Wind Mechanics
When Mariner 9 first entered the Martian orbit in the Fall of 1971, a planet-wide dust storm was raging throughout the entire planet. This dust storm started two months earlier and was detected from Earth-based observations. The dust storm appeared through the telescope as a yellowish cloud that rapidly covered the entire planet. The dust storm was one of the earliest pieces of evidence that aeolian process was operating on the Martian surface. It is one of the few geological processes that remains active even at the present time. Similar to the Earth, aeolian processes can cause erosion to the surface as well as create new landforms by deposition. Even though Mars has a smaller surface gravity, it also has a lower atmospheric pressure, so very large wind speeds are necessary to move dust particles and transport them across the planet. Typical wind speeds in the Martian atmosphere exceed 200 km/hr (or 125 miles/hr). Gusts can often reach 500 to 600 km/hr (or 300-375 miles/hr). Typical Wind speeds at the surface during a dust storm, as measured by the Viking landers, are typically about 30 m/sec (or 67.5 miles/hr). With wind speeds as high as these, there is no question that dust particles can be blown off the ground and produce dust storms.