Grain size ratio may be crucial for the fluid initiation of martian sand grains

Abstract

The active Aeolian sand movement on the Martian surface has been puzzling scientists for several decades because the existing theories do not support the initiation of sand grains under the current Martian environment. In this study, I theoretically calculated the fluid threshold (u⁎ft) for the initiation of sand grain movement by considering the gravitational force, the drag force, the lift force, and the cohesive force. For a fixed grain diameter, u⁎ft calculated by considering the wind turbulence is generally less than the values obtained from well-known initiation threshold expressions. It also reveals that wind turbulence has a significant impact on u⁎ft of grains with diameters of greater than 50 μm on Earth and for those with diameters of greater than 500 μm on Mars. The grain size ratio (the ratio of a grain diameter on the bed surface versus the averaged diameter of a mixed sand bed) is a key parameter in determining u⁎ft of mixed sand beds. On Mars, the lowest u⁎ft, which occurs for mixed sand beds with average grain diameters of 50–150 μm, could be 0.6–0.7 m/s when the optimal grain size ratio is determined from the given grain size ratio range. Thus, the results of this study not only quantitatively agree with recent findings from wind tunnel experiments but also provide a theoretical explanation for the observations of active sand movement on Mars.