The Threshold Wind Speed For Dust Storms On Mars

Mars is a desert planet and has a very thin atmospheric layer. There are many observations about Mars'dust storms transmitted to Earth since Mariner 9 got into orbit around Mars at 1971. There are advantages for dust storms on Mars, but the threshold wind speed for a dust storm to start is much higher than on Earth due to its low air density. Viking 1 and Viking 2 had been working on Mars for some years and measured the wind speed needs to be 30 m/s at 2m height for dust storms to happen. The wind at this speed amounts to hurricane of level 12 on Earth. In this paper, we calculate the theoretical value of threshold wind speed for dust storms on Mars through constructing a model of wind-blown sand on the surface of sanded ground. Based on the wind profile of neutral atmospheric boundary layer, it is concluded that the wind speed of 28.7m/s at 2m height is needed for sand of diameter of 100μm to start moving on Mars. This wind speed is just the threshold wind speed for dust storms to start, because sand of diameter of 100μm is the most easily starting on Mars.In this paper, we also investigate the process of sand saltation, and calculate the particle diameter of suspending dust in Mars'atmosphere when dust storms happen. It is concluded that, when wind speed is at the threshold value, sand needs to roll distance of one particle diameter and then saltates through colliding with other particles. The dust of small particle diameter is not easy to start moving because of cohesive force, but it can be ejected by collision of the first saltating sand with the ground. It is calculated that the particle diameter of suspending dust is less than 31μm when wind speed is at the threshold value.