pathfinder, July 1998 Almost twenty years subsequent to Viking lander operation, a more sophisticated lander was successfully deployed. Known as Pathfinder, this ground-based system was equipped with an ASI/MET weather monitoring system that recorded temperature (results from which are quoted earlier), barometric pressure, wind speed and direction. Additionally, a digital camera developed by Peter Smith and colleagues at the Lunar and Planetary Laboratory, University of Arizona, provided stereo imaging. Each "eye" of the stereo camera was associated with a filter wheel. Twelve geology filters covering 443 nanometers to 1003 nanometers, including three repeated for stereo coverage, were loaded into those wheels (in addition to eight solar filters and a close-up diopter); their bandwidths range from 19 nanometers to 41 nanometers. Most "news-worthy" of Pathfinder's systems was a mobile robot, Sojourner, itself capable of imaging and mineral identification. Prior to Pathfinder's launch, I, along with Stephen Metzger, then a doctoral student at the University of Nevada, Reno, proposed to NASA what we felt would be a necessary image processing technique to "see" dust devils on Mars against a heavily dust-laden background sky. Although the Viking landers did not provide sufficient resolution to clearly image dust devils, their images revealed a sky so dust laden that its color was red to reddish-white. Given that Martian dust is also red to reddish-brown, resulting from oxidation of iron minerals, consequently Martian dust devils were likely to have a relatively high visible red signature, we felt that image differencing would be necessary to reveal dust devils against the background sky.
Composite (blue, green, and red filter) image of Big Crater from the Mars Pathfinder camera. The hill in the distance has been alternatively referred to as "Far Knob" and "Misty Mountain." Notice the hazy atmosphere. The haze is entirely attributable to dust. Some researchers have speculated that if not for the dust, the Martian sky would be almost black due to the low atmospheric density. Image courtesy of NASA. Two of the filters on each eye of the Pathfinder camera were visible blue and visible red. We hypothesized that Martian dust devils would have a higher red reflectance, due to the color of the dust, and a lower blue reflectance. Martian dust is red to bright red, not blue. Consequently, we believed that a Martian dust devil should appear as a brighter feature in the red image and a darker feature in the blue image. On this basis, we designed a process involving the subtraction of a blue-filter image from a red-filter image to enhance and reveal dust devils against the background Martian sky. Experiments applied to digital images of Earth dust devils suggested that this technique would be successful. Nevertheless, our proposal to be involved in the Pathfinder mission to look for dust devils in this manner was assigned relatively low priority by NASA and we were not funded.
After applying the sky-image subtraction, then red-minus-blue differencing, two large dust devils are revealed in the Big Crater image. Image courtesy of the author and NASA. During the Pathfinder mission, the ASI/MET instrument recorded several "events" that involved a brief, sudden drop in temperature with associated changes in barometric pressure, wind speed, and wind direction. These "events" lasted only for a short duration of time, and one of only a few plausible explanations for these events was the passage of a dust devil directly over the instrument. This could not, however, be confirmed by the Pathfinder camera. Although dust devils were the most likely cause of the ASI/MET instrument fluctuations, images from the |
