Mars Global Surveyor's (MGS) Mars Orbiter Camera (MOC) captured this view of a dust storm within the Ius and Melas Chasms of the Valles Marineris trough system on May 16, 1999.
The dust storm is seen in the lower 1/3 of the image. It occurs at the junction between eastern Ius Chasma and western Melas Chasma. The apparent motion of the storm is approximately from the south (bottom of image) toward the north. The dust cloud forms a sharp front along its northern margin, which is seen along the north wall of Ius and Melas Chasms--in fact, at the time the image was taken, the dust had advanced up over the north wall of Melas Chasma (upper portion of lower right third of image) and was advancing across the upland that separates this chasm from western Candor Chasma. For a clear-atmosphere view of western Candor and Melas Chasms, see "Western Melas and Candor Chasms, Valles Marineris, MOC2-105, 25 March 1999".
For scale, note that the large crater south of Hebes Chasma, Perrotin, is about 95 kilometers (59 miles) across. Bluish-white clouds in the image are interpreted to consist of water ice. The pink/red clouds of the dust storm occur closer to the ground, at a lower altitude than the water ice clouds.
One of the most interesting aspects of this dust storm is that Valles Marineris was observed to have a dust storm at exactly the same time of year, one Martian year ago. During its approach to Mars, MOC obtained a picture of the planet on July 2, 1997, just prior to the Mars Pathfinder landing. At the time, it was winter in the southern hemisphere, and dust clouds were observed within Valles Marineris. The picture is seen in "Mars Orbiter Camera Views Mars Pathfinder Landing Site,MOC2-1, 3 July 1997". It will be interesting to see if similar storms occur within the Valles Marineris 1 and 2 Mars years hence. The next times will be in early April 2001 and mid-February 2003.
Malin Space Science Systems and the California Institute of Technology built the MOC using spare hardware from the Mars Observer mission. MSSS operates the camera from its facilities in San Diego, CA. The Jet Propulsion Laboratory's Mars Surveyor Operations Project operates the Mars Global Surveyor spacecraft with its industrial partner, Lockheed Martin Astronautics, from facilities in Pasadena, CA and Denver, CO.