Planetary saltation: Should we care about cohesion?

Sediment transport in saltation is an important driver of the morphodynamics of planetary sedimentary surfaces and particularly responsible for the formation and evolution of aeolian ripples and dunes. When estimating the incidence and persistence of saltation on extraterrestrial planetary bodies, geomorphologists usually ask by how much the atmospheric winds on such bodies exceed the threshold value required to initiate saltation, a question that is inherently linked to the cohesiveness of a body's surface sediments. For example, there is currently an ongoing controversy about the saltation initiation threshold on Saturn's moon Titan because of strongly varying estimations of the cohesiveness of Titan's soils. If the value of this threshold is outside a certain relatively small range, the currently leading explanation for an observed mismatch between Titan's dune orientation and the predominant atmospheric wind direction is thought to break down. Here we put up for discussion an alternative viewpoint on the importance of cohesion and saltation initiation. First, we briefly review experimental and theoretical evidence from the literature suggesting that, in the field (in contrast to wind tunnel experiments), saltation is almost always easily initiated, which means that one mainly needs to understands whether saltation can be sustained once initiated. Second, we present results from DEM-based numerical simulations suggesting that saturated saltation, in particular the smallest wind speed at which it can be sustained (i.e., the cessation threshold), is almost unaffected by cohesion. Third, we show a simple theoretical conceptualization that explains these numerical results and, when implemented in an analytical model, captures existing cessation threshold and saltation transport rate measurements. Finally, we show that the predictions of this model are consistent with several direct and indirect observations associated with extraterrestrial saltation, including the orientation of Titan's dunes.