Lifting and Transport of Martian Dust by the Ingenuity Helicopter Rotor Downwash as Observed by High‐Speed Imaging From the Perseverance Rover.

Martian atmospheric dust is a major driver of weather, with feedback between atmospheric dust distribution, circulation changes from radiative heating and cooling driven by this dust, and winds that mobilize surface dust and distribute it in the atmosphere. Wind‐driven mobilization of surface dust is a poorly understood process due to significant uncertainty about minimum wind stress and whether the saltation of sand particles is required. This study utilizes video of six Ingenuity helicopter flights to measure dust lifting during helicopter ascents, traverses, and descents. Dust mobilization persisted on takeoff until the helicopter exceeded 3 m altitude, with dust advecting at 4–6 m/s. During landing, dust mobilization initiated at 2.3–3.6 m altitude. Extensive dust mobilization occurred during traverses at 5.1–5.7 m altitude. Dust mobilization threshold friction velocity of rotor‐induced winds during landing is modeled at 0.4–0.6 m/s (factor of two uncertainty in this estimate), with higher winds required when the helicopter was over undisturbed terrain. Modeling dust mobilization from >5 m cruising altitude indicates mobilization by 0.3 m/s winds, suggesting nonsaltation mechanisms such as mobilization and destruction of dust aggregates. No dependence on background winds was seen for the initiation of dust lifting but one case of takeoff in 7 m/s winds created a track of darkened terrain downwind of the helicopter, which may have been a saltation cluster. When the helicopter was cruising at 5–6 m altitude, recirculation was seen in the dust clouds. Plain Language Summary: Mars is a dusty planet with dusty atmosphere, and dust is a major factor in the weather. Weather events, from large storms to small dust devils, raise dust in their winds, but the conditions needed to lift the dust remain elusive. We used video of six flights of the Ingenuity helicopter, taken by Mastcam‐Z on the Perseverance rover, to document when and where dust lifting occurred. We found that the helicopter sometimes kicked up dust when it was cruising >5 m above the surface, and that it always did so when it was 1.4–3.6 m above the surface as it landed. Some of the dust was likely lifted when the winds moved sand particles, and the sand dislodged the sticky dust, as in some current models. However, some dust lifting happened with lower winds, and likely happened when large aggregates of dust (sometimes called "dust bunnies") were dislodged and broke up. Key Points: Six flights of the Ingenuity helicopter were documented with video acquired by Mastcam‐Z on the Perseverance rover in Jezero crater, MarsDust mobilization was expected when the helicopter was below 1‐m altitude but was observed at low wind speeds from >5 m altitudeRecirculating dust clouds unexpectedly formed while the helicopter was at >5 m cruising altitude [ABSTRACT FROM AUTHOR]