Fig. 3: Dune length (blue-to-yellow circles) and large-ripple wavelength (light purple¹ and dark green circles³⁶) decreases with increasing atmospheric density.

Ripple data follows a previously derived scaling relationships for drag ripples⁵; best-fit power law to an updated global compilation of dark-toned large-ripple wavelength has an exponent of −0.63 (green dashed line)³⁵. The width of the hydrodynamic anomaly (gray shade) was calculated for a 125 μm grain size⁷ and assuming that wind shear velocity is at the transport threshold⁴⁹ (Methods), using the model of ref. 12. The statistical significance of the regression slope was computed using a Wald Test, and errorbars (narrower than line width) indicate a 95% confidence interval. Each dot in the figure represents the 2nd percentile dune length in equal-area bins (60 km²).