Scaling of equilibrium planetary saltation transport

Aeolian sediment transport in saltation shapes erodible surfaces and affects the dust cycles and climates of planetary bodies. For the approximately unidirectional near-surface winds often temporarily prevailing in planetary atmospheres, saltation transport approaches an equilibrium state when given enough fetch to adapt. However, predictions of even this arguably simplest transport state have relied on oversimplified physical models or empirical models derived exclusively from measurements under Earth's atmospheric conditions. Here, we use grain scale-resolved sediment transport simulations to derive general scaling laws for equilibrium planetary saltation transport. The simulations, consistent with terrestrial measurements, cover seven orders of magnitude in the particle-fluid-density ratio s, ranging from water to extremely rarefied air on Pluto. They reveal that the saltation threshold exhibits a parabolic dependency on the grain size, with a pronounced threshold minimum that scales as s^1/3. In contrast, previous studies reported a s^1/2-scaling and substantially larger threshold values for nonequilibrium conditions. Furthermore, the simulations reveal that the saltation mass flux and grain impact energy flux, which is responsible for the emission of soil dust into a planetary body's atmosphere, obey scaling laws resembling the classical law by Ungar and Haff (Sedimentology 34, 289-299, 1987), but with nonconstant scaling coefficients proportional to s^1/3. Our results, summarized in phase diagrams for the cessation threshold, mean mass flux, and dust emission potential, are consistent with several geomorphological observations across Solar System bodies, such as the eastward propagation of Titan's dunes despite predominant westward winds.