Aeolian saltation on Mars at low wind speeds

Abstract

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.

Sedimentary processes of the Bagnold Dunes: Implications for the eolian rock record of Mars

“…No spatial variation in grain size was observed across the ripple. A similar size distribution from this area was also found by Sullivan and Kok [].…”

Chemistry, mineralogy, and grain properties at Namib and High dunes, Bagnold dune field, Gale crater, Mars: A synthesis of Curiosity rover observations

“…Overall, there is little or no evidence for dust in the MAHLI images of the Bagnold sands. Collectively, observations of the Bagnold sands at Namib and High dunes are consistent with observations made from orbit and from the ground of sand transport and bedform motion, i.e., present‐day wind‐driven activity [ Silvestro et al ., ; Bridges et al ., , 2017; Lapotre et al ., , Sullivan and Kok , ].…”

Numerical Study of Shear Stress Distribution Over Sand Ripples Under Terrestrial and Martian Conditions

“…Although our simulated ripples are quite small compared to the large ripples observed on Mars (e.g., Lapotre et al, ), the smaller shear stress at the crest may explain why impact ripples can be larger on Mars. Specifically, the ripples on Mars can still grow due to saltation that occurs at low wind speeds above the impact threshold (Sullivan & Kok, ). Thus, the large ripples on Mars can form under the same impact mechanism acting for small ripples without the need for the wind‐drag formation hypothesis (Lapotre et al, , ).…”

“…On Earth the ratio is u * ti / u * t ≃ 0.8 while on Mars it is much smaller, u * ti / u * t ≃ 0.1 (Kok et al, ). It has been suggested that this difference can explain the large aeolian activity observed on Mars despite the low predicted wind velocities (Kok, ; Kok et al, ; Silvestro et al, ; Sullivan & Kok, ). Global climate models for Mars, because of their low spatial resolution, generally do not predict u * > u * t , and thus, no sand transport should occur (Basu et al, ; Chojnacki et al, ).…”

“…The few and infrequent wind measurements made by Mars landers have also found that winds on Mars rarely exceed the fluid threshold needed for initiation of sand transport (Holstein‐Rathlou et al, ; Schofield et al, ; Zimbelman, ). Recently, a mechanism was identified for low‐flux sand transport on Mars occurring close to u * ti without the need for gusts above the fluid threshold to initiate the saltation (Sullivan & Kok, ). This mechanism is based on sporadic saltation clusters formed above the impact threshold but well below the fluid threshold, over long fetch lengths where the grains hop on the surface with increasing energy due to the low Martian gravity.…”