Dust
Sunshine warms the Martian atmosphere and causes the air to move, lifting the dust off the ground. Mars is dryer and colder than Earth, and dust raised by these winds tends to remain in the atmosphere because there is no rain to wash it out.
On Mars, a dust storm can develop in a matter of hours, and envelope the entire planet in a Global Dust Storm.
In 2001, A Global Dust Storm covered the entire planet within a few weeks and lasted for months.
In the image above, a huge dust storm races across on Mars. Note wispy clouds at top right.
The Mars Rover, Spirit, took photographs of its own solar panels - note the build up of dust.