One of the benefits that was taken from the NASA - Mars Global Surveyor (MGS) - Mars Orbiter Camera (MOC) Extended Mission was the opportunity to observe how the Red Planet's weather changed from one Martian Year to another. This picture that shows us the Giant Shield Volcano Arsia Mons (which is large enough to cover almost the entire American State of New Mexico and is the only one of the Major Tharsis Volcanoes that is located South of the Equator) was taken on June, 19th, 2001, such as the same day when the Martian Southern Spring Equinox had occurred. On this particular day, the MGS - MOC Wide Angle Cameras documented, as you can see, an unusual Spiral-shaped Cloud that formed within the approx. 110 Km wide Summit Caldera of the Giant Shield Volcano.
Since this Cloud was bright both in the red and blue images acquired by the Wide Angle Cameras, the NASA Scientists speculated that it was probably (and mostly) made of of fine Dust Grains rather than microscopic Water Ice Particles. Furthermore, the Cloud's spin could have been induced by the action of suddenly formed and extremely powerful Swirling Winds that generated on the Inner Slopes of the Caldera Walls - and which were (most likely) caused by the strong difference existing between the temperature of the (however extremely thin) air that was near the Walls themselves and the temperature existing on and/or near the Floor of the Summit Caldera.
A plausible alternative to the abovementioned hypothesis is that the Swirling Cloud might have been induced by a Vortex that formed when the Winds (which, in any case, must have formed within the Caldera) blew up and over the Caldera itself. As a matter of fact, many similar Spiral Clouds were seen, after this one, inside the Arsia Mons' Caldera for several days, but it was impossible to find out if such a Spiral Clouds were the result of a single (and very persistent) phenomenon which lasted throughout that time, or if the Clouds, in some way, "regenerated" themselves every afternoon.
This frame has been colorized in Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - Mars Global Surveyor Orbiter and then looked down towards the Surface of Mars), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team.
