The red planet is known to experience global dust storms, but their trigger isn't yet understood. On average the martian atmosphere is 20 times dustier than Earth's arid regions, therefore the big challenge is to explain how the thin martian air is able to erode and carry so much fine dust (dust is actually tougher to erode than sand grains, which are much larger but don't hide and cling to the surface as do fine clay dusts). A variety of explanations were proposed but none had been adequately demonstrated in nature.
Dust devils have been seen in orbital images of Mars from Mariner 9 onwards. They have been recognised in Viking orbiter images, and most noticably seen in Mars Global Surveyor MOC images, like the one below. MOC has seen dust devil tracks over large areas of Mars, such that what was once thought rare is not seen as common! Generally martian dust devils appear similar to Earth ones, although a bit larger. Thermal vortices were sensed at surface level by Viking lander meteorology data, and they have also been noted by the Mars Pathfinder meteorology sensors, as shown on the right. Beagle 2 also had meteorology sensors, and would hopefully have seen then (the landing site is prime dust devil terrain; flat dusty, lots of sunlight). The MERs, Spirit and Opportunity, don't have meteorology sensors, but rest assured researchers are looking at the images returned, to see if any can be seen using the technique that was applied to Pathfinder!
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NASA/Malin Space Systems |
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In 1996 Carr and Metzger (MPF Participating Scientist proposal, 1996) predicted geomorphically that dust devils would exist at the Mars Pathfinder Ares Vallis site and its meteorology and imaging systems should be able to detect the vortices, and in 1997 the MPF ASI-Met team announced that the appropriate meteorological signatures were common there (Schofield et al., 1997). In early 1998, Metzger et al. (1999) developed image processing techniques with the IMP camera data that identified several active dust devils around Pathfinder. As detailed in the paper, this involved subtracting a known 'sky' colour from the image, and then contrast stretching the result. To date several dust devils in the Pathfinder imaging data have been found, the first of which is shown on the left (this vivid images was processed by Jeff Johnson). |
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Technique... Dust devils were not obvious in imagery returned by the Pathfinder lander, despite the detection of wind vortices by ASI-Met and IMP team use of conventional image processing techniques (such as contrast manipulation). It was possible that the dust devils were spectrally indistinguishable from the reddish color of the Martian sky due to scattering caused by suspended dust with a high reflectance in red wavelengths and a low reflectance in blue wavelengths. Metzger and Carr predicted that an increased concentration of dust, as in a localized dust devil plume, blocks both background red and blue light from traveling toward the IMP camera. Whereas sunlight illuminating the dust column effectively scatters red wavelengths toward the camera and replaces much of the lost background red light, the dust's poor reflective efficiency in blue wavelengths does not substantially replace the background blue light blocked by the column. Therefore, an occultation feature is produced in the blue image. Due to the strong general Martian dust haze, however, the general "emphasised" dust haze must be subtracted, using sky flats, from the scene being searched. This removes ubiquitous dust reflectance except any concentrated plumes, such as a dust devil (Metzger et al., 2000). |
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Some results...
The color
panorama below is of the view to the southwest of MPF overlain with images of
five martian dust devils, including the South Twin Peak feature. Several
visualization techniques by the different authors account for the different
color sets (Metzger et al., 2000).
On the right is an animation is of a small
martian dust devil (15 m wide, at least 250 m tall) moving to the south (left)
on the leeward side of South Twin Peak. The three frames represent the red,
green, and blue-filter images taken 20 seconds apart, shortly after noon, as
the column moved across the marscape at 0.5 m/s.
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What would it be like? On Mars it would be a slow and soft experience; the temperature would rise a degree or two, the wind would pick up, and the sun would haze...but Mars dust devils are large and gentle compared to Earth, so you wouldn't get blown over, and the encounter would take a while, maybe up to a minute - compare this to Earth. Mars is so dry, and with all the dust around, you might get electrically charged...possibly...no-one knows for sure. Perhaps it would look like a glow discharge (pic or technical paper), or maybe like St Elmos fire (or here). When in the centre of the dust devil, there would be clear skies for a while, and low winds, and a small pressure drop. Eventually the dust devil would completely pass, and the wind would swing back to its normal direction, and the temperature and pressure would return to normal.
