Recently I read a book about Mars, and it says that global sandstorm blows from southern hemisphere to northern hemisphere at perihelion. It's because the southern part is more heated at that position and the closer distance to sun gives it a high temperature, thus sublimating large amout CO2 in the southern polar cap. This increase in amount of air blows from southern hemisphere. Here is my question: why doesn't sandstorm blow in another way at aphelion? When mars is farther from sun, its southern hemisphere is farther than northern one, so less heated. More CO2 is deposited back to the southern polar caps. At the sane time, northern ice cap absorb less air back relatively. Why doesn't this difference around the globe create global sandstorm as well?