Extended Data Fig. 7: Trajectory analysis example.

(a-c) Each point on the lines with color corresponding to the colorbar on the left are for a trajectory of a 1 mm quartz grain at average Earth conditions leaving the bed with an ejection velocity of v_↑ from the horizontal axis. The solid black line denotes the impact threshold friction velocity, while the black dot and the corresponding dashed black lines denote the unique pair of the friction velocity and ejection velocity at the impact threshold. (a) The ratio of the ejection to impact speeds for a trajectory. (b) The impact angle for a trajectory, with the cyan line indicating the ejection angle. (c) The ratio of the ejection to impact speeds for a trajectory, normalized as if the impact angle was fixed (θ_↓ = − 10^∘), e₁₀ (Methods). The green line (also in (d)) is the ‘target’ restitution coefficient for this case using the empirical relation found in Extended Data Figure 4c. (d) The minima for each line in (c) plotted against the friction velocity. We define the impact threshold as the intersection of the trend and the green line.