Interesting question. While the pressure at the surface is one percent or less of earth's surface atmospheric pressure, the gradient decreases more slowly at Mars than at earth. The main ingredient in the equation is the gravity of Mars, which is less than earth's gravity and cannot pull the atmosphere in so tight. Orbital altitude at Mars is twice as high as at earth because the pressure at altitude is higher. Another factor could be that the sunlight is weaker at Mars and would not drive the gas away from the planet as quickly as it does at earth, which would tend to let light concentrations continue to exist at high altitude.
Ok, now you have done it - I have to go down to the lab and play with STK for a while. (sattelite tool kit)