Windblown Ripples at Mars-Like Pressures

Low-pressure wind tunnel experiments revealing the origin of Mars's large windblown ripples — MARSWIT experiments.

Windblown Ripples Under Earth-to-Mars Atmospheric Pressures

Sand ripples record past and present interactions between planetary surfaces and atmospheric flows. On Mars, enigmatic meter-scale windblown ripples have long been assumed to be impact ripples — bedforms shaped by grain collisions during saltation. However, ground and orbiter observations challenged this interpretation, suggesting instead a drag-ripple origin driven by aerodynamic instabilities, analogous to current ripples in water.

This project used the Mars Surface Wind Tunnel (MARSWIT) at NASA Ames Research Center to reproduce Mars-like conditions in the laboratory and directly observe how large windblown ripples form and evolve from a flat sand bed across a range of atmospheric pressures (50–1020 mbar).

Windblown Ripples on Earth and Mars

Windblown ripples under varied pressure conditions. A Two scales of windblown ripples atop Namib dune, Gale Crater, Mars (Curiosity rover Mastcam mosaic mcam005410, sol 1192; credit: NASA/JPL-Caltech/MSSS). B Impact ripples in the Mesquite Flat Sand Dunes, Death Valley, California, USA (image modified from Alvarez et al., 2025).

Experiment — Bed Evolution at 100 mbar

The video below shows a side view of the sand bed evolving under active saltation at 100 mbar, capturing the growth of two distinct scales of ripples from an initially flat surface.

Side view of sand bed evolution at 100 mbar in the MARSWIT. Two scales of windblown ripples grow simultaneously from an initially flat bed under turbulent airflow at Mars-like atmospheric pressure.

Key Findings

  • Impact ripples and large ripples arise from two distinct bed instabilities. Two scales of ripples appeared simultaneously from a flat bed at low pressures, with no intermediate wavelengths ever observed. Large ripples formed far faster than coalescence of small ripples could account for, directly ruling out the impact-ripple hypothesis.

  • Aerodynamic roughness length (z₀) increases as atmospheric pressure decreases. Measurements over equilibrated rippled beds under active saltation show that z₀ grows systematically with decreasing pressure.

  • In martian sand sheets, z₀ is likely controlled by form drag. Rather than grain-scale roughness or the viscous sublayer, the shape of the ripples themselves dominates the aerodynamic resistance of the bed.

  • z₀ can be up to two orders of magnitude larger on Mars than is typically assumed, while flow remains smooth. Values can reach up to 1 cm — far exceeding the flat-bed assumptions used in current wind-speed predictions and global circulation models for Mars.

  • Large martian ripples are drag ripples. Their size, formation mechanism, and associated sand fluxes are all consistent with aerodynamic (drag) ripple theory.

  • Alvarez, C.A., Lapôtre, M.G.A., Swann, C., & Ewing, R.C. (2025). Ripples formed in low-pressure wind tunnels suggest Mars's large windblown ripples are not impact ripples. Nature Communications, 16, 2945. DOI
  • Alvarez, C.A., et al. (2025). Aerodynamic roughness of rippled beds under active saltation at Earth-to-Mars atmospheric pressures. Nature Communications. DOI