Aeolian saltation on Mars at low wind speeds

Laboratory experiments indicate that the fluid threshold friction speed, u *tf , required to initiate fully developed aeolian saltation is much higher on Mars than on Earth. A discrepancy exists between Mars climate models that do not predict winds this strong and observations that sand‐sized particles are indeed moving. This paper describes how wind friction speeds well below u *tf , but above the impact threshold, u *ti , required to sustain saltation, can initiate sustained saltation on Mars, but at relatively low flux. Numerical experiments indicate that a sand grain on Mars mobilized sporadically between u *ti and u *tf will develop, over fetch lengths longer than generally available within low‐pressure wind tunnels, trajectories capable of splashing grains that propagate saltation and collectively form a cluster of saltating grains that migrate downwind together. The passage of a saltation cluster should leave behind a narrow zone of affected surface grains. The cumulative effect of many clusters represents a low‐flux phenomenon that should produce slow changes to aeolian bedforms over periods in which winds remain close to u *ti and never or rarely reach u *tf . Field evidence from small impact ripples along rover traverses is consistent with effects of saltation at these low friction speeds, without obvious evidence for events ≥ u *tf . The potential utility of this grain mobility process is that it can operate entirely at more common winds well below u *tf and so help explain widespread sand movements observed on Mars wherever evidence might be mostly absent for u *tf being exceeded.