Why does Mars have dust storms that can cover the whole planet?

Mars has a very thin atmosphere, but its surface is covered with fine dust that can be lifted by winds. When sunlight warms the surface unevenly, it creates pressure differences that stir the atmosphere and lift more dust, sometimes in a self-reinforcing cycle. Because Mars lacks large oceans and has a dry, dusty surface, once a storm starts it can spread widely. Global dust storms are most likely during the southern spring and summer, when Mars is closer to the Sun in its elliptical orbit. These storms can block sunlight and affect solar-powered spacecraft, as seen during the 2018 event that affected the Opportunity rover.