Supporting data for the paper: "Grain size ratio may be crucial for Martian sand initiation". Each file contains the relevant data for the corresponding figure within the paper. Each sheet either named by the panel it describes or by the data it provides. For further information, please contact: Lin-Tao Fu (fultofficial@hotmail.com) This study concerns the fluid thresholds under the current Martian environment by a theoretical method. This file contains the data of figures 2-6 in the main text and figure S1 in supporting information. The conditions at which the data were obtained are clearly stated in the file. The physical variables as well their units are also described in the file. Fig. 2. Threshold friction velocity versus grain diameter for uniform cases on Earth. (A) Comparisons of calculated fluid thresholds with both semi-empirical and experimental general thresholds. (B) Effects of the selection of a bed surface on the calculated fluid thresholds. Fig. 3. Threshold friction velocity versus grain diameter for uniform cases on Mars. (A) Comparisons of calculated fluid thresholds with semi-empirical general thresholds. (B) Effects of the selection of a bed surface on the calculated fluid thresholds. Fig. 4. Fluid thresholds versus grain size ratios on both Earth (A) and Mars (B). Fig. 5. Minimum fluid thresholds versus bed grain diameter within the given range of grain size ratio on both Earth and Mars. Fig. 6. Effects of boundary layer thickness on calculated fluid thresholds. Fig. S1. Effects of lift force on calculated fluid thresholds in uniform grain size cases on both Earth (A) and Mars (B)