Comment on "Minimal size of a barchan dune"

Area of Science:

• Geophysics
• Planetary Science
• Fluid Dynamics

Background:

• Dune formation size and minimal size scale with flux saturation length, defined as the relaxation length of the slowest mode towards equilibrium transport.
• The Parteli, Durán, and Herrmann model predicts zero saturation length inversely proportional to wind shear stress, a prediction challenged here.

Discussion:

• The Parteli et al. model is shown to be inconsistent, as grain inertia limits relaxation rates even under high wind, preventing saturation length from reaching zero.
• The model's refutation of dune size scaling with drag length, based on Martian dune wavelengths, is contested.
• The estimation of Martian saltating grain size (500 microm) is questioned due to straightforwardness and conflicting rover imagery.

Key Insights:

• Grain inertia imposes a lower limit on relaxation rates, contradicting the Parteli et al. model's prediction of zero saturation length.
• Martian rover images reveal smaller grain sizes (87±25 microm) and millimeter-scale hematite spherules, indicating smaller grains are the primary saltators.
• The presence of large, infrequently entrained spherules does not support the hypothesis of past strong winds acting on large grains.

Outlook:

• Revisiting dune formation models with a focus on grain inertia and realistic Martian grain size distributions is crucial.
• Further investigation into the dynamics of mixed-grain-size sediment transport in aeolian environments is warranted.
• Understanding the interplay between wind shear, grain properties, and saturation length is key to accurately modeling dune evolution on Earth and Mars.