In December 1999, the MOC team finally had an answer! A dust
devil, shown in the above figure, was caught in the act of creating a
swirly, dark streak! An eerie sensation washed over the first team
members who saw this picture---here was an event on Mars "caught
in the act" just hours before the picture was played back to Earth.
A "smoking gun."
The first dust devil seen making a streak---located in Promethei
Terra---was traveling from right (east) to left (west). A columnar
shadow was cast by sunlight coming from the upper left. This
shadow indicates the true shape of the dust devil. The bright dust
devil itself does not look like a column because the picture was taken
from a camera looking straight down on it. The dust devil is less than
100 meters (less than 100 yards) wide and the picture covers an area
approximately 1.5 by 1.7 kilometers (about 1 by 1 mile).
Dust devils are spinning, columnar vortices of wind that move
across the landscape, pick up dust, and look somewhat like miniature
tornadoes. Dust devils are a common occurrence in dry and desert
landscapes on Earth as well as Mars. They form when the ground
heats up during the day, warming the air immediately above the
surface. As the warmed air nearest the surface begins to rise, it
spins. The spinning column begins to move across the surface and
picks up loose dust (if any is present). The dust makes the vortex
visible and gives it the "dust devil" or tornado-like
appearance. On Earth, dust devils typically last for only a few minutes.
Photo Credit: NASA/JPL/Malin Space Science Systems