  Aeolian sand transport is a major process shaping landscapes on Earth and on
diverse celestial bodies. Conditions favoring bimodal sand transport, with
fine-grain saltation driving coarse-grain reptation, give rise to the evolution
of megaripples with a characteristic bimodal sand composition. Here, we derive
a unified phase diagram for this special aeolian process and the ensuing
nonequilibrium megaripple morphodynamics by means of a conceptually simple
quantitative model, grounded in the grain-scale physics. We establish a
well-preserved quantitative signature of bimodal aeolian transport in the
otherwise highly variable grain size distributions, namely, the log-scale width
(Krumbein phi scale) of their coarse-grain peaks. A comprehensive collection of
terrestrial and extraterrestrial data, covering a wide range of geographical
sources and environmental conditions, supports the accuracy and robustness of
this unexpected theoretical finding. It could help to resolve ambiguities in
the classification of terrestrial and extraterrestrial sedimentary bedforms.
