Astronomy Picture of the Day
March 8, 2013

Swirling Winds at Gusev (Sol 421)
Swirling Winds at Gusev (Sol 421)

Credits: NASA/JPL - MER Spirit - Credits for the additional process.: Dr Paolo C. Fienga/Lunar Explorer Italia/IPF

Dust Devils were caught while running across the Plains of Gusev Crater by the NASA - Mars Exploration Rover (MER) Spirit (now decommissioned) during the 421st Sol (such as March, 10, 2005) of its Mission. Today's APOD, is an image-mosaic obtained from pictures taken by the Rover's Navigation Camera (or NavCam for short). Several views of the Region of Gusev Crater which were obtained from orbit, often showed countless Dark Streaks marking the Landscape (Streaks that were identified as the Tracks left by Dust Devils in those places where they were strong enough to remove the Surface Dust and so to show the - relatively - darker Soil that lies below), but this image-mosaic represents the first time that active Dust Devils have been seen and photographed directly from the Surface of Mars.

Scientists have considered several causes for the occurrence of these (often, but NOT always) small Wind-related Phenomena, and the most reliable explanation is based on the fact that Dust Devils always occur when the Sun heats the Surface of Mars (as well as of any other Celestial Body possessing an Atmosphere: let's not forget that Dust Devils, in fact, do occur even on our Home Planet Earth!). After some time of continuous Solar Illumination and subsequent Heating, the Surface, once is warm enough, is in condition to heat the Layer of Atmosphere that is closest to it, so that the warm Air rises in a whirling motion, lifting up and stirring Dust, Ashes and small particles of Sand, and acting, more or less, like some sort of "Miniature Tornado" (or a - relatively - "Small Hurricane"). However, another explanation that can still be considered (even though we, as IPF, believe that it is quite unlikely) says that a Flow Structure might develop over Craters as the Winds' Speed increase. As the Winds pick up, the turbulence eddies and then rotating columns of air form. As these columns grow in diameter, they become taller and gain Rotational Speed. Eventually, they become self-sustaining and the Wind blows them down range.

One Sol before the images forming this mosaic were taken, the Power Output from Spirit's Solar Panels went up by about 50% just when the amount of Dust that had set on the Panels decreased. Was this a coincidence, or did a "helpfu"l Dust Devil - litterally - blow over Spirit and lifted off some of the Dust? Wahatever the truth may be, by comparing the separate images that were taken by the Rover's different Cameras, team members estimate that the Dust Devils visible here (and indicated by the red arrows) moved of about 500 meters (1640 feet) in the 155" (seconds) that passed between the frames taken by the Navigation Camera (NavCam) and the Hazard-Avoidance Camera (HazCam); that equates to about 3 meters per second (such as approx. 7 miles per hour). The Dust Devils caught in these frames appear to be about 1100 meters (such as almost three-quarters of a mile) from the Rover.

This frame (which is an Original NASA - Mars Exploration Rover "Spirit" b/w image-mosaic published on the NASA - Planetary Photojournal and identified by the ID n. PIA 07457) has been additionally processed and then colorized in Absolute Natural Colors (such as the colors that a normal - meaning "in the average" - human eye would actually perceive if someone were on the Surface of Mars, near the NASA - Mars Exploration Rover (MER) Spirit, and then looked towards the Horizon and Sky of Gusev Crater), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team.

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