Three generations of devils: The largest black and white image is a MOC image that shows dust devils making some rampage on a Martian plain. However, the MGS mission was not the first mission to detect these phenomenons. On the upper left image, dust devils were already spotted during the Viking mission (Viking orbiter image), and on the upper right, this image taken at the Pathfinder landing site in 1997 shows that they are a permanent and global occurrence. Dust devils, with dust storms, are the main agent of removal and deposition of dust on Mars. They are most common on Mars and they are pointing out a potential danger for future missions: the existence of zones of dust accumulation hazardous to land in, as well as daily passages of these small tornadoes that can pile up dust on top of solar panels, and instruments. They are a hazard that needs also to be considered for future human exploration.
Source: NASA JPL, Viking, Pathfinder, and MGS mission.