Dust devils are little tornadoes that pop up in dry, windy places. Mars is one big dry, windy place, so it was no surprise that the Viking Orbiters snapped pictures of hundreds of dust devils in the 1970s. "With the Orbiters looking down on top of them, they look like little white dots," says James Carr, a geological engineer at the University of Nevada. Surprisingly, NASA scientists never saw any dust devils on the ground in images from either the Viking or Pathfinder landers. This is because the lower Martian atmosphere is chock-full of dust; the sky at the horizon is essentially the same color as the dust devils, making them almost invisible. Carr, however, thought of a way to spot them. Whereas the light at the horizon is a mixture of red dust and blue sky, the dust devils are pure red, so by subtracting out the parts of the horizon that were both red and blue, only the dust devils should remain. Carr and his graduate student Stephen Metzger studied hundreds of Pathfinder images and so far have found five hidden dust devils. Though small, Carr says, these little whirlwinds can have a huge impact: under the right conditions, one can trigger a Mars-wide storm that fills the entire planet's atmosphere with dust.