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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.