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. Plain Language Summary Wind-blown sand grains on Mars create ripples and dunes and erode landscapes. Wind tunnel experiments have shown that much stronger winds are required on Mars than on Earth to start most grains moving. There is little evidence for winds this strong on Mars, yet orbiters and rovers observe changes to ripples and dunes. How could sand be moving routinely on Mars even though winds required to initiate this process are rare? A small fraction of sand grains starts moving at lower wind speeds, well below the high wind speeds where most sand starts to move, and this phenomenon turns out to be important. In this paper, a computer program simulated what happens to a sand grain sporadically mobilized under low winds on Earth and on Mars. The results differ between the two planets. On Mars, a grain sporadically mobilized at low wind speed will hop along the ground with increasing energy, eventually gaining enough energy to splash other sand grains vigorously at each bounce to cause additional sand to move collectively in a low-rate process that should result in gradual surface changes to ripples and dunes during prevailing low winds. Primarily, this is an effect of low Martian gravity. Slow ripple migration in the presence of wind direction changes should allow cross-oriented patterns of simple ripples, and this is observed at the Bagnold dunes, Gale crater. These results could help explain widespread sand movements observed on Mars wherever evidence might be mostly absent for very strong winds.