Morphology of Barchan Dunes on Earth and Mars: Classification and Scale‐Invariance

Morphological characteristics were measured for barchan dunes on Earth (2,686 dunes in 30 barchan fields) and Mars (720 dunes in 10 barchan fields) using satellite images. The data were used to (a) develop a new barchan classification system; (b) compare characteristics of barchans on Earth and Mars; and (c) assess whether barchans, in bulk, display allometric or scale‐invariant characteristics. Dimensional metrics were obtained for the width and length of barchan bodies, the width and length of barchans including the horns, and the length of each horn. Dimensionless metrics were derived for the ratios of the body width to the width between the tips of the horns (width ratio), the length of the entire barchan to the length of the body (length ratio), and the length of the longer horn to the shorter horn (symmetry ratio). The width, length, and symmetry ratios were used to classify barchans into eight types and compare the characteristics of their distributions on the two planets. From this analysis, it was established that, statistically, barchans on Earth are distinctive from those on Mars based on the morphometrics, with terrestrial barchans being, on average, of smaller size and more often symmetrical, while Martian barchans more often have convergent horns that are short relative to the central dune body and are more often asymmetrical. The analysis further reveals that barchan planform morphology can be considered scale‐invariant, and we argue that body width is the most appropriate measure representing barchan size. Plain Language Summary Crescent‐shaped sand dunes (barchans) are widely found on Earth and Mars. Relative variations in the shape of these bedforms are thought to reflect differences in the planetary sand transport environment in terms of sand flux magnitudes, transport modes, and wind climate (speeds and directional variability). Comparing the shapes of barchans on Mars with those on Earth can generate hypotheses on how and why sand transport conditions and dune formation under the Martian atmosphere may be different. Satellite imagery analysis of 3,406 barchan dunes on both planets yields insights on planform dimensions (width and length of the dune body, lengths and divergence of the horns) as well as three shape indices (Width Ratio, Length Ratio, Symmetry Ratio) that were used to derive a classification of eight barchan types. Analysis shows that average barchan planform geometry displays fixed proportions over several orders of magn